Proton Synchrotron Division
Introduction
Following the celebration of the 40th anniversary of the PS at the end of 1999, the synchrotron this year entered its fifth decade of operation as CERN’s workhorse accelerator. The Division was once again fully occupied supplying its usual multitude of beams to the users, nevertheless finding time to work on other exciting subjects not directly linked to today’s beams, and including essential R&D aimed at the very long- term future of CERN. Nowhere else in CERN does one find such a diversity of topics being studied and machines being operated, which is what makes the Division so attractive. In April the ISOLDE technical team was welcomed into the Division, it having been decided by the CERN management that PS would be a more logical home for ISOLDE operation and development than its former home, EP Division. Then at the end of the year a group of RF experts from SL Division was also welcomed into the Division. Since it was no longer necessary to coax the best performance from LEP’s superconducting cavities after the closure of that machine, this team will become the nucleus for the construction of the new CTF3 test facility. The highlights of the year with regard to beams were the commissioning of the Antiproton Decelerator (AD) and the start-up of the neutron Time-Of-Flight (nTOF) facility. Both had been eagerly awaited by the physicists, who obtained exactly what they were expecting, although perhaps with just a slight delay. The year also saw the end of a decade of electrons/positrons in the PS for LEP, and the best run ever for heavy ions. This latter run was scheduled originally as the last, but it has now been agreed that ions will come back again in 2002/3, and of course later for the LHC. Work continued on improving the proton beam needed in the future by the LHC, and during this time a new intensity record was established for protons in the PS, and extracted from it. In summary, the year 2000 was an excellent one for the PS and for the Division.
Operation
Operating statistics for the different particle beams in the PS Complex in 2000 are presented in the tables below.
Proton Synchrotron Division 185 Operational statistics for lepton operation in 2000
Total number of hours scheduled for lepton operation, including expt. areas 6787 h Total number of hours achieved for lepton operation 6573 h Hours scheduled for lepton production for SPS/LEP, including setting up in PS 5242 h Hours achieved for lepton production for SPS/LEP 5041 h Electrons supplied to SPS/LEP 1.50 × 1017 Positrons supplied to SPS/LEP 1.38 × 1017
Operational statistics for proton operation in 2000
Total number of hours scheduled for proton operation 6477 h Hours scheduled for setting-up and machine development 441 h Hours scheduled for proton production for SPS 4038 h Hours achieved for proton production for SPS 3832 h Protons produced for SPS (at PSB extraction) 1.76 × 1019 Protons produced for SPS (at PS extraction) 1.59 × 1019 Protons for machine studies (at PSB extraction) 1.99 × 1018 Protons for AD (at PSB extraction) 2.62 × 1018 Protons for East Hall test beams (at PSB extraction) 7.74 × 1017 Hours scheduled for ISOLDE operation 3224 h Hours achieved for ISOLDE operation 3136 h Protons supplied by PSB for ISOLDE operation 8.73 × 1019
Operational statistics for Pb-ion operation in 2000
Hours scheduled for ion production for SPS 1184 h Hours achieved for ion production for SPS 1138 h Total charges of Pb53+ ions for SPS (at PSB extraction) 1.91 × 1016 Total charges of Pb53+ ions for SPS (at PS extraction) 1.4 × 1016
Operational statistics for AD operation in 2000
Hours scheduled for AD operation for physics 1482 h Hours achieved for AD operation for physics 1272 h
The Proton Run from March to September
The PS Complex started up at the beginning of March as usual, following the annual shutdown. The LPI machine was the first into action, but the proton Linac 2, the PS Booster and the PS itself were not far behind. Leptons were injected into the PS on 13 March with the protons following soon after. However, it quickly became apparent that there were some timing or synchronization difficulties affecting all machines. These problems were soon traced to an unforeseen instability in the TG8 timing units, which had been modified during the shutdown; the PS Complex has about 250 of them. When laboratory tests revealed that the problem could not be resolved easily, it was decided to go back to the old hardware/software configuration for each of
186 Proton Synchrotron Division the 250 units. Thanks to a big effort from the Controls group this change was made in less than a week, and the delivery of beam to users was not delayed.
LEP began operation on 3 April, and the East Hall beams were delivered as planned a week later. ISOLDE was also scheduled to begin operation then, and for ISOLDE this start-up was an important one as it was their first as part of PS Division. All went as planned and the PSB beams at 1.0 and 1.4 GeV, with nominal intensity up to 3 × 1013 protons/pulse, were sent to the ISOLDE target by mid-April. ISOLDE started by using the GPS target station, but one of the big priorities for the first half of 2000 was to complete the commissioning of the second separator (the HRS). This is important if 400 shifts of beam time are to be delivered to the ISOLDE users per year, as promised. This important milestone was passed at the end of May when the first scheduled HRS physics run took place.
The SPS proton physics programme was scheduled to start a week after Easter, but this did not mean that the SPS was idle until then. The PS had to supply a number of specialized Machine Development (MD) beams to the SPS machine physicists as part of the preparation of the SPS for LHC. On 2 May, the SPS began fixed- target physics operation with protons. This operation normally uses the standard five-turn proton extraction at 14 GeV from the PS. However this year, the SPS scheduled a two-week physics run to test some components of LHC detectors. These tests required the SPS to extract beam at 450 GeV, with the LHC-style 25 ns bunch spacing. As this bunch spacing is established in the PS, it was decided to perform this test run using a test version of the LHC beam in the PS, instead of the standard SPS proton beam. This mode of operation was extremely successful and will probably be repeated for similar runs in the future.
AD commissioning started on time although there had been a serious vacuum leak on a stochastic cooling kicker, and some alignment and vacuum problems with the electron cooler. However, this did not stop the AD team making excellent progress, and the goal of decelerating antiprotons to 100 MeV/c using both stochastic and electron cooling was successfully achieved by July. The AD requires two beams from the PS. First the machine is set up using 3.5 GeV/c protons, which are injected in the ‘wrong’ direction around the AD machine (i.e. they circulate anticlockwise). This has the advantage that the AD can be optimized for deceleration with a relatively intense proton beam rather than with low-intensity antiprotons. However, the beam cooling systems do not work in this mode. Therefore, the transverse emittance of the 3.5 GeV beam has to be very delicately controlled in the PSB and the PS, to allow subsequent deceleration in the AD. Then, to produce antiprotons, the AD needs a high-intensity 26 GeV proton beam incident on the antiproton production target. This beam is produced by injecting four PSB bunches into half of the PS circumference and then compressing them into a quarter of the circumference, just before ejection from the PS. This new scheme was made operational during the PS start-up and 1.5 × 1013 protons at 26 GeV were available for antiproton production for AD right from the start.
Every 12 weeks or so, the PS main power supply needs some regular maintenance because it is rotating machinery (and is now more than 30 years old). A 10-hour technical stop was scheduled for 7 June, in the shadow of which other interventions were carried out, including a large amount of scheduled (and unscheduled) vacuum work on Linac 2, the PSB and the PS. All the PS Complex water stations were also stopped for routine maintenance and inspection. In spite of the large amount of work carried out, all the beams were back again by early evening. For the PS, this was simply a continuation of what had been already operational: protons for SPS, leptons for SPS/LEP, slow extracted beams for DIRAC and the other East Hall experiments, the AD production beam, and a number of MD beams for both PS and SPS studies. At the
Proton Synchrotron Division 187 Booster, the ISOLDE programme at the GPS station was in full swing and, throughout June and July, the commissioning tests of the HRS continued.
During June, a lot of effort was put into completing the commissioning of the AD machine, in order to meet the re-scheduled deadline for starting AD physics on 10 July. By 14 July, the ASACUSA team was able to announce that it had made ‘a complete scan of a 597 nm resonance’. This was done with only 10 AD antiproton shots in a single afternoon. During the preliminary AD test run in December 1999, a similar scan had taken 24 hours. However, there was still a lot of work to be done, and all through the summer the small AD team shared the beam time with the physics experiments to complete the AD commissioning phase. This sharing of beam time between experimental physics and the machine studies programme was very successful, and by the end of the year the AD was running with over 85% antiproton beam availability for the experiments out of the 110 scheduled hours each week. Although the AD cycle is still a factor of two longer than planned, the decelerated intensity is more than double the design intensity, which allows the AD users to profit fully from this unique machine.
By August the SPS machine development experts were ready to begin studying the ‘full’ LHC beam in their machine. There were some concerns at the PS about the long 26 GeV flat-top on the LHC cycle, which means that the r.m.s. current in the ‘8-loop’ pole-face winding exceeds the maximum allowed value if there are too many of these cycles in a single PS supercycle. Fortunately it was possible to increase the limit on this r.m.s current just enough to allow the required three consecutive LHC cycles. However this is still a severe limitation, and modifications will have to be made in the future. Another major development on the LHC beam was the demonstration by the RF specialists of triple bunch-splitting in the PS. We have grown accustomed to the complex RF gymnastics in the PS, but this is probably the most complex yet! By injecting six PSB bunches into the PS and then splitting them successively into three bunches and then two and two again, we obtain 72 bunches with the correct 25 ns LHC bunch spacing. This scheme has the advantage that the longitudinal beam emittance is kept very closely under control and it is much easier to give the SPS the correct longitudinal emittance. This ‘new’ LHC beam was successfully used by the SPS for their long MD at the beginning of November and has become the ‘standard LHC beam’. The scheme is so flexible that it was possible to supply the SPS with another new beam having 50 nsec bunch spacing, with less than three days’ notice. It is interesting to note that this reduction in longitudinal emittance for the SPS has given indications that the LHC beam in the PS might also be affected by trapped electrons.
The Pb-Ion Run from September till the End of the Year
After a three-day stop from 11 to 13 September, the PS Complex started supplying Pb ions to the SPS for setting up, and then for ion physics from 18 September, in parallel with the other users, in particular LEP. This run lasted until 27 November, and was the best ion run the SPS users have ever had, thanks, amongst other things, to the very stable beam supplied by Linac 3, PSB and PS.
After a much-discussed one-month extension, LEP took its last beams from the PS early in the morning of 2 November. This ended a period of more than 11 years of lepton acceleration in the PS. LPI had delivered its electrons with the high efficiency and quality we have come to expect until the very last moment of LEP operation on 2 November. The end of LEP, however, did not mean that the LPI could take a rest. Immediately after the LEP stop, LPI was put to use supplying electron beams for a number of experiments which are testing LHC equipment using the LIL electron beam and the synchrotron radiation emitted from EPA. This run ended
188 Proton Synchrotron Division on 21 December. Originally this should have been the end of LPI operation, but the LEP extension meant that LPI has to be put back into service again for six weeks at the beginning of 2001 to complete these measurements. The conversion of LPI to become the CTF3 facility can only begin after that.
In November, after a number of delays to ensure that the radiation safety criteria were correctly met, the first operation of the nTOF facility began. Initially this zone was supplied with low-intensity pulses (less than 4 × 1011 protons) at 20 GeV, which are accelerated on the same cycle as the normal East Hall beam. However, at the very end of the run, a full-intensity test of the facility was made with a few hours of operation at 7 × 1012 protons/pulse, using dedicated 20 GeV cycles. Following this run, the PS was shut down but the Booster continued for several days, operating at 600 MeV for some dedicated ISOLDE tests. This mode of operation ended on 7 December.
Amidst all this activity on new beams, there was still time to improve the existing beams. Thanks to a number of improvements in the PSB and a lot of painstaking work by the PSB team, the Booster was able to supply over 3.3 × 1013 protons/pulse regularly to ISOLDE. The PS also took advantage of this increased intensity, and in August a new all-time intensity record was set in the PS with 3.5 × 1013 protons injected and 3.305 × 1013 accelerated and ejected at 14 GeV, a full 6% increase over the previous record dating from 1997.
Lepton Operation
The LEP Pre-Injector (LPI) provided beam to PS, SPS, LEP and to the LPI experimental areas for 7032 hours in 2000. Most of the time (5544 hours) was dedicated to providing beam for LEP physics, for which the LPI availability remained very high, greater than 98%. Owing to the great flexibility of the pre- injector, synchrotron light from the accumulator ring was also used, in parallel with the LEP fillings, to study photo-desorption effects in LHC components. Beam was also sent to the LEA area simultaneously with, and completely in the shadow of LEP, thus permitting further progress on technical studies for LHC detectors without bothering the LEP users. Altogether 4992 hours of operation were recorded for the synchrotron light facilities, 960 hours for the LEA area, and 72 hours with the HSE beamline. Intensive machine studies were performed during the year with the LPI to investigate the performance of the linac (LIL) and of the Electron Positron Accumulator (EPA), both of which will be transformed and rearranged into the CLIC Test Facility (CTF3) in 2001. Because of the extension of one month given to the LEP physics programme, the series of studies for LHC using the LIL areas was not completed, and it will therefore be necessary to run LIL and EPA for some weeks next year in order to complete the measurements.
Proton Operation
Proton operation in the complex went smoothly in 2000, and as mentioned above, new record intensities in the PS were obtained. An important improvement was made to the transfer line between Booster and PS in that the yokes of the trajectory-correcting dipoles were changed from solid to laminated. This, together with a change of their supplies meant that the trajectories can now be corrected from pulse to pulse (PPM), which allows a much better control over the beam at the entrance to the PS and helps with the extremely tight emittance tolerances required by the LHC.
Proton Synchrotron Division 189 An interesting new feature was a special run made after the end of the normal physics schedule, when a dedicated period of operation of the Booster was given over to ISOLDE tests using a 600 MeV extracted proton beam instead of the normal 1 or 1.4 GeV. The aim was to compare the yields of particular isotopes from one target unit using different proton energies; such a direct comparison had never been made before. To do it, however, required establishing the extraction and transport parameters for the Booster and the transfer line to ISOLDE. All went extremely smoothly, and the results showed that in general ISOLDE has nothing to gain by using a 600 MeV beam, even for species close to stability. Another interesting study made with the PSB concerned the possibility to pulse it faster, twice as fast in fact, at 0.6 s; such a pulse rate might become interesting in later years when the LHC demands on the supply of protons become high.
Pb-Ion Operation
Although it was not needed for delivering beam to the ion experiments until much later in the year, the heavy-ion Electron Cyclotron Resonance ion source, ECR4, started up at the end of March. The aim was to study light ion yields with a new, moveable extraction geometry. Ions of helium, oxygen and argon were extracted and accelerated in the RFQ. But the RFQ can only accelerate ions with an incident energy of 2.5 keV/u, so the extraction voltage from the source is not optimal for these light ions, hence the interest in being able to adjust the extraction gap externally without breaking the vacuum. A prototype extraction was designed using a simplified electrode (easier to machine) but early tests with high voltage gave severe discharge problems. The extraction is embedded in the solenoidal fringe field of the source, and a standard extraction puller electrode works perfectly under these conditions, so it was a mystery why the new geometry did not work. However, before the problem could be solved, the Pb-ion running period had to be prepared.
The operational period started in mid-September. Unlike 1999, the start-up proved to be very smooth and the ECRIS reached its normal operating conditions in about 24 hours. The source was again adjusted to favour stability over absolute intensity, and ran for just over 40 days providing a virtually constant current of 22 to 23 mA. At this point it proved necessary to reload the source with lead for the remaining eight days of the run. During the whole ion run the beam availability out of the linac was over 99%. In the PSB and the PS, operation for the ion beams has become virtually a routine procedure after an initial check by passing the ion beam through the complex during an MD session. The PSB still continues to suffer from transmission problems due to machine vacuum conditions, a situation exacerbated by the presence of a large percentage of high-intensity ISOLDE pulses in the supercycle. This phenomenon hid a vacuum leak in one PSB ring for some time until a leaking valve was discovered and steps were taken to reduce its impact on the beam.
After the physics run, a final attempt was made to solve the problems with the adjustable extraction geometry by adding a piece to the puller, to simulate a collar on the normal puller. The behaviour of this extraction was now comparable to the standard one, to the relief of all concerned, but it is not yet understood why a small addition in a virtually field-free region should have such a major effect. Bearing in mind the future use of LEIR as an accumulator for ions for LHC, the Linac 3 ion beam was used to test ion-induced desorption from a variety of pre-treated vacuum chambers. Pb27+ and Pb54+ ions were used, with repetition rates varying from one pulse every 1.2 s to 1 pulse every 16.8 s.
190 Proton Synchrotron Division Experimental Areas Operation
East Hall
The East Hall was very busy throughout the year, with secondary beams delivered for LHC tests to 33 different teams sharing the four beam lines. Irradiations of detector components were also provided for a large fraction of the year in the two irradation areas now available in the Hall: one of these provides 24 GeV/c protons in t7, and another provides parasitic neutrons behind the DIRAC beam dump.
But the Hall is used not only for tests. There are now two fully-fledged experiments taking data. One is the DIRAC experiment (PS212) which measures the lifetime of π+π– atoms as a stringent test of medium-range QCD. The other is the newly-approved HARP experiment (PS214) which had a one-month technical run in the autumn. This experiment will use the t9 area to its full potential throughout 2001 in order to get precise measurements of the production cross-section of pions from protons on various target materials, as a function of the incident proton energy. These results are eagerly awaited since they affect how a Neutrino Factory may be designed.
AD Hall
The three AD experiments ASACUSA, ATHENA and ATRAP were all installed last year. This year saw the completion of the set-ups and the installation of the RFQD in the ASACUSA area. For the future, the DEM line will be prepared as a zone where machine development measurements may be made, and perhaps very small, temporary experiments may be accommodated.
ISOLDE Hall
The ISOLDE experimental Hall is full, and one can hardly see where any new apparatus could be installed. Nevertheless the REX experiment build-up continues, although it is nearing completion. As part of the ISOLDE Consolidation effort next year, the ISOLDE experimental Hall will be equipped with an access control system, and the main door through which equipment enters will become double such that cleanliness in the Hall can be maintained.
LEA (LIL Experimental Area)
LIL provides an electron beam of 500 MeV to the LEA zone whose intensity, pulse duration and repetition rate can be adjusted according to the user’s requests. An LHC detector group (CMS) made use of the beam for 960 h in parallel with the beam to LEP. The group continued previous work with electrons to measure on-line the damage created by radiation in quartz fibres. Successful runs gave very good results for 0.3 mm and 0.4 mm core diameter quartz fibres. The hope is to be able to select the best type of fibre for the CMS forward calorimeter which is very sensitive to the electromagnetic component of hadronic showers.
Proton Synchrotron Division 191 SLF (Synchrotron Light Facility)
The two synchrotron light facilities (SLF) on EPA ran for 4992 h during the year. One of them (SLF92) was used by the COLDEX experiment. The experimental results confirmed that the gas desorption induced by synchrotron radiation from surfaces kept at 4 K is acceptable for the LHC. A new beam screen for the LHC composed of co-laminated copper/steel with a sawtooth profile was developed. The gas desorption reduction in the presence of photons was confirmed. The other line (SLF42) was used by the photon-scrubbing experiment. Studies were made on copper/steel co-laminated material at room temperature for the straight sections around LHC detectors. Photoelectron yield measurements were performed together with forward reflectivity measurements. Photon and electron scrubbing studies were also continued. All these parameters are crucial for the LHC machine.
HSE line
As last year, the EPA extraction line (HSE) was used to continue studies on RF shielding and skin effects for the LHC. RF shielding exists for finite cavities when the inside of the chamber is completely coated with 2 µm of titanium. However the ceramic chambers foreseen for LHC extraction kickers will use stripes. Further studies were requested with the HSE line. One ceramic chamber was coated on the inside and a second one was not; a magnetic probe (1 GHz) monitored both. First results showed that with one stripe only, the current does not use the path with the smallest resistance.
ISOLDE
Operation
Responsibility for the operation, maintenance, and development of ISOLDE, the isotope separator on-line facility, was transferred to PS Division from EP on 1 April 2000. A new section was created in the PP group for the ion-source and target unit production, while the remaining staff was integrated in other PS groups. In this new organization the OP group is responsible for ISOLDE operation. The long-term aim is that the facility will operate like any other PS machine from the main control room, with support from the PS expert groups as necessary. This solution will take time to implement and only a first step was taken towards it in 2000 with the introduction of some limited support from the PS Booster operators to the ISOLDE engineers in charge (EICs). Nevertheless, this year a record number of protons (a total of 8.1 × 1019) at different energies (0.6, 1.0 and 1.4 GeV) were sent to the separators, a record number of ISOLDE shifts (both for physics and for target developments) were achieved, and a record number of interventions in radioactive areas were made.
The goal of the operational year 2000 was to run a full physics programme with 300 shifts on the General Purpose Separator (GPS) and to commission the High Resolution Separator (HRS) in low resolution mode for 100 shifts. In reality, ISOLDE delivered 345 shifts of which 296 were pure physics shifts and 49 were for machine development and target tests with radioactive beam. The GPS delivered 295 shifts and the HRS 50 shifts, which is only half the expected amount. The reason for this was a malfunctioning front-end which could not be repaired during the on-going physics run. In the user community, 41 experiments were ready to take up to 492 shifts at the beginning of last year. The physics co-ordinator managed to distribute the
192 Proton Synchrotron Division 345 shifts available to 38 of these experiments, an excellent achievement. For 2001 there are experiments waiting for 600 shifts, a normal amount of backlog which will give the physics co-ordinator sufficient freedom to distribute the planned 400 shifts efficiently.
A total of 37 ISOLDE target and ion source units was irradiated during the year, 25 on the GPS and 12 on the HRS. This permitted a record number of physics shifts to be achieved. This success was due to the extension of the run as a single user during the 600 MeV test, to the absence of ‘critical day’ interruptions, and to the use for the first time of PPM mode between the two separators.
Targets
Altogether 29 targets were assembled and tested off-line in 2000. On average over the last five years, more than 11 physics shifts were obtained for each target unit produced. In 2001, with the arrival of the second technician in charge of target assembly, the goal of 40 targets/year will be within reach. The production of targets in recent years is summarized in Fig. 1.
30 20 10 0 1990 1992 1994 1996 1998 2000 2002
Fig. 1: Number of target units produced as a function of the year.
During 2000, 35 target and ion-source units were put on the separators, and the 11 different target materials used were oxides (titanium, calcium, zirconium, cerium and thorium), carbides (lanthanum and uranium), molten metals (lead and tin), metal foils (niobium and tantalum). The 2 µm Ta-foil targets developed in 1999 were used twice in very successful runs to produce neutron-rich beryllium and lithium isotopes. The ISOLDE Resonant Ionization Laser Ion Source (RILIS) was used in conjunction with tungsten and niobium high-temperature cavities for the ionization of beryllium, gallium, tin, indium, thallium and lead isotopes. The ion-sources of the units used on-line are listed in the table below.
Ion sources used during 2000
Number of units Ion source Transfer line
11 W-surface 5 Nb-surface 3 Plasma MK3 Temperature-controlled 4 Plasma MK5 High temperature 14 Plasma MK7 Cooled
Proton Synchrotron Division 193 Out of the 11 target materials listed, cerium and thorium oxides were synthesized and tested for the first time. Fission of thorium and uranium can be induced in many ways (by neutrons, photons or charged particles). Prototypes were designed to test fast-neutron-induced fission on thorium oxide and uranium carbide targets. The high-energy proton-to-neutron converter consisted of 10–20 cm long, 10 mm diameter tantalum or tungsten rods bombarded with 1 or 1.4 GeV protons. The design allows one to direct the proton beam either onto the target, or onto the nearby converter. For the first time, the fast neutrons and high-energy proton production cross-sections of neutron-rich isotopes of xenon, krypton and caesium were directly compared. It turns out that the evolution of the cross-section for very neutron-rich isotopes is in first order independent of the mechanism inducing the fission. As the converters are much more robust towards thermal shocks than ISOLDE targets, higher proton beam power could be handled by them in the future. The development of target materials and ion-sources was greatly helped by contributions from the EURISOL project members.
Front-ends
ISOLDE front-ends are designed as disposable items with a planned lifetime of the order of five years. Currently there are three front-ends, but the one on the HRS (No. 2) is unreliable and will be removed in the next shutdown and sent for disposal. There will then be no spare. The first of a new generation of front-ends is under construction but it will not be ready until the beginning of 2002, so that we will be forced to run in 2001 with the existing front-ends. The way the target couples to the front-end is shown in Fig. 2.
Grounded x-y-z movable extraction electrode –60 kV Vacuum vessel Electrostatic quadrupole triplet
Target Ion source
Turbo molecular pumps (2 × 1000 l/min)
Fig. 2: An ISOLDE target and ion-source unit with the front-end.
The extraction electrode x,y,z movement is the only moving part in the vacuum chamber of the ISOLDE front-end. Its task is to adapt the beam optics to the different ion source characteristics and to cope with small misalignments. The cage of one of the ball bearings of the z-positioning screw broke on front-end No. 2 but
194 Proton Synchrotron Division could be repaired after two years of radioactive cooling. This operation required dismantling in the ventilated hot cell. Over 6 man-months and 10 mS integrated dose were needed to successfully replace the faulty ball bearing. Sadly, shortly after the first real target was irradiated when it was back in service, uncontrolled sparking degraded the titanium tip of the extraction electrode and caused increased friction in the movement mechanism. The friction torque was larger than that of the clutch, thus forcing us to replace it by a direct connection. This front-end then worked, but could not be considered reliable.
Consolidation Project
The 10 years since the move of ISOLDE to the Booster, together with recent budget constrains, has led to a situation where a consolidation programme is urgently needed to guarantee the future of the facility. Furthermore, the European legislation for radiation safety has recently been revised. To bring the level of radiation safety at ISOLDE in line with this new legislation, important investments have to be made. A consolidation plan stretching over three years has been established for the facility. It will be run as a project involving two main activities. The first addresses the radiation safety issues and involves an extension and improvement of the present buildings; this is to ensure that the target area and a new workshop extension can be classified as a class A radioactive laboratory, and that the experimental hall can be upgraded to a class C laboratory. The second activity is the technical consolidation, involving among other things the building of a new spare front-end, the high-resolution mode of the HRS, renovation of target manufacturing facilities, and improvements of the radioactive beam quality. The project will be financed partly by the ISOLDE collaboration.
nTOF
At the beginning of November, a beam of 20 GeV/c protons was delivered for the first time to the target of the nTOF experiment (neutron Time Of Flight) in the TT2A tunnel. The effort spent preparing this beam beforehand bore fruit because the requested intensity was quickly obtained (a few 1011 protons) in the required 25 ns bunches. This low-intensity beam is practically ‘free’ because it is produced during the same PS magnetic cycle as that which supplies the normal East Hall beam; in fact it comes from the beam accelerated in one of the Booster cycles that is unused during a normal East Hall PS cycle. At the same time, the designers of the neutron-producing Pb target were able to verify that the number of neutrons emitted, and their energy spectrum, were as expected.
Following this milestone, efforts concentrated on producing a beam of the same energy but of much higher intensity for nTOF, up to 7 × 1012 protons in bunches of 25 ns. This beam, which is at the limit of what the PS can produce, requires very delicate adjustments to the PS, particularly at low energy where there are strong space-charge effects, and at transition where there is beam break-up instability. However, the beam was produced, transported, and measured thanks to the active collaboration of almost all the PS groups. It was then used, right at the end of the year, to measure the heating effects in the Pb target, which had been a cause for concern. Both the new proton beams will be used in 2001 to supply the many users eagerly waiting to start their neutron measurements in the nTOF experimental area, about 200 m downstream of the production target.
Proton Synchrotron Division 195 Antiproton Programme
AD
The AD project reached its successful conclusion and was commissioned and turned over to the users as a going concern. Beginning in April, soon after the PS complex start-up, the AD set out on a long and intense period of machine development, whose goal was to improve the performance obtained in 1999 and reach the design specifications in time for the start of the physics programme foreseen for 10 July. The list of areas needing improvement was quite long: the ring optics and acceptance, the antiproton deceleration efficiency, cooling at several beam energies, re-bunching prior to ejection at 100 MeV/c, set-up of the ejection lines, and shortening of the AD cycle.
After extensive work on the ring optics at both high and low energies which resulted in higher transverse acceptances as well as a better understanding of the optics at low energy, a regular antiproton deceleration cycle was established. The stochastic cooling system, which already worked satisfactorily at 3.5 GeV/c was now optimized at the 2 GeV/c flat-top and as a result, the final beam emittances were then well below the design values. Electron cooling improvements yielded beam emittances at 300 MeV/c close to the design values. By mid-June the electron cooling started to work satisfactorily at the lowest flat-top, and beam emittances after cooling approached the design specifications. This was the first time in the history of the AD that beam cooling had worked at 100 MeV/c. By the end of June, the first re-bunched beams could be ejected after further work on the first harmonic RF-system.
The first antiproton beams of the new millennium for physics were delivered on 10 July. First to receive the 100 MeV/c antiprotons were the ATRAP and ASACUSA Collaborations, followed some two weeks later by the ATHENA team. After that, each Collaboration ran with exclusive beam time for a period of two to eight hours before passing on to the next team. The normal running schedule was from Monday morning to Friday evening, but in order to increase the efficiency and reduce the downtime, the ACR crew effort was increased from October onwards, when nights were introduced on the shift rota. In September, a short shutdown was made to allow the installation of the RFQD which is part of the ASACUSA experiment. This was followed by two weeks of dedicated AD machine development time.
Continuous evolution towards the design goals was maintained by programming machine development periods in parallel with the physics; this was done on a weekly basis until the AD shutdown on 30 November. The characteristics of the extracted beam were gradually improved over this period. Areas of improvement included beam intensity and stability, orbit stability, extracted bunch length and time stability, electron cooling, and the AD cycle length. Towards the end of the run, the AD regularly extracted >2× 107 antiprotons in a 330 ns long bunch every 112 seconds. Record intensity of the extracted beam at 100 MeV/c was 2.7 × 107 per bunch. Some time was also spent on studies of the ‘stacking’ mode, in which the AD accumulates several batches of antiprotons at 3.5 GeV/c before deceleration and extraction. The beam availability for physics during the period from July to November was 85.9% with a total of 3540 hours logged for the whole year.
To round off a successful year, the AD went ‘live’ in November for the second time. Just as in May, the AD was again the guest star in a series of Webcasts within the framework of the ‘Live from CERN’ project. This time, the theme was ‘the antimatter factory’: a ‘mission impossible’ team of three students from the
196 Proton Synchrotron Division Geneva region was sent around CERN in search of the necessary ingredients for anti-hydrogen production. Once the mission was completed, an alien turned up in the studio looking for antimatter for his spaceship which was unfortunately out of fuel! This adventure was followed by a large audience placed in the CERN Microcosm studio, the observation tower in Tampere (Finland), the assembly hall of the University of Bari (Italy), the Exploratorium in San Francisco (USA), and in classrooms all over the world. A question-and- answer session was then held between the audience and the engineers and physicists working on the AD machine and its experiments. All the CERN participants enjoyed it and we hope the audiences did too.
RFQD
The decelerating RFQ, designated in CERN as RFQD, successfully delivered low-energy antiprotons to its ASACUSA users. The design was based entirely on calculations and was built without a prototype. Initially some discrepancies with theory appeared in the form of a much higher than expected RF power requirement, and a distorted field. However, this can be attributed to the great length of the device (3.6 m corresponding to 2.4 RF wavelengths) which makes the structure very sensitive to slight tuning variations. It also reduces the resolution of full-length simulations, because of the practical limit on the number of mesh points that can be used. The problems were solved by upgrading the RF amplifier chain to higher power, by additional copper plating, and by geometry adjustments to the 34 internal RF cells.
Fig. 3: The RFQD installed in the AD hall.
Proton Synchrotron Division 197 The RFQD was brought to full RF field at the beginning of the summer, then packed and transported to the Tandem accelerator of the University of Aarhus in Denmark, for a short period of beam tests with 5 MeV protons. This beam offered the advantage of a much higher repetition rate than available with AD, together with the possibility of using standard proton diagnostics such as Faraday cups. The tests were entirely successful, and the RFQD was immediately packed and shipped back to CERN.
After reinstallation in the AD hall (see Fig. 3), physics runs started at the beginning of November. Antiprotons, decelerated to kinetic energies of 15 keV, were routinely provided with transmissions of 50–70% of the theoretical figure and ‘publishable’ results were rapidly obtained by the physicists of the experiment ESA, part of the ASACUSA Collaboration. Precious experience with the new hardware was gained at this time, but a dedicated measurement period is foreseen immediately after the start-up in April 2001 to consolidate the project.
Diagnostics
The maintenance and operation of the numerous (~ 1000) beam measurement devices spread out along each of our accelerators and transfer lines is a permanent source of concern, in order to ensure that the operators have reliable and precise tools. This is especially true for the stringent conditions imposed by LHC beams. This year it was, in addition, necessary to finish the AD instrumentation and to equip the new nTOF facility. For this latter, the effect of the high charge in each beam bunch means that existing diagnostic equipment had to be modified. Also, with the arrival of ISOLDE in the Division, the specific nature of their instruments necessitated a review of both hardware and software issues in order to formulate a consolidation plan for 2001. As LPI will be converted into CTF3 next year, a facility with yet further new characteristics, a number of existing LPI instruments will have to be adapted and new ones created.
For AD, the problems encountered with the measurement of the closed orbit under the extreme conditions of deceleration were solved. The measurement of the beam intensity and its longitudinal properties using the new Schottky pick-ups are now available, but instrumentation for the transverse parameters of the decelerated beam are still being developed. For the tests of the RFQD with protons at Aarhus University, extremely sensitive monitors were constructed to permit digitization of the beam profiles for very low energy, low intensity beams. These helped a great deal in making the RFQD operational so rapidly.
To prepare the PS for the LHC era, efforts concentrated on the ‘fast blade’ or ‘guillotine’ monitor, which allows us to measure the distribution of transverse amplitudes of the beam. The mechanisms were installed in the Booster during the shutdown, as was the control electronics. First results were encouraging and point the way to improvements of both the software and the electro-mechanics. Mounted in one PSB ring, the monitor will be a reference for comparison with other measurements of the transverse beam properties, in particular the beamscope. Concerning the eight new fast wire scanners delivered by TRIUMF, the mechanical elements were installed in the PSB and the control electronics is being fabricated for testing early in 2001. The SEM grids in the measurement line after the Booster were replaced by a model with finer resolution so as to measure the LHC beams more precisely.
198 Proton Synchrotron Division Emittance preservation is essential along the accelerator chain for LHC, and to this end, instruments to measure the matching between the PSB and the PS are under study. In particular, the comparison in the PS between a multiturn SEM grid and a quadrupole pick-up are continuing. The results so far are in excellent agreement, which opens the way for non-destructive and permanent measurements of the transverse beam properties at injection to the PS in the LHC era. In the same way, wide-band pick-ups were placed in the transfer line from PS to SPS so as to observe the positions and intensities all along the train of 72 bunches as it goes to the LHC. Complete tests of the digitization system will be made next year. Meanwhile the development of slow current transformers for LHC continues, and comparisons between the prototypes made at CERN and commercial equipment are being made.
Accelerator Controls
The exploitation of the control system driving the PS Complex on a day-to-day basis is in the hands of the CO group. This includes the support to the operators of an efficient ‘on-call’ team, the continuous introduction of changes when requested by the operations crew, and the upgrading of the software and hardware components for better reliability or more suitability to the needs. This activity proved once again to have been very successful since the fault rate attributed to the controls in 2000 reached a record low level of 0.2% over the whole year. The AD machine remained the most demanding machine of the PS Complex in terms of day- to-day care and follow-up of its evolving operating conditions. The ISOLDE controls were officially integrated with the arrival of the ISOLDE technical team in April 2000, but remained under the responsibility of a few specialists because of their specificity.
Developments in 2000 were all aimed at improving the reliability and overall performance of the control system, whilst reducing the manpower and operating costs necessary to run it. On the hardware side, the network was completed with the extension of the structured network cabling to both ISOLDE and LPI. The replacement of the VME front-end crate controllers with PowerPC-based units was extended to all the PS machine VME control crates. The TG8, the basic component of the PS timing system, was completely redesigned and successfully tested in operation. In addition, a project was started to deal with the introduction of industrial equipment interfaces in the PS system (PLCs, Profibus, etc). Local file servers were exchanged for Linux machines, and the controls database was installed on a new machine under IT Division responsibility. Finally, in the framework of the controls desktop migration, two consoles of the PS main control room were successfully converted to standard PC workstations running Linux, thus paving the way to a complete migration in 2001.
On the software side, the ‘middleware’ layer developed in collaboration with SL Division has been implemented, based on CORBA and MOM technologies. First tests were made successfully at the end of the year, opening the way to its installation in the PS environment in the first half of 2001. The architecture of the central sequencing system needed by PS and SPS to satisfy the LHC requirements was defined, and the implementation phase was started. Most of the existing XWindows/MOTIF applications programs were ported to Linux. Lastly, the Java programming environment was further developed using commercial tools, and the Accelerator Software Component (ASC) layer was broadened to provide applications programmers with easy and powerful program objects.
Proton Synchrotron Division 199 In order to modernize the ISOLDE control system, a new infrastructure for the front-end computers was defined which is compatible with the existing PS controls, and takes account of the wishes of the ISOLDE users. This project will be implemented in two stages in 2001 and 2002. It is based on the DSC/VME PS standard infrastructure with the addition of industrial components. The necessary know-how for the latter has to be acquired, and issues related to the integration of these units in the existing control system have to be understood. To this end, a dedicated test stand is being set up in the laboratory. In 2001 a selection of power supplies for the ion source and the lenses will be implemented, using PLCs and I/O modules distributed on a PROFIBUS fieldbus. For the CTF3 project, the realization of new controls has started on the basis of the existing LPI control infrastructure. All new equipment installed will use the same components as the other accelerators of the PS Complex.
Finally, it should be noted that the CO group has the task of providing the Division with its desktop computers. As every year, the oldest PCs were replaced, so that upgraded Pentium 133 MHz machines are now the lowest level of PS desktop computer. New public printers were installed and the video equipment in the conference rooms was renewed. As a test for the deployment of Windows 2000 (delayed by IT Division to 2001), a few machines were migrated in order to gain experience and help elaborate a procedure for the global deployment, but this was not without difficulty.
Consolidation of the PS Complex
Our consolidation efforts were particularly active in the area of power converters. For example at the Booster, new power converters for the BTP line adapted for PPM were commissioned, and preliminary studies were made of the regulation electronics of the Booster main power converter, aimed at testing the feasibility of pulsing with 0.6 s cycles at 1.4 GeV in the future. At the PS, development work for a new B-train electronics was started, as was a new gate control set for the PS main generator and control logic (using PLCs) for the rectifiers, in order to improve stability and reproducibility.
Kickers and septa were also involved in the consolidation programme. Modifications to the KFA71-79 electronics were made to permit true PPM operation for these modules, improvements were made to the PSB transfer kicker pulse flat-top to give it a < 1% ripple, and a new compact PCI-based control front-end for the ISOLDE HV target pulsers was designed and installed. The spare electrostatic septum PE.SEH 31 was installed during the shutdown, but then a new-generation electrostatic septum PE.SEH 23 was designed, constructed and built; this uses standard vacuum seals, improved vacuum equipment, and has in-vacuum bake-out lamps. The septum is ready for installation in the next shutdown. For the magnetic septa, the biggest job concerned the recuperation of the highly radioactive SMH16 which had been taken out of the PS ring in January 1999 after some laminations sheared off, blocking the gap. New coils were manufactured and installed, and the new, improved magnet is now fully tested.
Finally, our campaign to remove old cables came to its end with further work in the Booster galleries, and in several odd corners of the PS Complex. Of order 1000 cables were removed which filled four large skips, and the work represented roughly two man-years of effort.
200 Proton Synchrotron Division PS Protons for LHC
With virtually all the planned hardware already installed, the project of upgrading the PS Complex as LHC proton pre-injector is coming to an end. The last major item still in the pipeline, the closed-circuit PSB water cooling and air conditioning system, was put into operation in March 2000 and works as expected. Although the water inlet temperature of the PSB magnets eventually reached ~ 27°C during the summer season, no adverse effects on the PSB beams were observed.
A series of machine development sessions was carried out with a view to producing the ‘nominal’ LHC beam at 26 GeV/c, i.e. an intensity of 1.1 × 1011 protons/bunch, a transverse normalized r.m.s emittance ≤ 3.0 µm, a 25 ns bunch spacing, and a bunch length ≤ 4 ns (to fit into the SPS 200 MHz buckets). Already by the end of 1999, a beam with the nominal intensity and transverse emittances of ~ 2.5 µm (better than specified) was successfully produced, demonstrating that the scheme (involving two-batch filling of the PS at an increased energy of 1.4 GeV) works in the transverse plane. However, the 4 ns bunch length (involving non-adiabatic bunch manipulations based on new 40 MHz and 80 MHz cavities) could not be achieved with the nominal intensity. The problem was that the original scheme was based on a debunching–rebunching process in the PS which is plagued by longitudinal microwave instabilities leading to an uncontrolled blow-up in momentum spread, and hence to bunches with excessive length.
Although this instability may probably be tackled by a systematic effort to reduce the PS impedance (e.g. by dismantling the 114 MHz lepton cavities), a radically new way to produce the LHC proton beam in the PS was proposed. It is based on the recently invented triple-bunch splitting which was successfully demonstrated in 1999, as well as on the more conventional splitting of one bunch into two. This scheme (Fig. 4) consists of the following steps:
– the PSB provides two batches of three bunches each, filling six out of seven PS buckets (i.e. the PS RF system is tuned on harmonic 7), thus leaving a void in the PS ring;
– the six bunches are split into 18 by triple-bunch splitting which are then accelerated to 26 GeV/c;
– on the PS ejection flat-top, the 18 bunches are sliced (by two steps of double-bunch splitting), into 72 bunches 25 ns apart, with a void of 12 empty buckets. The RF harmonic is 84. Note that a 20 MHz cavity is required for generating the intermediate RF harmonic 42;
– the 72 bunches are shortened to 4 ns by means of the 40 MHz and 80 MHz systems followed by extraction, as in the old scheme. The kicker risetime is placed in the beam void, thus enabling ejection without mis-steered bunches.
In a crash programme, a prototype 20 MHz cavity was assembled using obsolete equipment, and the low- level electronics for all the splitting operations was prepared. After a series of machine studies on this new scheme, a truly nominal LHC beam, featuring the correct transverse properties as found in 1999, but with a bunch length of ~ 3.8 ns at nominal intensity, was achieved (see Fig. 5) for the first time in November 2000. This new scheme offers an unexpected fringe benefit: by omitting one or several PSB rings, bunch trains and holes of different length can be generated. Moreover, other bunch spacings (50 ns, 75 ns, 100 ns) are feasible. This additional feature proves invaluable for studying electron cloud effects in the SPS (a potentially serious problem for the LHC); these phenomena strongly depend on the bunch population, spacing, and length of bunch train which can now be varied. The only major drawback of the new scheme is that it requires ~ 15%
Proton Synchrotron Division 201 more protons per PSB bunch, with a concomitant increase of the space-charge tune shifts in both PSB and PS. This virtually excludes the production of the very-high-brilliance ‘ultimate’ beam for LHC (1.6 times the bunch intensity within the same transverse emittance) with the present systems.
PS ejection: 320 ns beam gap 72 bunches on h = 84 in 1 turn
Quadruple splitting at 25 GeV
Acceleration of 18 bunches on h = 21 Triple splitting at 1.4 GeV
PS injection: 6 (4 + 2) bunches on h = 7 in 2 batches Empty bucket
Fig. 4: The new scheme to generate the LHC beam (72 bunches, 25 ns spacing, bunch length ≤ 4 ns) without resorting to a debunching-rebunching procedure. Note that the ‘quadruple’ splitting is done in two steps of double-splitting.
Fig. 5: One of the 72 LHC bunches on the PS 26 GeV/c extraction flat-top, with a bunch length shorter than 4 ns.
202 Proton Synchrotron Division The ‘PS proton for LHC’ project finished officially at the end of 2000, but some items are still outstanding. On the hardware side, the eight PSB wire scanners provided by TRIUMF must be commissioned, and the PS transverse damper systems for both planes (to eliminate injection oscillations and damp transverse instabilities) must be purchased. In addition, in view of the recent success of the new RF filling scheme, two 20 MHz RF systems (tunable also to 13.3 MHz to allow an LHC bunch spacing of 75 ns) must be fabricated. Machine development sessions will be devoted to reducing the modulation in intensity between LHC bunches from the present ±20% to ±10%, which seems acceptable for the LHC. A bigger task, however, will be to produce the ‘initial’ beam which the LHC requires for the first years of collider running (with a luminosity of ~1033 cm–2 s–1). This beam features much smaller transverse emittances (25% of the nominal beam emittance), making the preservation of transverse emittance along the injector chain a major issue.
Ion Projects
PS Ions for LHC
A plan has been established to produce the first Pb ions in the LHC by the end of 2006. However, the main activity was to finalize the LEIR scheme for collecting the Pb ions needed by LHC, which opened the way to studies of options concerning the PS. Work on the improvement of the ECR source and on a Laser Ion Source continued in parallel. In addition, attention was paid to lighter ions, which have been requested by the ALICE experiment. Altogether six ion species have been specified in collaboration with the LHC specialists: lead, indium, krypton, argon, oxygen and helium. These ions give a reasonable cover of the periodic table, exist as (relatively) pure isotopes, are well-suited to an ECR-type source, and are acceptable for the experimenters. The intensities per bunch of the different ions that may be ‘digestible’ in the LHC have been worked out, and whereas for lead and indium they seem to be achievable, lighter ions are severely restricted by space-charge limits in the injector chain.
Now that the LEIR scheme is defined, limits in the PS have also been established and ways to improve the situation have been suggested. These include increasing the injection energy into the PS, and the use of a three-, two-, or even a one-bunch mode instead of the four-bunch scheme used for lead ions (i.e. where four bunches per PS cycle are prepared). The price to pay for the higher transfer energy is stronger septum and kicker magnets in the PS, apart from the higher demands on LEIR itself and the transfer line from LEIR to the PS. For the modes with fewer bunches, a longer filling time is required (more than 40 min per LHC-ring in the one-bunch scheme instead of 10 min for the four-bunch mode).
To avoid an intolerable emittance blow-up in the foil which converts Pb54+ to fully stripped Pb82+ after the PS, a low-beta stripping insertion in the TT2 transfer line (PS to SPS) has been designed, leading to β β π σ h ~ v = 5 m. It reduced the emittance blow-up by a factor of 4 to ~ 0.2 mm mrad (normalized, 1 emittance) compared to 0.8 π mm mrad in the present set-up used in the fixed target Pb runs, where the final emittance (about 4 π mm mrad compared to 1.5 π mm mrad for the LHC) is less critical. The beamline elements for the insertion (four extra quadrupoles and six power supplies) have to be found or ordered in 2001 if one wants to test the insertion from 2003 onwards. Emittance conservation is one of the challenges for the ions for LHC. To limit the emittance increase caused by stripping to the theoretical value of 0.2 π mm mrad, one needs a very delicate optical setting of the downstream part of TT2, which will require many long MD sessions. Hence it is essential to start this work as early as possible.
Proton Synchrotron Division 203 LEIR
The LEIR scheme was fully defined in 2000, after much discussion and analysis of the possible options. The aim was to define the scenario that will later be implemented for providing the ions that LHC will require in 2006. Different ways of accumulating the Pb ions (and also the other ions that seem now to be of interest to the experimenters) were considered. It has been agreed that the idea of moving Linac 3 to the South Hall to make for easier injection into LEIR should be abandoned. Rather, Linac 3 will stay where it is, and the part of the transfer line which is common to LEIR injection and transfer to the PS will be pulsed. A combined multiturn injection (both horizontal and vertical) will be installed in a straight section having large dispersion, while the 3-m electron cooling and the extraction will be installed in two opposite sections with zero dispersion (see Fig. 6). All the magnets, the majority of the instrumentation, and the vacuum chambers will be recuperated from LEAR. However, one of the principal problems to be solved is the quality of the vacuum required in LEIR. Tests are under way at Linac 3 of the out-gassing of vacuum chamber walls under the bombardment of Pb ions, and different surface treatments are under consideration.
E2BHN03 15 E2QFN11 E2BHN04 EJECTION, dispersion ~ 0 m E2QFN12 E3QFN01 E2QFN13 E3BHN01
10 E3QNN02
BHN10 QFN11 QFN12 QDN11 QDN13 QDN14 QDN12 KFH1214
E3QNN03 SMH1113 BHN40 E3QNN04 5 QDN42 QDN21 QFN42 QFN21
SMH42
0 SEH40
INJECTION, dispersion ~ 10 m QFN41 QFN22 –5 QDN41 QDN22
BHN30
BHN20 QDN32 QFN32 QDN34 QDN33 QFN31 QDN31 –10
ECOOLING, dispersion ~ 0 m –15 –10 –5 0 5 10
Fig. 6: The proposed LEIR layout.
204 Proton Synchrotron Division Laser Ion Source (LIS) Studies
The aim of the LIS experiment is to demonstrate that such a laser ion source can provide a reproducible ion pulse at high pulse rate, and can be an economic and reliable candidate as the source of heavy ions for LHC. The experiment is run by an international collaboration between CERN and Russian Institutes (ITEP and TRINITI). It is partly financed through the International Science and Technology Centre (ISTC) and INTAS (International Association for the promotion of co-operation with scientists from the newly independent States of the former Soviet Union).
During 2000, further progress was made towards a full-scale source, and it is anticipated that next year the full system will be tested at CERN. In TRINITI, a 95 J CO2 laser pulse was shaped to 10–15 ns length and focused to a power density of some 1013 W/cm2 on a target, thus generating an expanding plasma. Measurements show that more than 2.6 × 1010 ions of Pb25+ will be extractable from the plasma in a pulse of 5 µs, the time needed to fill one PS Booster ring. The value expected originally was only 1.4 × 1010 ions. Also in Russia, the manufacture of the 100 J laser pulse amplifier is advancing, and the commissioning of sub- systems started in the autumn of 2000. The objective is to build a device that delivers more than 106 shots without intervention, at a repetition rate of 1 Hz. After pre-commissioning in Russia, this device should be delivered to CERN in the first half of 2001.
At CERN, most of the effort was put into preparatory work for integrating the new high-energy laser in the present set-up, and to convert the source for 1 Hz operation. In parallel, new plasma and ion beam diagnostic tools were tested successfully. These were a polychromator measuring X-ray spectra from the laser-generated plasma through the Bragg effect (in the framework of the INTAS collaboration), and a sapphire crystal detector allowing beam profile studies of ion currents at very low beta. Furthermore, to get a first idea of what a LIS could provide in the way of medium-heavy ions with mass around A = 110, a silver target was bombarded with our present, low-energy laser. The result was that approximately 3 × 1010 ions of Ag19+ could be delivered in a pulse of 5 µs through a standard extraction aperture.
CLIC
The CLIC Study
The Compact LInear Collider (CLIC) being studied at CERN is a facility that might be built after the LHC. It is an e+e– collider based on an RF system working at 30 GHz, and intended to reach a centre-of-mass collision energy of 3 TeV and a luminosity of 1035 cm–2 s–1. The work on this facility converged last year to a possible design which is now documented in a CERN report published in July 2000, under the title ‘A 3 TeV Linear Collider Based on CLIC Technology’ (CERN 2000–008). The principle of this technology is a two- beam scheme, where a high-current beam decelerated in a drive linac is used to provide the RF power to feed the main linac. The production of this high current involves many complex components such as a delay loop and combiner rings, and it is far from trivial. The beam of interest for the physics is then accelerated in the main linac at modest intensity, but to very high energy (1.5 TeV). Separate electron and positron main linacs fire their beams towards one another, and the beams collide at the interaction point (IP) which is where the experimental apparatus is situated, assumed to be located in the vicinity of the injection complex. At the energies proposed, the whole facility is about 40 km long. During the year, working groups on technical
Proton Synchrotron Division 205 services and engineering aspects were created, involving mainly ST and EST Divisions, to prepare a CLIC conceptual design. They cover the study of a site in the Geneva region, and of a possible layout. They also include feasibility investigations for the civil engineering, electrical power distribution, cooling, ventilation, etc. Taking into account the geological constraints, the tunnels would be on average ~ 110 m underground. So far this has resulted in two possible implementation scenarios, in which the injection systems and the IP are both assumed to be located within the existing CERN site. Meanwhile, work continued on important beam dynamics issues and on critical RF components such as the power extracting transfer structures (PETS) and the accelerating structures at 30 GHz, by using the test facility CTF2. Plans became more concrete for the successor facility, CTF3, which uses much of the LIL injector material liberated by the closure of LEP.
Positron production has been studied and the peak energy deposition density simulated. A new tracking code has been developed on the basis of the KEK code used for the B-factory. The phase space distribution of the final state, as well as the positron yield (0.3 e+ per e– and per GeV) has been obtained. A damping ring optics was designed, based on a TME arc lattice and using established optimization algorithms. In this design, the intrabeam scattering growth rates are orders of magnitude higher than the synchrotron radiation damping. In addition, the estimated growth times are about 0.6 turn at 10–9 torr for the fast beam-ion instability, and 14 turns for the electron-cloud instability. As a consequence, another option is being explored of a ring with a much larger circumference, in which radiation damping, intrabeam scattering and quantum excitation would occur almost exclusively in the long wiggler sections. For the main beam, bunch compression is needed in order to produce a bunch length of 30 µm. The drive beam generation requires that the bunch length be either stretched (to limit the coherent synchrotron radiation effects) or compressed (to optimize the power transfer to the main beam). A magnetic insertion, capable of achieving both these functions by changing only the quadrupole strength was therefore studied. As a challenge, this approach was first applied to the preliminary design of the CTF3 transfer line between the Delay Loop and the Combiner Ring. Solutions were indeed found which satisfy the geometrical constraints and allow a bunch variation of ±1.6 mm around the nominal value, with acceptable quadrupole gradient changes.
The effects of transverse wake-field in the drive beam decelerator were investigated for the four- waveguide structures. In particular, the dependence of beam stability on the frequency of the transverse mode and the length of the decelerator was simulated, to detect the amplification of any initial jitter at the other end of the decelerator. The results show that a deviation in frequency from the fundamental mode by 2% generates a large, asymmetric increase of the amplification factor between 3 on one side and 10 to 100 on the other side of the minimum. Studying the beam stability, the envelopes obtained after trajectory correction are found almost to fill the aperture at the low energy end. Remedies would be an increase of the decelerator length or an improvement of the correction algorithm. Simulations with the six-waveguide structures showed that the beam is significantly more stable in this case. Concerning the PETS, fabrication studies have advanced; with a 1 m- long PETS, power levels above 200 MW for 10 ns long pulses are expected in the test facility.
In the main linac, the control of the bunch-to-bunch energy spread requires compensation of the beam loading. A new method has been developed which consists in generating a ramp in the RF output of the PETS by modifying the time structure of the drive beam. The final pulse has a modulated bunch distribution whose density is lowest at the head of the pulse and grows towards a constant value in the core. This corresponds to a current ramp that produces a ramp in the PETS power output. A full bunch-to-bunch energy spread of less than 5 × 10–4 can be obtained, which is below the target value of 0.1%. Since high-gradient accelerating structures are the key components of the main linac, efforts focused last year on understanding their performance limitations. This was motivated by the discovery of surface damage in prototype 30 GHz
206 Proton Synchrotron Division accelerating sections tested in the past in CTF2, by reports of similar problems encountered in the X-band by the NLC study, and by re-calculation of the pulsed surface heating. Theoretical work centred on the analysis of the options available to optimize the structures, which involved extensive use of the program HFSS. In parallel, experiments were planned to determine the breakdown signatures, to study the physical process involved, and to discover the parameters that affect breakdown level. High-gradient tests have been carried out in a test stand installed in CTF2 (see below) and dedicated experiments on pulsed surface heating were carried out in the Russian National Institute of Physics. In an attempt to reduce this heating, slotted iris structures were further studied. Work on main-beam position monitors has also resumed, with the objective of extracting position signals from the accelerating structures directly.
At the interaction point (IP), the vertical spot-size increase due to synchrotron radiation in the detector solenoid limits the maximum acceptable crossing angle to 20 mrad for a 4 T solenoid field; this is also the minimum angle required. Several final-focus systems with finite dispersion across the final doublet were studied at 3 TeV but none reached the desired luminosity. Considering ionization heating and image currents, three or four locations in the final focus were found where spoilers could survive the impact of an undiluted beam, provided proper materials are used (carbon or beryllium). As a consequence of failures in the linac, the emittance may blow up by several orders of magnitude and this determines the minimum betatron amplitudes required at the collimators. Expressions for collimator wake-fields were compiled and programs for studying scattering by residual gas or thermal photons were installed. Investigations started on the design and layout of the final quadrupoles at the IP. Parameters were computed for γ-γ collisions. Vertical position displacements between the beam centres at the IP generate a loss of luminosity. To counteract this effect, related to beam jitter at the IP, fast position feedback systems have been modelled. They consist of correctors and beam position monitors located very close to each other on the same side of the IP.
CTF2 Experiments and Results
After the successful test of a string of four 30 GHz accelerating modules in 1999, the RF equipment was converted into a 30 GHz high-power test stand. For this conversion, the power extraction structure (CTS-L) was further improved and installed in the drive beam line of the first module, and a spare solenoid from LIL was mounted around it for improved beam focusing. During the year the CTS-L served as 30 GHz power source for all accelerating structure tests. It reliably delivered up to 150 MW. However, when the focusing field of the solenoid was applied, a shortening of the RF output pulse, together with an increase of the vacuum level was observed, probably due to multipactoring in the CTS-L.
The vacuum system and the instrumentation for the 30 GHz accelerating structure tests were considerably improved. Pumping and vacuum gauges are now installed upstream and downstream of the CLIC accelerating structure (CAS), in the feeding waveguide, and on the pumping chamber of the CAS itself. A bake-out at 150° C is now applied routinely and static vacuum levels in the 10–8–10–9 torr range are obtained. Wall current monitors (WCM) installed upstream and downstream of the CAS allow the measurement of dark current and of breakdown-related charge bursts. By applying time-of-flight techniques, the energy of the electrons constituting these charge bursts can be determined. Light emitted by the CAS during RF breakdowns into the beam pipe is detected with a fast photomultiplier, and first tests have been performed to measure mechanical vibrations of the CAS with accelerometers. An attempt to measure X-rays produced by dark current and/or RF breakdowns in the CAS failed, because of the strong X-ray background produced by beam halo losses of the nearby drive beam.
Proton Synchrotron Division 207 Three different CAS structures were tested. Two were disk-loaded waveguides of 28.5 cm active length and constant impedance. The main difference between these two CAS structures is the coupler design, which is the standard CTF one-port design in one case and a prototype two-port design in the other. The third structure was a 12.3 cm long planar structure built at the Technische Universität of Berlin, Germany. The single-port disk-loaded structure was limited by breakdown to a mean accelerating gradient of 60 MV/m, corresponding to a peak surface field of 265 MV/m; the two-port structure reached a mean accelerating gradient of 70 MV/m corresponding to a peak surface field of 266 MV/m. Both structures have the maximum surface field on the iris between the coupling cell and the first regular accelerating cell. Post-mortem inspection of both structures showed damaged surfaces only at this spot. The third structure tested, the planar one, reached an accelerating gradient of 50 MV/m. Its peak surface field remains to be calculated. Post- mortem inspection showed slight damage in the coupler region for this structure as well. All the field values quoted are measured for 15 ns pulse length, but higher values have been reached for shorter pulses.
Thanks to various refinements in the measurement apparatus and calibration techniques, it is now possible to achieve agreement to 1% between the measured probe beam acceleration and the acceleration predicted by the 30 GHz RF power measurement. A large quantity of data has been collected to characterize the RF breakdown process. One important observation is that these breakdowns give hardly any reflected power compared to more conventional RF cavities, but produce intense electron bursts. The intensity of these bursts is strongly correlated with a reduction of the RF power transmitted through the structure. Another particularly fascinating feature observed is that the light pulses emitted during RF breakdowns have a duration of hundreds of nanoseconds, which is much longer than the RF pulse itself.
Testing of a beam-powered single-cell cavity in the drive-beam line gave maximum surface fields without breakdown of 380 MV/m. Varying the temperature of this cavity in situ between –190°C and +200°C showed no significant variation of this maximum field.
The studies on coherent synchrotron radiation (CSR) effects in magnetic bunch compressors have continued with new experiments to measure the dependence of the emittance growth on the horizontal beam β-function. The design of a new CSR experiment to measure CSR shielding by the vacuum chamber has been completed, and the fabrication of the hardware components is under way; the four new dipole magnets needed will later be re-used in the injector of CTF3.
During the year 2000 no major modifications were made to the 3 GHz part of the CTF 2 beams. The modulators and klystrons, RF guns and 3 GHz travelling-wave structures worked very reliably, which allowed an operation that was more stable and efficient than in past years. A new, VME-based, DAQ system for the streak camera was successfully put into operation and replaced the obsolete PC-based system.
CTF3
The preliminary phase of the new CTF3 test facility was completely defined, so that work can start on modifying the LPI complex as soon as it becomes free in April 2001. The main goal of this preliminary phase is to demonstrate the technique of funnelling bunch trains of electrons to multiply the bunch repetition rate by up to a factor five at low bunch charge, compatible with the present LIL accelerating structures. This is in preparation for the RF frequency multiplication scheme using an electron beam, to be demonstrated in later phases.
208 Proton Synchrotron Division A new electron gun will be installed, which is presently being built by the LAL laboratory under a Collaboration agreement, closely following the design used for the CLIO machine (Collaboration pour un Laser d’électrons dans l’infrarouge à Orsay). Construction is on schedule, with installation at CERN foreseen for June 2001. The linac will be shortened to eight accelerating sections for which the design of the new optics and layout are complete. Modifications to the present EPA ring have been defined, in order to make the lattice isochronous with a dispersion-free injection region. A solution for the optics of the transfer lines has also been developed, and detailed drawings of the new layout have been prepared. Finally, an extensive machine development programme was pursued at LPI in preparation for the modifications foreseen, and for commissioning of the new machine. These included measuring LIL lattice parameters, beam emittances, bunch lengths and the EPA circumference, and testing the linac modelling and ring isochronicity.
For later phases of CTF3, a new front-end will be required, consisting of a high-current gun operating at a voltage of 140 kV, a pre-buncher, a sub-harmonic buncher, and a travelling wave buncher. For the hardware of the gun, an existing diode assembly is being modified by SLAC to fit the CTF3 requirements. LAL is starting studies concerning the gun electronics and the high voltage deck, as well as the design of the pre-buncher. SLAC has the responsibility for the optical layout of the injector and the work is well advanced. In parallel, the RF design of a 20-cell travelling wave buncher has started.
The existing LIL accelerating structures will need to be replaced by new structures, capable of accelerating the high bunch charge. A Tapered Damped Structure (TDS) has been developed, and a full-sized prototype was successfully tested with RF power up to 50 MW. An alternative design, a Slotted Iris Constant Aperture Structure, is also being developed. Computer simulations are very promising, and prototype work has started. A preliminary enquiry has been launched to produce these structures in industry, leaving open the choice between the two versions. The optics of the new linac will use triplet focusing, and the implementation details can now be finalized.
As RF power sources, the klystrons available from LIL will be used, together with a pulse compression system. This system has to produce longer RF pulses than the existing LIPS, with a flat-top in amplitude and phase. A scheme which uses a pre-programmed RF phase variation has been developed, and tests with RF power have started. Also, a very promising alternative to the twin-cavity system of LIPS has been developed, the Barrel Open Cavity, using only one cavity for storing the RF power; a model has been built and tested at low power.
For the later stages of CTF3 the INFN Laboratory, Frascati, will take responsibility for the delay loop, the combiner ring, the transfer lines, and the bunch compressors (both developing the layout and producing the hardware required). The optical layout of the combiner ring is well advanced, and the design of the delay loop has started. On the hardware side, a model of a strip-line kicker has been built, and studies concerning the RF deflectors and magnetic chicanes are progressing.
Proton Synchrotron Division 209 NUFACT (Neutrino Factory) Study
NUFACT Working Group
The Neutrino Factory Working Group (NFWG) created in May 1999 has been quite active in the year 2000. Contributions showing the latest ideas were made at various conferences and in particular at the NuFact00 meeting in Monterey. A CERN reference scenario has been worked out for a Neutrino Factory, and is shown in Fig. 7 in a schematic way. Collaborations have continued with many laboratories, in particular CEA, FZJ, GSI, IN2P3, INFN and RAL.
The first part of the reference scenario for a Neutrino Factory is the ‘proton driver’ which, in order to provide an intensity of 1021 neutrinos/year, should deliver a beam power of about 4 MW. In the CERN scheme, a linear accelerator delivering a beam of energy 2.2 GeV has been chosen. The reason for this choice is the SPL superconducting 2.2 GeV linac study that is based on the availability of LEP cavities, klystrons, etc. after the shutdown of that machine.
A possible H– linac 2.2 GeV, Accumulator4 MW ring + bunch
layout of a compressor p neutrino factory Magnetic horn capture Target Ionization cooling Drift
Phase rotation
Linac → 2 GeV µ Recirculating Linacs 2 → 50 GeV
Decay ring – 50 GeV ≈ 2000 m circumference
ν beam to near detector ν µ + ν π + µπ
ν beam to far detector
Fig. 7: The schematic NUFACT reference scenario.
This linac accelerates H– ions, operates at 75 Hz with a pulse duration of 2.2 ms, and has a beam current of 11 mA. The beam is injected into an accumulator (by charge exchange injection) and subsequently into a compressor ring to produce the necessary short bunch lengths of the order of nanoseconds. One hundred and forty bunches (at a frequency of 40 MHz) will be sent onto a target to produce short pion bunches. The pions will be collected and then subsequently decay into muons, which are captured. These muons must then have their energy spread and transverse emittance reduced, prior to their acceleration to 50 GeV. At CERN, there is
210 Proton Synchrotron Division considerable experience with magnetic horns for the collection of antiprotons, and in the conventional production of neutrino beams. It is therefore planned to investigate the possibility of using a magnetic horn for the pion collection in the Neutrino Factory.
Because of the high repetition rate and the large number of bunches, an RF system is proposed for the manipulation of the muons. The RF system will capture and phase-rotate the muon bunches, and will also be used in the process of ionization cooling of the muon beam. Further acceleration of the muons to 2 GeV is performed in a special linac with solenoid focusing up to around 1 GeV, followed by more conventional quadrupole focusing. Subsequent acceleration takes place in two Recirculating Linacs (RLAs) to an energy of 50 GeV. The muons are then injected into a storage ring (or, decay ring) where they are kept for the duration of their lifetime (1.2 ms at this energy). The muons decaying in the long straight sections of this ring produce the required neutrino beams. These straight sections point towards the neutrino detectors situated at suitably large distances.
The challenges on the accelerator side are enormous. The high intensities in the proton driver must be mastered with very low losses if hands-on maintenance is desired. The target problems (4 MW deposited) are more demanding than for a neutron spallation source, and the issues of ionization cooling must be addressed in order to increase the beam quality and intensity. The muon accelerators are also not easy, because acceleration must be rapid so as not to lose too many particles during their short lifetime (the muon lifetime is 2.2 µs at rest). Some of the technical problems are being addressed in experiments at CERN: for example, a modified 200 MHz cavity with locally-enhanced fields has been tested successfully in a high radiation area near the AD target zone; and a pion production experiment (HARP) made its first run, to obtain data that will allow an optimized target design. In outside collaborations, CERN has participated in a RAL experiment at TRIUMF (MUSCAT) to make measurements on muon scattering which will yield important data in the context of ionization cooling; and target experiments are in preparation at BNL (as well as at ISOLDE) to study the interaction with the high-intensity proton beam.
Superconducting H– Linac (SPL)
The suggestion was made some time ago to re-use the large amount of RF hardware from LEP, once it reached the end of its life, to build a new linear accelerator for H– ions. This machine could deliver 4 MW of beam power at 2.2 GeV, and could be the proton driver for a Neutrino Factory. If it were located at CERN it would dramatically improve the performance of the PS complex for all users of high beam intensity (i.e. LHC, CNGS, and AD) and also for the ISOLDE community whose medium-term plans for a second-generation Radioactive Beam Facility could then be realized on-site. The details of the SPL design have been refined during the year and a block diagram is shown in Fig. 8. The low-energy part using room-temperature structures has been optimized, and the debunching line at high energy has been properly designed. Superconducting multi-cell cavities for β = 0.8 and β = 0.7 have been built and have demonstrated useable gradients exceeding 9 MV/m and 5 MV/m respectively. The design of the chopper and its driving amplifier has progressed in close collaboration with other teams (Los Alamos and CEA, Saclay). The beam dynamics has been analysed for the whole accelerator and a satisfactory layout for the CERN site has been found which minimizes the cost of civil engineering work by re-using existing tunnels. A conceptual design report (CERN 2000–012) has been produced.
Proton Synchrotron Division 211 45 keV 7 MeV 120 MeV 1.08 GeV 2.2 GeV 13 m 78 m 334 m 345 m 3 MeV 18 MeV 237 MeV389 MeV
H– RFQ1 chop.RFQ2 DTL CCDTL β 0.52β 0.7 β 0.8 LEP-II dump
SourceLow energy section DTL Superconducting section
Stretching and collimation line
PS/Isolde
Accumulator ring
Fig. 8: The SPL block diagram.
Collaborations
Innovative ECRIS
The Innovative ECRIS Collaboration, with major funding from the Fifth Framework Programme of the European Commission and involving CERN, GSI (Darmstadt), INFN LNS (Catania), CEA (Grenoble) and UJF-ISN (Grenoble) made significant progress in 2000. Following the commissioning and testing of the 28 GHz gyrotron in Grenoble, the microwave power-pack was installed on SERSE, the superconducting ECRIS in Catania. In CW operation, the frequency scaling of the intensity (proportional to the square of the frequency) for a given magnetic field configuration (i.e. the confinement) was quickly confirmed. However, the resonant magnetic field at 28 GHz is about 1 T, and given the limitation on the maximum field available, it proved difficult to drive the source into saturation when trying to optimize the confinement. Additional tests also demonstrated the possibility to have an afterglow mode of operation, although the pulse was somewhat shorter than expected and somewhat less stable than the beam from the ECR4 Pb-ion source. The next stage of the project, installation of the 28 GHz microwave source on the normally-conducting ECRIS PHOENIX is now under way, with results expected in 2001. These results will form the basis for a design for a source giving 10 times the present intensity in linac 3 for the future CERN ion programme.
LIBO
The LIBO (LInac BOoster) project for cancer therapy aims to build a 3 GHz proton linac in order to boost the beam energy of a cyclotron from 50–70 MeV to 200 MeV. Such cyclotrons exist in many hospitals and small laboratories throughout the world, and the aim is to be able to treat deep-seated tumours with the higher energy beam. The project is a collaboration between the TERA Foundation, CERN, and the INFN (Milan and Naples). The PS Division has offered support in the RF design, low-level measurement and power testing at LIL of the prototype of the first LIBO module (62–74 MeV). This module is 1.2 m long and is composed of four tanks of ‘side-coupled’ cells. The tanks are connected by three off-axis bridge couplers that leave space
212 Proton Synchrotron Division for permanent quadrupoles on the beam axis, while constituting a single RF structure. During the year 2000, the 103 half-cell units that make up the module were brazed together, and a series of RF measurements and adjustments, before and after brazing, allowed an excellent field symmetry on axis of ±2.8% to be obtained. After the vacuum tests at the end of October, the module was installed in the LIL gallery and powered using a spare modulator-klystron (at 35 MW peak power).
The conditioning was rapid, and the module finally reached an accelerating gradient of 25 MV/m, which is well in excess of the design value of 15 MV/m. Indeed, the conditioning had to be stopped for lack of time, but there was clearly potential for increasing the gradient even further. These excellent results open the way to a re-design of LIBO for higher gradients, making the structure more compact and more suitable for hospitals. The next step for the LIBO module will be to move it to Catania, where it will be tested on a cyclotron beam, powered by a klystron provided by the medical accelerator firm Scanditronix, in the framework of a collaboration with TERA. In parallel, the technology will be refined and documented, in order to transfer it to industry.
Controls for IHEP Protvino
The controls collaboration is focused on the U-70 accelerator complex upgrade, to provide the IHEP institute with a simple, efficient control of their accelerator complex. The progress of the project is reviewed regularly by a common management meeting; the expected completion date has been confirmed for the year 2002, according to the updated planning of 1999. During the year, the activity concentrated on two major systems, the controls of the ejection and of the correction power supplies. These systems were provided with the full power of the tool-kit developed in previous years. A new timing system was included in this renewal. The autumn run was then used to validate the functionality and the reliability of the system. After that, the new facility was handed over to the operating team, and the U-70 physicists immediately benefited from the quality of their new control system.
On the hardware side, the mass production of the embedded micro-controllers was started, and will continue next year. Installation of the MIL1553 field bus, timing system and VME crates connected to the control system’s LAN was made as necessary. On the software side, the enrichment of generic control programs using the newly-available hardware facility continues. Populating the real time database, which is the kernel of the control software, shows the power and the flexibility of the ‘operation régime’ concept provided by this system.
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