Particle Accelerators © Gordon and Breach, Science Publishers Ltd. 1976, Vol. 7, pp. 163-175 Printed in the United Kingdom
SOR-RING An Electron Storage Ring Dedicated to Spectroscopy
1 T. MIYAHARA,t§ H. KITAMURA,t ) s. SATo,t M. WATANABE,t s. MITANI,2) E. ISHIGURO,3) T. FUKUSHIMA, T. ISHII,4) SHIGEO YAMAGUCHI,S) M. ENDO,§6) Y. IGUCHI,7) H. TSUJIKAWA, T. SUGIURA,§ T. KATAYAMA, T. YAMAKAWA, SEITARO YAMAGUCHI and T. SASAKI§8) Institute for Nuclear Study, University ofTokyo, Tanashi, Tokyo, Japan
(Received February 25,1976; infinalform June 1,1976)
A 300-MeV electron storage ring to be used exclusively as a synchrotron-radiation source for spectroscopy has been constructed in the Institute for Nuclear Study (INS), University of Tokyo, Tanashi. Its useful spectral range lies between 40 and 2200 A. The 1.3-GeV electron synchrotron of INS currently being operated for high-energy-particle experiments serves as an injector. Electron beams are extracted from the synchrotron at 300 MeV, transported about twenty meters, and injected into the ring at the rate of one pulse per second. In the test operation a current of 10 mA was stored in a filling time of 10 minutes. The lifetime measured at a current of 1 mA was 2.5 hours, while the design goal determined by the Touschek effect is 100 mA with one hour life, for oper ation at 300 MeV. Increase of operating energy up to 375 MeV is feasible with a minor modification of the present design.
INTRODUCTION hand, converting electron storage rings under taken or constructed primarily for colliding-beam Electron storage rings have been developed in their experiments into machines dedicated for spectro history of the last two decades mostly as colliding scopy, was also reported, for instance, by the beam machines for experiments in high-energy Physical Sciences Laboratory, Wisconsin," and physics, but recently, their importance as light LURE (Laboratoire pourl'Utilisation du Rayonne sources in the vacuum ultraviolet region has been ment Electromagnetique), Orsay.S more and more recognized by workers of other SOR-RING is a 300-MeV electron storage fields of natural science. 1 Existing storage rings, ring constructed at the Institute for Nuclear Study for instance, DORIS in Hamburg and SPEAR (INS), University of Tokyo, Tanashi, to be used in Stanford have established their synchrotron as a dedicated synchrotron-radiation source for radiation facilities for applications in spectroscopy, spectroscopy in the vacuum ultraviolet region. crystallography, biology, and so on. 2,3 On the other The construction of SOR-RING was proposed by INS-SOR in 1965. It was undertaken, as a dedicated machine from the beginning. Construction started t Postgraduate research fellow of Institute for Nuclear in 1971 and was completed in 1974 with the first Study (INS). t Institute for Solid State Physics (lSSP), University of successful test filling. Tokyo. The present paper describes the design principle, § College of General Education, University of Tokyo. the instrumental details of the ring and the beam 1) On leave from Department of Physics, Kyoto University. transport system, the problems encountered in the 2) Research Institute for Atomic Energy, Osaka City University. course of construction, and the results of test 3) Department of Applied Physics, Osaka City University. operations. The spectral intensity of synchrotron 4) Department of Physics, Tohoku University. radiation and the main features of the ring are 5) Department of Physics, Tokyo Metropolitan University. given in Section 2 and Section 3, respectively. 6) Present address: National Institute of Radiological Sciences. The features of various parts of the ring are 7) Institute for Optics, Tokyo University of Education. discussed separately: the magnet system in Section M) Visiting staff of INS. 4; the rf system in Section 5; the vacuum system in 163 164 T. MIYAHARA, et ale
Section 6; beam transport system in Section 7; p is given by" monitors of beam characteristics in Section 8. The results of test operations are briefly discussed N(A) = 7.86 x 1011J(mA) [E(GeV)r in Section 9. The main parameters of the ring [p(m)J2 are tabulated in Table L x A(A)(~)3 foo K 5/3('1) d'1, (2-1) AelA
TABLE I where N(A) is the number of photons of wavelength A, emitted per second in a width of 1 A into a Design parameters of the SOR-RING horizontal angle of 1 mrad along the orbit; y and Ac Energy of stored electrons E = 300 MeV are defined by Stored current I = 100 rnA 10 Number of stored electrons No = 3.6 X 10 E A- 4np -3 L y==--2' C--3- (2-2) Beam lifetime > 1 h (at 100 rnA) me Y Radius of curvature in a bending magnet p = 1.10 m K 5I3(1]) is the modified Bessel function of fractional Field strength of a bending magnet B = 9.09 kG Field index in a bending magnet 11 = 0.5 order of the second kind. Total orbit length C=17.4m The operating energy and the orbit radius of Length of a bending magnet L B = 0.864 m SOR-RING have been decided to be 300 MeV and L = 0.10 m Length of a quadrupole magnet F 1.1 m, respectively, from consideration of the L = 0.20 m D spectral range of interest and the limited available Length of a straight section L s = 1.31 m Revolution frequency fo = 17.24 MHz space. For this energy and radius, the practical Resonant frequency of the rf cavity f = 120.66 MHz spectral range of radiation emitted from the ring Harmonic number h = 7 extends down 40 A for 300-MeV operation as will V > 7kV rf voltage RF be seen in Figure 1, which shows the spectral Radiation loss per turn per electron Urad = 0.65 keV intensity distribution given by Eq. (2-1) with Numbers of betatron oscillations Vx :::::::: Vz :::::::: 1.2 Average of f3 x 2 < Px < 2.5 m p == 1.1 m. The available spectral range extends Average of f3z 2 < pz < 2.4 m even to 20 A by increasing the operating energy. Range of the field gradient of a 2 The orbit radius of 1.1 m leaves the possibility of quadrupole magnet 0
A: E = 350 MeV of the radial oscillation with the vertical oscillation. B: E =300 MeV The use of quadrupole magnets with variable C: E =250 MeV field gradients enables us to choose an operating point suitable for exciting difference resonance. The beam cross section is also made large if the dilation or momentum-compaction factor of a machine is made large. This is attained by weak 101° focusing. Therefore, SOR-RING was designed as a ~ machine of a weak-focusing type with variable field E -0 gradients. tU '- the value of the field index, n, of the bending E magnet in SOR-RING was chosen close to 0.5 for o~ 9 u 10 fast damping to occur." In a weak-focusing system QI ~ where the value of n is 0.5 and the focusing force "-en c of the quadrupole magnets is weak, the only 0 '0 possible case for the excitation of difference .J::. 0- 8 resonance occurs for Ivx - vzl = o. In SOR-RING 10 the designed values of the betatron number are Vx = Vz = 1.20. The time constants of oscillation damping are 54 msec for both radial and vertical oscillations and 27 msec for the synchrotron oscillation. The operating energy, the beam cross section and the current decay can be varied Wavelength according to the users' requirements. Such tun ability will be useful, for instance, for suppressing FIGURE 1 Spectral distributions of synchrotron radiation higher order diffraction from a grating by reducing from SOR-RING, (p = 1.1 m) for several electron energies. The ordinate represents the number of photons of wavelength electron energy in the ring, or for controlling the A, emitted per second in a width of 1 Ainto a horizontal angle cross section of the beam so as to increase lifetime, of 1 mrad along the orbit by a current of 1 rnA. The abscissa or sometimes for increasing luminosity of the light represents the wavelength. The peak in the curve for 300 MeV source with a sacrifice of lifetime. is at the wavelength of 172 A. It isa great advantage ofthe dedicated low-energy electron storage ring that users can stay close to the 7 1 ring to carry out the optical alignment and 3.8 X 10 sec.- • rnA- 1. SOR-RING is designed so as to achieve the storage of a current of 100 rnA. measurement except during injection. It also reduces the loss of radiation intensity, which is by In this case, the design lifetime is one hour which no means negligible if one has to work at long corresponds to the rate of decrease of 1.6 percent in a minute and is long enough to perform ordinary distance from the source separated by radiation optical measurements including modulation shields. It is often the case in the larger machines. spectroscopy. The layout of the components of the ring is shown in Figure 2. The magnet system is of a separated function type and consists of eight identical bending magnets, B 1 - Bs, and four triplet focusing systems, QI-Q4' each of which contains two 3 MAIN FEATURES OF SOR-RING radial-focusing quadrupole magnets, QF' and one vertical-focusing quadrupole magnet, QD. The In a low-energy electron storage ring the lifetime of odd-numbered straight sections 81-87 which are the stored beam is limited mainly by the Touschek not occupied by quadrupole magnets are used for effect. Since the Touschek lifetime is proportional mounting pumping stations and the rf cavity. to the reciprocal of the density of stored electrons, Electrons extracted at 300 MeV from the electron the beam cross section should be made large at synchrotron are injected at the first straight high-current operation, so that the Touschek section 8 1 . Four synchrotron radiation ducts lifetime is long. The large beam cross section is R 1 - R4 are attached to the bending sections attained by difference resonance through coupling B 3 - B 6 . Each of the radiation ducts is followed by a 166 T. MIYA'HARA, et al.
TP RF CAVITY
ELECTRONS
1m
TP
FIGURE 2 Structure of SOR-RING. Sl-SS: Straight sections, each being 1.31 m long. B1-Bs: Bending magnets. QI-Q4: Triplets of quadrupole magnets. IP: Sputter-ion pump. TP: Titanium sublimation pump. MB: Mechanical
booster. RP: Rotary pump. R 1-R4 : Ducts for synchrotron radiation. ·PM DUCT: Duct for measuring stored currents, by the photoelectric method. Injection is made through the section S 1 where a pulsed bending magnet (PB 2 in the text) is placed. The rf cavity and the perturbator magnet (K 2 in the text) are located in the sections, S3 and Ss, respectively. differential pumping unit with two pumping pump, and a mechanical booster are designated as stages evacuated by sputter-ion pumps for separat IP, TP, RP, and MB, respectively. In addition ing the optical instruments from the ring vacuum. to the four pumping stations in the straight sections The vacuum system has to provide a low equilibrium distributed pumping-elements are installed in all pressure with the full load of 100 rnA stored in the the bending sections. A design goal of the ultimate ring. In Figure 2, pumping units including an ion pressure under full load is 1 x 10- 9 Torr with pump, a ·titanium-sublimation pump, a rotary overall pumping speed around 4500 liters· sec- 1. SYNCHROTRON RADIATION SOURCE 167
4 MAGNET SYSTEM deviation of 2 % within the radial aperture of 80 mm. In QD the effective gradient length with the 4.1 Bending Magnets same maximum deviation is 21.9 em within the same radial aperture. The eight radial focusing The bending magnet is C-shaped, and machined magnets and the four vertical focusing magnets from iron block with low carbon content. The have their respective common power supplies. magnetic field strength for 300 MeV is 9.09 kG Quadrupole magnets were mounted with an along the central orbit and the maximum attain expected tilt about the longitudinal axis, ~ Relative variation of field strength ~BIB among '5 '- eight magnets is less than 10- 3, which is estimated CIJ J:l from an error in the gap between the polefaces. E z:::J The eight identical bending magnets are con 1.15 nected electrically in series from a common power supply so that the current of each bending magnet is varied simultaneously. The measured standard deviation of the field strength over all the bending magnets is ±0.08 %, well within the accuracy required from the orbit design. The effective length 1.15 1.20 1.25 1.30 of a bending magnet is 90.6 ern, while the length of Number of radial betatron oscillation the pole piece along the central orbit is 86.4 em. The FIGURE 3 The numbers of radial and vertical betatron effect of this difference will be described in Sec. 9. oscillations determined by the field gradient, KF or KD . Each curve corresponds to a constant value of K F or K D represented in units of m - 2. The operating point is shown as Po, where V ~ V ~ 1.20 and K ~ K ~ 4.6 m ":'. 4.2 Quadrupole Magnets x z F D
Quadrupole magnets are used in the form of Figure 3 shows the change of Vx and Vz with the triplet system. The pole pieces of a focusing magnet focusing force ofquadrupolemagnets as parameters. QF and a defocusing magnet QD have lengths of In the figure, K F and K D represent the field 10 em and 20 ern along the orbit, respectively. The gradients of the quadrupole magnets, QF and diameter of the circle inscribed within the faces of QD' respectively. We find in Figure 3 that the ~ ~ the four pole pieces is 75 mm. The intersection values of K Q corresponding to Vx Vz 1.20 are of the surface ofa pole piece with a plane perpendicu 2 K ~ K ~ 4.6 m- . (4-1) lar to the orbit is hyperbolic. The field gradients in F D QF and QD were found constant to 5%within the The range of the field gradient was preferred to be horizontal space of 80 mm width, which is the 2 O 5 RF SYSTEM 3 An rf system which was formerly used for the INS electron synchrotron," was converted to be used in the present ring with a change in accelerating E C electrodes. The resonant frequency is 120.66 MHz. .2 U The corresponding harmonic number is seven. c ::::J The dilation factor was estimated to be about 0.9. '7 c=. For this value of the dilation factor, the minimum gap voltage required for sufficiently long quantum lifetime is 7 kV. The cavity is of a self-oscillating re-entrant type with a one-turn short loop for fine adjustment for frequency change due to beam loading or to the thermal expansion of the cavity. The resonant frequency is variable over the range Path of an electron of ±500 kl-lz around the central frequency, and the FIGURE 4 The p-function in the unit lattice of SOR-RING. adjustment is made automatically by a frequency The abscissa represents one eighth of the design orbit consisting comparator. The frequency stability is within of a half of a straight section, a bending magnet, and a half of a quadrupole magnet section. Two broken lines show planes of ±0.01 %,- which guarantees the stability of the mirror symmetry. B, QF and Qn represent the bending magnet, orbit, and accordingly, the stability in the spatial the quadrupole magnet for radial focusing and that for vertical position of the radiation source. focusing, respectively. The cavity is made of a copper plate 3 mm thick and a pair of stainless-steel electrodes. One side of the cavity is insulated from the other with thin teflon sheets and also from the doughnut with 4.3 Nonlinear Component ofthe Field ceramic tubing. In order to suppress the multi pactoring discharge, which causes outgassing from The allowable range of the shift, Av, in the operating the accelerating electrodes, a dc bias-voltage of point caused by the nonlinear component of the 2 kV is applied across them. field is limited by the requirements from the beam cross section and the efficiency of injection. The range of the field nonlinearity permissible for 6 VACUUM SYSTEM keeping a tolerable injection efficiency is estimated to be 5 % for quadrupole magnets and 18% for 6.1 Lifetime bending magnets, Figure 5 shows the radial distribution of the field indices of bending magnets, The lifetime of the beam depends mainly on three effects, scattering by residual gas, quantum fluctua B 1 and B4 • In the figure, the ordinate represents the field index. .The abscissa represents the distance tion and electron-electron scattering (Touschek from the center of curvature of the equilibrium effect).Among these three effects;the lifetime due to orbit. The curve given by the full line shows the quantum fluctuation is more than a day if the rf field indices of the magnets without the shims, voltage is more than 7 kV. Secondly, the calculated which were incorporated later in the pole pieces of lifetime associated with the scattering by residual the magnets for extending the spatial range of gas at the pressure P is given by uniform field index. For this extension of the range, LR = 7.7 x 10- 5 X p-l(sec). (6-1) we corrected the field in the B1 by shims with 10mm width and 1.3 mm thickness glued at the outside It is more than 20 hours, if the pressure is less than edges of the pole pieces. Shims with 10 mm width 1 x 10- 9 Torr. Thirdly, the Touschek lifetime at and 0.6 mm thickness were glued at the inside 100 rnA is estimatedto be" edges of the pole pieces of B . The radial distribu 3 4 LT = 3.73 X 10 sec, (6-2) tion of the field indices of B 1 and B4 with shims are shown by the chained line in Figure 5. The curve assuming the volume of one bunch to be 1.7 x appears to indicate that the correction has been 2.4 x 600 mm ', Thus, the lifetime of the beam is made effectively. mainly determined by the Touschek effect, if the SYNCHROTRON RADIATION SOURCE 169 ------60 ----.+------60 ------ ", //'/ / / . ' , / / / / / ' "/ ' / / "',"" , , , / ' / / iO.61 FIELD INDEX - 10 - 'shim 4 3 without shims with shims 2 with shims -2 without shims .~ -3 -4 FIGURE 5 Radial distributions of the field indices in the bending magnets, B 1 and B4 , without shims (full line) and with shims (chained line). The ordinate represents the field index. The abscissa represents the distance from center of the curvature of the equilibrium orbit. At the top, the cross section of the pole piece is illustrated. pressure is less than 1x 10- 9 Torr. Equation magnet is mounted. Taking the sizes of these (6-2) gives the Touschek limit of current lifetime obstacles into account, the injection point is placed product of about 0.1 A . h. 35 mm away from the central orbit. Also, con sidering the beam size at injection, we have chosen 6.2 Chamber Size a width of 80 mm as the horizontal size of the doughnut in the ring. At the point of injection, various obstacles such The vertical size of the chamber is determined as a magnetic shield and a septum of the pulsed by the initial amplitude of the vertical betatron bending magnet are placed. The location of these oscillation, which is related to the magnitude of obstacles relative to the beam positions is shown in coupling of betatron oscillations and the operating Figure 6, where the cross sections of the obstacles point. The magnitude of coupling or the operating and of the injected and stored beams are illustrated point is changed by the effect of the nonlinearity in a simplified manner. The ultrahigh vacuum in of the field as the oscillations damp. 10 This in turn the doughnut should be separated by the wall from causes the change in the ratio of vertical to radial the lower vacuum in which the pulsed bending amplitudes. We choose the operating point so 170 T. MIYAHARA, et al. Magnetic shield from the system is 8 t QTH ~ 9 X 10- Torr . liters . sec-i. I i Stored Thus, we obtain the rate of the total outgassing with ._beam. _ a stored current of 100 rnA as QT = QTH + QSR i t ~ 2.8 X 10- 7 Torr . liters . sec-I. Gap of PB2 Chamber wall 6.4 Distributed Pumps FIGURE 6. Cross-sectional view of the injection section Sl' The finite conductance of tubing reduces the The transported beam is injected through the gap of PB 2 • The effective pumping speed of the system, while the distance between the stored beam at the central orbit and the outgassing from the bending sections is appreciable center of the gap at the exit of PB2 is 35 mm. The distance between the stored beam and the chamber wall is 25 mm. at a stored current of 100rnA. As most of the photo desorption from the chamber wall takes place in vacuum chambers with low conductances, it is that the ratio at injection is smaller than 0.72 and necessary to reinforce the pumping ability by the final value corresponding to the completely distributed pump-elements built in between the damped state is 1.0. In _SOR-RING, the initial pole pieces of bending magnets. This technique amplitude of the radial betatron oscillation is has been well established and adopted in many 19 mm; the vertical size of the injected beam is other electron storage rings. 12 Figure 7 shows the 5 mm. These values and the ratio between the cross-sectional view of a vacuum chamber in a amplitudes of the radial and vertical oscillations bending section. The distribution of magnetic give us the required vertical size of the doughnut field strength in the chamber at an electron energy as 35 mm. of 300 MeV is also given in the figure. There, the abscissa represents the distance from the 6.3 Estimation ofthe Rate ofOutgassing POLE PIECE Outgassing is due to both ordinary thermal re TITANIUM leasing of molecules from the surface of the vacuum J chambers and photo-desorption of molecules in vPOLVIMIDE duced by synchrotron radiation. Photo-desorption 'J is caused by the secondary process in which photo []I] \ electrons from the surface of vacuum chambers \ producedby synchrotronradiationdetachadsorbed 'I \, ANODE CELL molecules as they leave or hit the surface. We ELECTRON studied the photoelectric yield of stainless steel, BEAM type 304, used in the present vacuum system in the vacuum ultraviolet for various incident angles. ,kgauss We found the average yield in the present case is about 0.12. From this value the rate of photo 9 desorption caused by synchrotron radiation is 8 estimated as 7 QSR ~ 1.9 X 10- 7 Torr· liters· sec-I. 6 A typical rate of thermal outgassingfrom stainless 5 steel, type 304, baked out at high temperatures for 1150 1100 1050 mm sufficiently long time is 1 x 10- 1 2 Torr· liters sec - 1 . em - 2 at room temperature. 11 Since the FIGURE 7 Cross-sectional view of a vacuum chamber in a bending magnet. The radial distribution of the magnetic field at total surface area of the vacuum chambers in an electron energy of 300 MeV is also given. The ordinate cluding distributed pumps in the SOR-RING is represents the field strength. The abscissa represents the distance about 9 x 104 cm', the rate of thermal outgassing from the center of the curvature. SYNCHROTRON RADIATION SOURCE 171 center of curvature of the central orbit. It should be and a mechanical booster along with a liquid noticed that the distributed pumps are mounted nitrogen trap. The vacuum chambers except for outside of the orbit, they have been placed inside the four straight sections, Sb S3' S5' and S7, were of the orbit in other storage rings. It has been heated by a current of 1200-2000 A directly flowing intended in the present design to irradiate the pump through the chambers. Since each of the four element by synchrotron radiation rather than to straight- sections mentioned above was heated avoid it. Since the total power of the radiation is with a mantle-heater, a bypass of copper-bar was only 8 W in each bending section for the current attached parallel to such a straight section. of 100 rnA, the temperature increase in pump Vacuum chambers in quadrupole magnets were elements due to irradiation is not serious, but bypassed by iron-bars to protect the chambers from rather profitable because it ~ill cause a mild overheating. The in-situ baking-out described above bakeout of the pumps whenever electrons are permits a small gap between pole pieces of a stored. magnet and gives a high efficiencyof heating. After Each anode cell is perforated with a number of the system was cooled down to room temperature, small holes through which synchrotron radiation the pressure reached 1 x 10- 10 Torr in 48 hours as well as neutral gas molecules are introduced and to 7 x 10- 1 1 Torr in 72 hours. into the plasma region inside the cell. These holes increase the effective aperture of the pumping element by a factor of two, and high rate of photo 7 BEAM TRANSPORT SYSTEM ionization due to the radiation introduced into the cell will help to maintain the discharge even in 7.1 Extraction ofElectrons from the Synchrotron ultrahigh vacuum. Its overall speed for eight bending sections is estimated as approximately Figure 8 shows the schematic illustration of the 1200 liters . sec- 1. In a test without the electron beam transport system and the ring. In the figure, beam in the ring, they worked well over the pressure K 1 refers to a fast kicker magnet, K 2 to a perturba 5 10 range from 10- to 10- Torr. The ultimate tor magnet, PB1 and PB2 to pulsed bending 10 pressure was 5 x 10- Torr when other pumps magnets, Bt1 and Bt2 to ordinary bending magnets, were suspended. Ion clearing electrodes were not Qt 1 -Qt6 to quadrupole magnets, Z 1 and Z 2 adopted in the SOR-RING. to vertical steering magnets, CM 1-CM6 to Ceren kov monitors, EM to an electrostatic induction 6.5 Total Pumping Speed monitor, and PM to a photomultiplier. As the electron synchrotron is usually operated with its The pumping speed required for the vacuum system maximum energy between 0.6 and 1.3 GeV;. at 1 x 10- 9 Torr is the extraction of electrons at the instantaneous energy of 300 MeV from the synchrotron has been ST = QT/P = 280 liters· sec-I, made by the use of a fast kicker magnet K 1 located when the system is in its final equilibrium, and it in a straight section of the synchrotron. K 1 should be much higher in transient periods when deflects electrons moving along the central orbit desorption of gases from surfaces is not yet by an angle of 12.3mrad. This amount of deflection exhausted. In order to obtain this pumping speed, displaces the beam by 50 mm outward at the four sputter-ion pumps with nominal pumping entrance of the pulsed bending magnet PB1 speed of 128 liters· sec- 1 each and four sublima located in the next straight section. The required 1 tion pumps of 700 liters· sec- each are mounted risetime of K 1 is about 60 nsec and the duration in four straight sections. By adding the pumping of the flat top of the pulse should be longer than speed of the distributed pump the total nominal 60 nsec. With a lumped magnet working as an pumping speed of the system is estimated as inductor, a risetime shorter than 60 nsec with the 4500 liters· sec-I. duration of the flat top as long as 100nsec has been obtained, although distributed magnets has been 1 3 6.6 Baking ofthe Vacuum Chambers used generally for such a purpose. K 1 is 30 em long, and has a ferrite core and a one-turn coil. The whole system was baked out at temperatures of The magnetic field strength to deflect 300 MeV 200° - 300°C for 72 hours, during which the electrons by an angle of 12.3 mrad is 410 G for a rough pumping was performed by a rotary pump magnet length of 30 em and the corresponding 172 T. MIYAHARA, et at. cemented with epoxy resin, and extracted out of the electron synchrotron. PB 1 is excited with a silicon-controlled-rectifier pulser, which produces a current in a half-sinusoidal wave of 1 kHz. K 1 is excited at the moment when the field of PB 1 almost reaches its peak intensity. 7.2 Beam Transport The extracted electrons pass through two bending magnets, Bt 1 and Bt 2 , and also through six quadrupole magnets. The deflection angle of Bt1 is 47° and the deflection angle of Bt2 is 680; the total length of the beam transport system is about 20 m. Each of Bt1 and Bt2 has a field index of 0.5 5m and the bending radius is 1 m. The initial radial size of the beam just before extraction is estimated to be 14 mm. The maximum radial size of the transported beam is limited to 35 mm, which is within the diameter of the beam pipe, 45 mm. For the cancellation of the effect of energy dispersion, a doubly achromatic system is adopted so that the off-energy function u and its derivative u' become zero at the exit of the second bending magnet Bt 2' This is accomplished by adjusting the 1.3GeV ELECTRON field of two quadrupole magnets, Qt 1 and Qt 2 . SYNCHROTRON In order to match the jJ-functions of the electron synchrotron to those of the ring, we used four quadrupole magnets Qt 3-Qt6 . At the injection FIGURE 8 Schematic illustration of the beam transport point a small polyimide w-indow is fixed to separate system and SOR-RING. K 1 : Fast kicker magnet. PB1 and PB z: Pulsed bending magnets. K z :Perturbator magnet. Bt 1 and the ultrahigh vacuum in the doughnut from the Bt z ' Bending magnets. Qt 1-Qt6 : Quadrupole magnets. CM1 ordinary vacuum in the beam-transport system. CM6: Cerenkov monitors. PM: Photomultiplier. EM: Electro Since electrons are injected through the window, static induction monitor. Z 1 and Zz: Steering magnets correct a certain amount of energy dispersion and small ing the beam vertically. Angles of deflection of the beam are also shown. angle scattering are inevitable. Thus matching of u at the injection point is not achieved at present. Bt 1 and Bt 2 are electrically connected in series to a common power supply. A correction coil R, pulse current is 490 A. In the pulse generator for operating independently of both Bt1 and Bt 2' KIa 12 m coaxial line with a characteristic is attached to Bt 2 to make fine adjustment of the impedance of 27 n is used as a pulse-forming field strength. Two steering magnets, 2 1 and 2 2 , network. It is charged up to 26.4 kV and discharges are used to correct the vertical deflection. 2 1 is through a triggered electric spark. The switching located between PB1 and Bt b and 2 2 is located by the spark is stabilized by dry nitrogen flowing between Qt 3 and Qt 4 . through the spark gap separating copper-tungsten electrodes. The time jitter was around ±5 nsec up to operation more than 5 x 105 times. The 7.3 Injection details of the extraction system will be discussed elsewhere. 14 A pulsed bending magnet PB2 is placed just in The 300-MeV electrons deflected by Klare front of the injection point. Figure 9 shows the further deflected by an angle of 12.5° by a septum PB2 magnet and its surroundings (seealso Figure 6). magnet PB1 with -a core of silicon-steel laminae PB 2 deflects incident electrons by 14° making their SYNCHROTRON RADIATION SOURCE 173 SUBLIMATION PUMP SPUTTER ION PUMP POLYIMIDE WINDOW CENTRAL ORBIT _ ---r- 35mm t PULSED BENDING MAGNET (PB2) 10cm FIGURE 9 Pulsed bending magnet PB 2 at the injection point and its surroundings. trajectories parallel to the central orbit of the ring. following as an optimum set of parameters: It is driven by a silicon-controlled-rectifier pulser ~ ~ Vx Vz 1.20, (7-1) . like that used for PBl The core of PB2 is the same ~ X K 2 16 mm, (7-2) as PBl and the rate ofoutgassing from the core is t f ~ 370 nsec, (7-3) appreciable. Therefore, PB2 is placed in a separate chamber with a small polyimide window through where X K2 is the maximum displacement of the which electrons are injected into the ring. orbit caused by the field of the K 2 magnet. When electrons are injected into the ring, the K 2 is of a vacuum-cored typevsince the use of equilibrium orbit at S1 is slightly shifted outward ferrite is inadequate in the ultrahigh vacuum from the designed central orbit by the field of a because of its high rate of outgassing. It produces perturbator magnet K 2 located at S5' Electrons a uniform field of 230 G with an accuracy of ±2 % are injected at the moment when the field of K 2 in a radial width of 80 mm. The pulse generator is at its maximum. The fall-time t f of K 2 is much for K 2 consists of an ordinary tank circuit with longer than the period of revolution. In order to L, C and R, and generates a damped oscillatory obtain a high efficiency of injection, we choose the current with a period of 1.5 usee. The K 2 system 174 T. MIYAHARA. et al. is switched by a stabilized triggered spark. The system with Cerenkov monitors, denoted as time jitter is around ±.5 nsec. eM1-eM 6 in Figure 8. Each Cerenkov monitor The K 1, K 2, PBland PB2 systems are controlled consists of a sensor 4 mm in diameter made of a by a counter circuit including a preset counter. quartz or lucite bar and a photomultiplier. The The preset counter produces two pulses, L 1 and L 2 , sensor can be moved vertically and horizontally by using reference pulses from the electronsynchrotron. a remote control. The reference pulses include a zero-field pulse, The stored current in the ring is measured by a which indicates the starting time of acceleration photomultiplier and an electrostatic induction in the synchrotron, and voltage-to-frequencypulses, monitor, denoted as PM and EM, respectively, which indicate the instantaneous energy ofelectrons in Figure 8. PM is located at a small duct attached in the synchrotron. L 2 triggers K 1 and K 2 at the to the vacuum chamber in the section Bland moment when the energy of the electrons in the detects synchrotron radiation with high sensitivity. synchrotron reaches 300 MeV. L 1 triggers PB1 and PB2 and is produced 250 usee before the triggering of K 1 and K 2' This time lag is.necessary for the 9 OPERATION AND PERFORMANCE half-sinusoidal magnetic field of 1 kHz in PB1 and PB 2 to reach their peak field strength. After confirming that all the parts of the ring worked satisfactorily, the test of the beam transport system was carried out. It was successfully done 8 BEAM MONITOR and 3OG-MeV electrons were injected to the ring. We observed the position and profile of the trans A current of 10 rnA was stored using parameters of ported beam at several points in the beam transport the magnets slightly different from the design FIGURE 10 Overall view of the SOR-RING during the test operation in December 1974. SYNCHROTRON RADIATION SOURCE 175 values. The designed and actual parameters are as ACKNOWLEDGEMENTS follows: The authors owe very much to three organizations: the Institute designed actual for Nuclear Study (INS) and Institute for Solid State Physics Field strength of the bending (ISSP), University of Tokyo, and the Institute of Plasma magnet 9.09 kG 8.79 kG Physics (IPP), Nagoya University, for their effort in promoting Field gradient of the the plan positively, and in particular, to Professor M. Sakai magnet QF 4.6 m- 2 4.7 m- 2 (INS), Professor T. Suzuki (ISSP) and Professor K. Husimi (IPP), the directors of these institutes. Field gradient of the Preliminary design studies and construction were financed 2 2 magnet Qn 4.6 m- 5.3 m- mainly by the Ministry of Education as a project under the budget of the Institute for Nuclear Study from the fiscal year The actual field strength of the bending magnet 1970 to 1973, while they were additionally sponsored by the was smaller than the design field. This is due to Mitsubishi Science Foundation in 1970 and 1971. the fact that the effective length of the bending The authors enjoyed collaboration with M. Muto, H. magnet is longer than the design length by 42 mm, Sugawara, I. Nagakura, T. Matsukawa and T. Shibaguchi during the period of construction, and it is greatly appreciated. as mentioned in Section 4.1. As is shown in Figure 5 T. Sasaki also wishes to thank Professor E. M. Rowe, Physical the observed value of n is lower than the design Sciences Laboratory, University of Wisconsin, and Dr. H. value by 20 ~~. As was described in Section 4.2, Pingel, DESY, Hamburg, for valuable information. We also the effective gradient length of a quadrupole thank Professor R. HaenseI, Universitat Kiel, and Dr. C. Kunz, magnet is also longer than the design value. Using DESY, for their continual encouragement and substantial help in sending requested materials. the actual values of the effective length of the bending magnet, the field index, and the effective gradient length of the quadrupole magnet, we obtain the calculated values of the actual operating ~ ~ REFERENCES point as Vx Vz 1.25. A more accurate measure ment of the betatron numbers by the rf-knock-out method will soon be made. About 10 minutes 1. C. Kunz, Vacuum Ultraviolet Radiation Physics, ed. E. E. Koch, R. Haensel and C. Kunz, Pergarnon/Vieweg, 1974, were required for filling the ring up to 10 rnA. p.753. Typical lifetime at a stored current of 1 rnA was 2. K. C. Holmes, ibid., p. 809. 2.5 hours at an average pressure of 1 x 10- 8 Torr 3. P. Eisenberger, B. Kincaid, S. Hunter, D. Sayers, E. A. and an rf voltage of 7 kV while the base pressure Stern and E. Lytle, ibid., p. 806. 4. E. M. Rowe and F. E. Mills, Particle Accelerators, 4, 211 of the ring was 1 x 10- 9 Torr. This lifetime is (1973). close to the lifetime t R of the scattering by residual 5. F. Wuilleumier, "Le Rayonnement synchrotron emis par gases. The pressure during the storage of a current les anneaux de stockage dOrsay (ACO et DCI)," LURE of 10 rnA was 2 x 10- 8 Torr in the present test Report 74/03 (1974). filling. This is due to radiation-induced outgassing 6. J. Schwinger, Phys. Rev. 68, 1912 (1949). 6 7. A. A. Kolomensky and A. N. Lebedev, Theory of Cyclic and its rate was estimated to be 2 x 10- Torr Accelerators, North-Holland, 1966, p. 222. 1 3 liters· sec- . rnA-1. This value is about 10 8. Annual Report INS, University of Tokyo, 1960. p. 31. times as large as the value estimated in Section 6.3. 9. Uta V6Ikel, DESY Report 67/5, March, 1967. This result is expected, since the time-integrated 10. H. Zyngier and E. Cremieu-A1can, ed. Proc. Intern!' current of the ring is still small.P Symposium on Electron and Positron Storage Rings, G. Leleux, SAC LAY (1966), VII.b.3. The rf voltage of 7 kV is sufficient for 300-MeV 11. G. Moraw and R. Dobrozemsky, Proc. 6th Internl. Vacuum operation with relatively low stored current. A Conqr. 1974. Japan. J. App!. Phys. Suppl. 2, pt. 1, 1974, test of increasing the energy of the stored electrons p.261. to 330 MeV was made successfully, but the rf 12. J. Cummings, N. Dean, F. Johnson, J. Jurow and J. Voss, J. Vac. Sci. Tech. 8, 348 (1973). voltage was insufficient above 330 MeV. A new rf 13. E. B. Forsyth, Brookhaven National Laboratory, Accelera system, which is now under test, can supply an rf tor Department Internal Report EBF-2, March 18, 1963. voltage higher than 20 kV and operation at 375 14. H. Kitamura, T. Miyahara, M. Watanabe, T. Katayama MeV will be feasible. Figure 10 shows an overall and T. Sasaki, JJAP, 15, 1349 (1976). view of the SOR-RING during the test operation 15. Proposal for a high-energy electron-positron colliding-beam storage ring at the Stanford Linear Accelerator Center, in December 1974. In conclusion, we summarize Stanford University, Stanford, California, September. the major parameters of the SOR-RING in Table I. 1966.