Energy Recovery Linacs As Synchrotron Light Sources Sol M. Gruner & Donald H. Bilderback (From the SLAC Beamline, Vol

Energy Recovery Linacs as Synchrotron Light Sources

Sol M. Gruner & Donald H. Bilderback

(From the SLAC Beamline, Vol. 32, Spring 2002. The full issue may be downloaded from www.slac.stanford.edu/pubs/beamline)

A PERIODICAL OF PARTICLE PHYSICS

SUMMER 2002 VOL. 32, NO. 1

Editor BILL KIRK Guest Editor HERMAN WINICK Contributing Editors JAMES GILLIES, page 6 JUDY JACKSON, AKIHIRO MAKI, MICHAEL RIORDAN, PEDRO WALOSCHEK Production Editor AMY RUTHERFORD Editorial Advisory Board PATRICIA BURCHAT, DAVID BURKE LANCE DIXON, EDWARD HARTOUNI ABRAHAM SEIDEN, GEORGE SMOOT ESRF HERMAN WINICK APS SPring8 Illustrations TERRY ANDERSON UHXS Distribution CRYSTAL TILGHMAN 700 m

The Beam Line is published periodically by the page 14 Stanford Linear Accelerator Center, 2575 Sand Hill Road, Menlo Park, CA 94025, Telephone: (650) 926- 2585. EMAIL: [email protected] FAX: (650) 926-4500

Issues of the Beam Line are accessible electroni- cally on the World Wide Web at http://www.slac. stanford.edu/pubs/beamline. SLAC is operated by Stanford University under contract with the U.S. Department of Energy. The opinions of the authors do not necessarily reﬂect the policies of the Stanford Linear Accelerator Center.

Cover: Aerial photograph of the Stanford Linear Accelerator Center and the Stanford Synchrotron Radiation Laboratory, one of several laboratories around the world working on the development of advanced X-ray light sources. See articles on pages 6 and 32. page 32

Printed on recycled paper CONTENTS

2 BEAM LINE EDITOR RENE DONALDSON RETIRES

SPECIAL SECTION: ADVANCED LIGHT SOURCES

3 FOREWORD 22 ENERGY RECOVERY LINACS The prospect for powerful new sources AS SYNCHROTRON LIGHT of synchrotron radiation SOURCES Herman Winick Pushing to higher light source performance will eventually require FEATURES using linacs rather than storage rings, much like colliding-beam storage rings 6 INTERMEDIATE ENERGY are now giving way to linear colliders. LIGHT SOURCES Sol M. Gruner & Donald H. Bilderback The high performance and relatively moderate cost of these 2.5–4 GeV 32 X-RAY FREE-ELECTRON machines make them the popular LASERS choice, with six now in construction With advances in technology, it is now and more proposed. possible to realize the dream of a fully Jeff Corbett & Thomas Rabedeau coherent X-ray laser. Claudio Pellegrini & Joachim Stöhr 14 THE ULTIMATE HARD X-RAY STORAGE-RING-BASED LIGHT SOURCE 6–8 GeV rings with circumference of about 2 km or more could provide X-ray brightness that signiﬁcantly exceeds that of any present facility, TBA (optical unit) X rays opening new research opportunities. Main Linac Pascal Elleaume 5–7 GeV page 22 Injector Dump

42 THE UNIVERSE AT LARGE People, Places, Papers and Power Virginia Trimble

56 CONTRIBUTORS Rene Donaldson Retires

After 35 years in the main stream of the American program of particle-physics and accelerator research, SLAC Beam Line Editor Rene Donaldson has retired as of February 2002. Rene’s career in scientific publication began with a brief stint at the Lawrence Berkeley Laboratory in 1967. She then moved on to Fermi National Accelerator Laboratory near Chicago, returning to Berkeley in 1985 as a key member of the Superconducting Super Collider (SSC) Central Design Group, directed by Maury Tigner of Cornell. Tigner has recently written of his work with Rene during those earlier days: While I have many fond memories of the times she was an essential part of the Snowmass organization the most vivid memories are of our time together at the SSC Central Design Group in Berkeley. Being a very visible activity on the national stage, we were under enormous pressure from many different interest groups and had many bosses to please. This made our public relations documents and reports of special importance and Rene handled all this with great aplomb and powerful attention without which success would have been impossible. Rene joined the staff of the Stanford Linear Accelera- Rene Donaldson in San Diego’s Old Town. (Photo by Tony Donaldson) tor Center in 1989, at a time when the original Beam Line was essentially an in-house newsletter aimed at the SLAC staff. She and Michael Riordan have been the central figures in transforming the Beam Line into a journal that during the last ten years has become a well-respected vehi- cle for communicating the purposes, achievements, and plans of our fields of study: fundamental and applied research in the physical sciences and the machines that make much of it possible. In all of this work—at Fermilab, Berkeley and Stanford—Rene has acted as writer, editor, illustrator, layout designer, publisher, conference coordinator, exhibit designer and whatever else she needed to be to get the job done. There is no one who has combined knowledge, skill, and energy any better. She leaves the fold with the thanks and best wishes of the physics community. We do, and shall, miss her. The Donaldsons (Rene and husband Tony) have now moved on to new quarters in the Arizona-New Mexico region, where there are strong ties to their lifelong interests in the beauty of the natural world and in the lives and art of early humans. The stage is set for a new chapter in the long adventure. Hail and farewell.

2 SUMMER 2002 FOREWORD HERMAN WINICK

This special issue of the Beam Line is con- viruses and proteins and facilitating cerned with the development of powerful drug design. new sources of X radiation for use in o Molecular environmental science, deter- advanced studies in the physical sciences. mining the chemical form of contami- Readers will be familiar with the fact that nants in soil and water—knowledge that synchrotron light sources have had a spec- is critical to developing strategies for tacular evolution. It all started from humble separation, remediation, and storage of beginnings about 30 years ago with parasitic contaminants. operations exploiting the radiation from o Materials science, understanding the bending magnets of colliding-beam storage structural and electronic properties of a rings—the first generation. In the 1980s wide range of materials including cata- dedicated facilities—the second generation— lysts, polymers and semiconductor were constructed in China, France, Ger- materials. many, Japan, the UK, and the US. The 1990s o Medical diagnostics and therapy, utiliz- saw the construction in these and more ing X-ray beams well-defined in energy countries (Brazil, India, Italy, Korea, Russia, and direction to minimize risk and dam- Switzerland, Taiwan and Thailand). Several age to healthy tissue. of these were more advanced rings with brighter electron beams and many straight STORAGE RINGS sections between bending magnets for inser- Compared with conventional X-ray tubes, tion devices to make even brighter X rays— synchrotron radiation offers extremely high the third generation. brightness, tunable, pulsed, polarized radia- This evolution has been driven by a veri- tion. The most advanced sources provide table revolution brought about by the use of 13 orders of magnitude higher brightness (a synchrotron radiation in several fields of measure of the concentration of the radia- basic and applied research and a growing tion) than the best rotating anode X-ray user community, now numbering about tubes. Around the world there are now 20,000 scientists worldwide. The reason is about 50 storage ring sources in operation, the capability of synchrotron radiation, over about 10 more in construction, and about a broad spectral range extending from the 15 more in various stages of design await- infrared to ultraviolet and X rays, as an ing construction approval. The most power- experimental tool. It provides information, ful facilities are the 6–8 GeV hard X-ray often unavailable by any other means, rele- (5–50 keV) rings in Europe, Japan, and the vant to the structure of matter (that is, the US with circumferences of 850 meters to arrangements of atoms in complex materi- 1450 meters. Some of these facilities con- als) and the electronic and magnetic proper- duct 30–60 simultaneous experiments and ties of matter. Impact has been strong in serve 2000 or more users annually. Yet the fields such as: user community continues to grow, leading o Structural molecular biology, including to the construction of new rings and the the determination of the structure of development of ideas for future sources

BEAM LINE 3 with even higher performance. The articles existing large tunnels now being used for in this issue describe some of the most B-Factories at SLAC and KEK. The challenges important directions for these future to be faced in realizing such an ultimate sources. The first two articles describe stor- machine are described in the article by age rings. Pascal Elleaume. INTERMEDIATE ENERGY LIGHT SOURCES LINAC-BASED MACHINES The high performance and relatively moder- In storage rings, quantum emission of radia- ate cost of these 2.5–4 GeV machines make tion from the bending magnets reacts on the them the popular choice, with six now in electron beam to produce energy spread and construction and more proposed. This popu- growth in beam size and emittance (the prod- larity is largely due to the high performance uct of beam size and divergence). This emit- of these machines, resulting from recent tance increases quadratically with electron technological developments as described in energy and with the third power of the angle the article by Jeff Corbett and Tom Rabe- of bend in each bending magnet. To mini- deau. These include shimming of undulator mize these effects the highest performance magnets to near perfection, which increa- rings use short bending magnets with low ses high harmonic content, and the use of magnetic field (to reduce radiation emission in-vacuum small gap, short period undula- by bending the beam gently in an arc with a tors. These developments allow these rela- large radius of curvature) separated by many tively low energy rings to deliver hard X-ray quadrupoles to refocus the beam after its (5–50 keV) performance that is closer to that dispersion in the bending magnets. This of the larger (6–8 GeV) rings than was previ- leads to geometries with many magnetic ele- ously thought possible. When operational ments and large circumference as described these new rings, including a replacement of by Pascal Elleaume. the venerable SPEAR ring, will go a long These fundamental limits on electron way in meeting the continued growth in beam brightness and pulse length in storage user demand around the world. rings can be overcome in linac-based light sources. The ultimate in large bending THE ULTIMATE HARD X-RAY STORAGE radius is a linac. With no bending magnets RING-BASED LIGHT SOURCE the beam properties are determined by the Possible future 6–8 GeV rings with circum- electron source. Rather than increasing ference of about 2 km or more could pro- with electron energy, emittance in a linac vide X-ray brightness that significantly decreases linearly with electron energy. exceeds that of any present facility, opening However, the precision with which bright new research opportunities. Plans are being linac beams must be made, accelerated, and made to eventually use the PETRA electron used in undulators poses severe challenges. ring at DESY for this purpose and perhaps Several relatively recent developments have one of these will eventually be built in the dealt with these challenges, opening a path

4 SUMMER 2002 to the use of high energy linacs to provide undulators it is possible to produce sub- higher performance than any storage ring. picosecond X-ray pulses with nine orders of One of these is the high brightness electron magnitude higher peak brightness than the source, particularly the laser-driven radio- best present third generation storage rings. frequency gun initially developed at Los The Linac Coherent Light Source project at Alamos National Laboratory. Another is the SLAC is on a path to operate such a machine improved capability to accelerate, compress, in 2008. A similar project is proposed at and transport such bright electron beams, as DESY, to couple an X-ray laser to the pro- demonstrated in the SLC beams at SLAC. A posed TESLA linear collider project. Claudio third is the precision undulator, as developed Pellegrini and Joachim Stöhr describe these in many light source facilities. In combina- projects, the challenges to be met, the extra- tion these developments have opened two ordinary properties of the radiation, and the main directions for future linac-based light new science that is envisioned. sources as described in the article on the Much of this new science exploits the energy recovery linac by Sol Gruner and sub-picosecond pulses of the X-ray FEL and Donald Bilderback and the article on the X- ERL. Interest in this new time domain for ray free-electron laser by Claudio Pellegrini X-ray studies has stimulated other approach- and Joachim Stöhr. es to achieving short pulses, although with lower intensity than ERLs or FELS. A tech- ENERGY RECOVERY LINACS AS nique for extracting radiation from a SYNCHROTRON LIGHT SOURCES 100 femtosecond slice of a stored electron To bring operating costs to a reasonable level beam is being pursued at the Advanced Light the energy is recovered from the electrons Source at Lawrence Berkeley National after they are used—a novel idea ﬁrst pro- Laboratory and elsewhere. The Sub- posed 37 years ago and now attracting much Picosecond Photon Source (SPPS) project, interest. The principle has been tested at now in construction at SLAC, will produce a low energy at the Jefferson Laboratory similar short pulse of X rays by compressing where a recirculating linac is used to drive the electron beam in the SLAC linac and an infrared free-electron laser. Sol Gruner using spontaneous radiation from an undula- and Don Bilderback describe the challenges tor at the end of the linac. These sources and opportunities provided by a higher will enable researchers to begin to get expe- energy ERL, delivering X rays with higher rience with X-ray pulses at the 100 fem- brightness and shorter pulses than any stor- tosecond level well before an X-ray ERL or age ring. FEL is available.

X-RAY FREE-ELECTRON LASERS The dream of a fully coherent X-ray laser seems now to be in reach. With bright 15 GeV electron beams and 100 meter long

BEAM LINE 5 Energy Recovery Linacs as Synchrotron Light Sources

by SOL M. GRUNER & DONALD H. BILDERBACK

YNCHROTRON RADIATION sources have proven to be immensely important tools Pushing to higher throughout the sciences and engineering. In conse- quence, synchrotron radiation usage continues to light-source grow, assuring the need for additional synchrotron radiation machines well into the foreseeable future. Storage rings, the basis for all major existing hard performance will SX-ray synchrotron facilities, have inherent limitations that constrain the brightness, pulse length, and time structure of eventually require the X-ray beams. Since large synchrotron X-ray facilities are expensive and take many years to bring to fruition, it is using linacs rather important to consider if there are alternatives to storage rings that offer advantages. The Energy Recovery Linac (ERL) is a proposed alternative synchrotron radiation source that than storage rings, will extend synchrotron radiation science into new realms not possible with storage rings while still being able to serve much like colliding- most existing storage ring applications. The usage of synchrotron radiation has become more spe- beam storage rings cialized and sophisticated. Although no single source will meet every need, the trend is towards higher brilliance and intensity, and shorter bunches. These trends are driven by a are now giving way desire to study smaller and smaller samples that require more and more brilliance, because tiny samples tend to have to linear colliders. both low absolute scattering power and require micro X-ray beams. An increasing number of experiments also require transversely coherent X-ray beams. It can be shown that transverse coherence increases with the brilliance of the source. Another trend is towards examination of time- resolved phenomena in shorter and shorter time windows.

22 SUMMER 2002 The time resolution of many experiments is ultimately limited by the duration of the X-ray pulse. If the time envelope of the X-ray pulse is sufﬁciently short and the source is sufﬁ- ciently brilliant, it can be shown that an increasing number of X rays will also be longitudinally coherent, that is, in the same quantum mode. So the optical characteristics generally desired from synchrotron radiation sources are brilliance and short pulses. These desired characteristics immediately translate into requirements on the electron bunches. The average brilliance scales inversely to the bunch transverse emittances and linearly with the average current. Peak values scale inversely with the bunch length. Thus, the most desired characteristics of an synchrotron radiation source are a high average current of very short, very low emittance bunches. With this in mind, let’s look at the characteristics of storage rings and linacs as sources of bunches for synchrotron radiation.

LL EXISTING HARD X-RAY (that is, photon energies greater than about 5 keV) synchrotron radia- Ation sources rely on storage rings. Storage ring technology is well-understood and is near fundamental limits of minimum transverse emittances, intensity and bunch length (see other articles in this issue). The limits arise from the requirement that the particles be stored in stable equilibrium orbits, despite energy and momentum perturbations arising from the emission of synchrotron radiation, the radio- frequency acceleration system, intrabeam scattering (Touschek effect), and small errors in the magnetic bending

BEAM LINE 23 and focusing systems, or their align- lattice, namely, use the bunches to ment, leading to coupling between produce synchrotron radiation long the horizontal and vertical orbits. before equilibrium is attained. In this These perturbations, which are spe- case, the bunch properties, and there- ciﬁc to the given lattice of magnetic fore the quality of the synchrotron optical elements that comprise the radiation will be limited not by the storage ring, damp the initial bunch lattice, but by the electron source emittance, cross-sectional distribu- itself. Modern radio-frequency and tion, and bunch length to equilib- direct-current photocathode-based rium values in a time which is typ- injectors can produce bunches which ically milliseconds long, that is, are both short and have small emit- many thousands of revolutions tances in all dimensions. Moreover, around the ring. While the equilib- properly designed linacs can accel- rium damping behavior endows erate bunches with very little emit- storage rings with excellent sta- tance degradation. An obvious bility, it also decouples the critical approach is then to couple a suitable characteristics of stored bunches photoinjector with a high energy from those of the initially injected linac to produce a stream of bunches. That is to say, bunches bunches which can then be passed which are injected into the storage through an undulator to produce bril- ring with smaller emittances or liant synchrotron radiation. The lengths than characteristic of the lat- drawback to this idea is that signif- tice will equilibrate to the larger val- icant currents (~100 mA) and ener- ues typical of the storage ring. As gies (~ 3-8 GeV) are usually needed to examples, values for some state- obtain adequately intense synchro- of-the-art synchrotron radiation tron radiation with state-of-the-art facilities are given in the table below. undulators. These values represent The long damping times of stor- enormous beam powers; for exam- age rings suggest a way around ple, a continuous 100 mA, 5 GeV the equilibrium limitations of the beam (such as a hypothetical continuously operating linac might Basic Parameters deliver) would carry 500 MW of Some basic parameters of a few existing third generation storage rings compared to power, equivalent to the power out- a possible Energy Recovery Linac in high-current and low-current modes. put of a large commercial electrical generating station. It would be eco- Ave. Horizontal nomically unfeasible to consume Energy Current Emittance Bunch Length this much electrical power on an Machine (GeV) (mA) (nm-rad) (ps fwhm) on-going basis. ESRF 6 200 4 35 In a storage ring, the requirement of continuously supplying such a APS 4 100 4 73 high power is circumvented by stor- SPring 8 8 100 6 36 ing the high energy particles and replacing only the power lost to syn- ERL (low emittance) 5.3 100 0.15 0.3 chrotron radiation, which is typi- -4 ERL (very low emittance) 5.3 10 0.015 0.3 cally ~10 of the power in a continuous beam, such as the linac example

24 SUMMER 2002 described in the preceding paragraph. Therefore it is only necessary to provide a relatively small amount of radio-frequency power to replace synchrotron radiation losses. Of course, it is possible to couple a low duty-cycle photoinjector, with con- comitant low average current, to a high energy linac, because the average power scales with the average current. This is exactly what will be done at the X-ray Free Electron Laser (XFEL) sources now TBA (optical unit) X rays being contemplated (see Claudio Pellegrini & Joachim Stöhr in this Main Linac issue). 5–7 GeV ET THERE IS A WAY to use high duty-cycle, high Yenergy linacs without con- Injector Dump suming large amounts of power: A schematic layout of an ERL synchrotron light source. 10 MeV electrons made in the Superconducting linacs may be injector are bent by a weak bending magnet into a few hundred meter long supercon- efficiently used as both linear accel- ducting linac and brought to full energy in a single pass. The electrons are then erators and decelerators–an idea sug- guided around a one-turn arc made, for example, of triple bend achromat (TBA) magnets with undulators producing the X rays. The electrons have a path length such that gested by Maury Tigner in 1965. they return out of phase with the linac and their energy is recovered before being When used as an accelerator, the steered to the dump at an energy of about 10 MeV by another weak field magnet. electron bunch gains energy at the The low energy at which the electrons are dumped greatly reduces the problem of expense of an electromagnetic (EM) residual radioactivity generated in the dump. field resonant in the linac; when used as a decelerator, the EM field range of 1010. This requires the use gains energy from the bunch. The of a superconducting (SC) linac to cir- distinction between the linear accel- cumvent the wall losses of normal erator and decelerator is simply one conducting linacs, such as the SLAC of bunch position relative to the linac. phase of the resonant traveling EM The figure shows a schematic lay- wave. For relativistic bunches, out of such an Energy Recovery Linac which, by definition always have (ERL) and illustrates how it can be speeds close to the speed of light, this used as a superior synchrotron radi- distinction is simply one of carefully ation source. A brilliant electron timing the arrival of the bunch at the injector is used to obtain relativistic entrance of a linac relative to the (say, 10 MeV) bunches with very low phase of the radio-frequency wave. transverse and longitudinal emit- In order to efficiently store the tances. These are then accelerated in energy recovered from the bunch, an SC linac to energies in the range it is necessary that the resonant Q of of 3–8 GeV. The resultant bunches the linac be very high, usually in the are then guided around a transport

BEAM LINE 25 loop, very much like a section of a The most convincing demonstra- third generation ring, consisting of a tion of ERL technology, however, has lattice of electron-optical elements been the commissioning in the last interspersed with undulators to few years of a 48 MeV, 5 mA ERL at produce synchrotron radiation. The Jefferson Lab as a driver for an loop length is carefully adjusted so IR Free Electron Laser (see figure on the bunches arrive back at the linac facing page). This user facility pro- 180 degrees out of accelerating phase. duced >1 kW of IR power on a routine They then decelerate through the basis. It’s energy recovery efficiency linac and emerge with the original is better than 99.97 per cent. (It injection energy, less synchrotron may be a good deal better. The radiation losses. These low energy actual efficiency has never been mea- bunches are bent aside by a weak sured to higher accuracy.) dipole magnet and sent to a beam ERLs have tremendous potential dump. Disposing of the electron as synchrotron radiation sources. As beam at very low energy results opposed to storage rings, the essen- in much less induced radioactivity tial bunch characteristics are deter- in the beam dump–an important mined primarily by the injector, not advantage of energy recovery. The by the entire ring. This greatly eases essential distinction between an ERL the task of upgrading the facility as and a storage ring, then, is that the injectors improve. Since bunches are electrons are recycled in a storage not stored, there is no decay of a fill. ring whereas only the electron Complex bunch trains can be read- energy is recycled in an ERL. This ily implemented by programming the allows bunch manipulations in an timing of the laser illuminating ERL which would be unfeasible in a the photocathode electron source. storage ring. Another major advantage is the abil- But can energy recovery be made ity to control the cross-sectional to work on a practical basis? A pio- properties of the bunch. In storage neering experiment was done in the rings, the bunches tend to be flat in mid-1970s with a SC linac at the cross section and have a vertical Illinois superconducting microtron. emittance which is orders of mag- Experiments at other labs followed, nitude smaller than the horizontal in concert with increasingly sophis- emittance. By contrast, the beams in ticated superconducting linacs and an ERL are naturally round with superconducting radio-frequency equal emittances in the horizontal structures developed, for example, at and the vertical, which can be very Cornell and Stanford. The CESR stor- advantageous for certain X-ray optics. age ring at Cornell, the CEBAF As discussed below, ERL sources are machine at the Thomas Jefferson being contemplated with flux and National Accelerator Facility (Jef- brilliance exceeding all existing stor- ferson Laboratory), and LEP at CERN, age rings. all of which are driven by SC radio- ERLs also lend themselves to the frequency structures, have proven production of very short X-ray the practicality of SC accelerating pulses. Studies of phenomena on structures for large machines. the 100 femtosecond time-scale are

26 SUMMER 2002 Injector IR Wiggler

Linac Optical System Dump

e– Recirculation Beam Line

Jefferson Laboratory IR-FEL. The Jefferson Laboratory IR-FEL is also a single-pass machine. Electrons from the injector are brought up to energy in the linac and then pass through a wiggler magnet to create the Infrared Radiation (IR). Mirrors at the ends of the wiggler magnet intensify the IR beam. The electrons circulate around the rest of the lattice before their energy is recovered in another pass through the linac, after which they are sent to the dump. (Source: Jefferson Laboratory web site) rapidly emerging as one of the next system driving the photocathode. frontiers of X-ray science. Our For example, the minimum emit- understanding of the initial events tance of a photoinjector is limited by in chemical and solid-state systems space charge effects near the pho- has been revolutionized within the tocathode electron source. The emit- last decade by spectroscopies with tance of the photoinjector can be de- femtosecond optical lasers. What is creased by sacriﬁcing photocurrent. lacking is the associated structural Thus one can envision operating an information. This may be obtained ERL in a high-flux, relatively low by pump-probe experiments in which brilliance mode or a lower-flux, systems are pumped with an opti- higher brilliance mode simply by cal laser and then probed, after an reprogramming the laser illuminat- adjustable time delay, with a very ing the photocathode. (As indicat- short X-ray pulse. Typical storage ring ed in the next two figures, in both bunches are some 20–100 picoseconds modes, the flux and brilliance can long, which severely limits experi- still be extraordinary). As another ments on faster time scales. By con- example, the repetition rate of trast, the bunches out of an ERL pho- pump-probe experiments with fem- toinjector may be only a few tosecond lasers is frequently lim- picoseconds long. Moreover, ERLs ited to low values (say 10 kHz) by the allow phase-space manipulations laser. In these cases it is desirable to with bunch compressors to create have a very short, high charge (for very short bunches. As an example, peak intensity) bunch at 10 kHz. One the CEBAF machine, which is based can imagine operating a general pur- on a multi-pass superconducting pose ERL with small charge bun- linac, has achieved bunches shorter ches at very high repetition rates (for than 100 fs rms. example, GHz) interspersed with Many novel possibilities are made high-charge bunches at 10 kHz. A feasible by the ability to adjust the “buffer zone” of a micro-second with charge and timing of bunches with no bunches on either side of the large appropriate manipulation of the laser bunch would allow isolation of the

BEAM LINE 27 X-ray pulse from the large bunches 23 0.015 nm at the appropriate experimental sta- 10 LCLS SASE 10 mA tion. Most other users with time- integration windows exceeding a few 0.15 nm 100 mA milliseconds would be insensitive to this bunch pattern. 25 m LTHOUGH consideration 1021 APS Upgrade of ERLs as synchrotron ) 2 5 m sources has been going on ESRF U35 A for only a few years, they have /mrad 2 already generated a great deal of excitement in the synchrotron radiation community. Nikolai Vinokurov 1019 2.4 m and Gennady Kulipanov and col- leagues at the Budker Institute of Nuclear Physics in Novosibirsk had suggested a specialized ERL machine in the mid-1990s. Geoff Krafft and David Douglas at Jefferson Labora- 1017 tory and Maury Tigner at Cornell, LCLS Spont. and perhaps others as well, were also thinking about the possibilities of more general purpose ERL synchro- Average Brilliance (photons/sec/0.1%/mm tron radiation sources. By late 2000, ERL synchrotron radiation projects 49 p Wiggler were being seriously considered 1015 at Cornell University (in collabo- ration with scientists at Jefferson 24 p Wiggler ERL 25m Laboratory), at Brookhaven Nation- al Laboratory (BNL) and at SP8 Lawrence Berkeley National Lab- APS oratory (LBNL). The focus of the 1013 CHESS Cornell effort is on a general purpose synchrotron radiation source; the BNL efforts are on ERLs for both 5 10 20 50 100 Photon Energy (keV) synchrotron radiation and heavy ion physics applications; and the Expected average brilliance (above) and coherent flux (facing page) for an ERL based on the Cornell/Jefferson Laboratory design described in http://erl.chess. machine being considered at LBNL cornell.edu/Papers/ERL_Study.pdf. The ERL may be operated either in a high-flux is specialized for the generation of mode (100 mA, 0.15 nm emittance), or, by reducing the photocathode current to short X-ray pulses at low duty cycle. decrease the photoinjector emittance, in a lower flux mode with very high brilliance Interested readers may find articles (10 mA, 0.015 nm emittance). on all three efforts in the March and May 2001 issues of Synchrotron Radiation News. The Cornell web site (http://erl.chess.cornell.edu) has

28 SUMMER 2002 descriptions of the Cornell/Jeffer- son Laboratory work, as well as 1015 LCLS SASE links to other ERL web sites. The projections given below are based on the ERL being designed by 0.015 nm 10 mA Cornell and Jefferson Laboratory staff. APS Upgrade The figures in these two pages 0.15 nm 100 mA show the average brilliance and coherent flux vs. X-ray energy for various machines and for a 5.3 GeV 1013 ERL for reasonable assumptions 25 m of photoinjector emittances and undulators. The curves show the expected ERL performance under U35 two modes of operation: A high current, “low-emittance” (100 mA; 5 m 0.15 nm-rad) mode and a lower current, even lower-emittance (10 mA; 1011 0.015 nm-rad) mode. It is assumed Coherent Flux (photons/sec/0.1%) that the SC linac operates at 1.3 GHz with every radio-frequency bucket 2.4 m filled, which corresponds to ERL 25m bunches of 77 and 8 pC, respec- tively. The advantage of high rep- SP8 etition rate operation is that a rea- APS sonable average current can be ESRF achieved with low-charge bunches, 109 which is desirable since space charge is the primary mechanism for emittance growth at the photo- 5 10 20 50 cathode. These bunches are con- Photon Energy (keV) siderably smaller than the ~1 nC performance levels. These issues concluded that the requisite perfor- bunches required by XFELs. For were explored in a workshop at mance levels can be achieved by most users, the ERL will seem like Cornell University in August 2000. straightforward improvements of a continuous source of X rays. No ERL has ever been constructed to existing technology and that no new Larger bunches at lower duty operate at these simultaneous val- break-throughs or radically new cycle—and with lower brilliance— ues of energy, average current and technology will be required. How- are likely to be possible for special emittance. Likewise, a photoinjec- ever, there are many issues—beam applications. The great advantage of tor with the requisite speciﬁcations stability, halo control, the negative an ERL is that beams for these spe- has never been built. The shortest effects of coherent synchrotron cial applications can be quickly pulses will present special chal- radiation, photoinjector design, implemented by simply repro- lenges, as will the X-ray optics and photocathode lifetime. which gramming the photoinjector. required to realize the full poten- will require prototyping before an There are numerous chal- tial of the synchrotron radiation optimum large ERL facility can be lenges to building an ERL to these beams. Signiﬁcantly, the workshop confidently designed. Fortunately,

BEAM LINE 29 almost all of these questions can be sources are directly transferable to addressed with a small, low energy the ERL. For these users, the design (for example, 100 MeV) ERL proto- of the beam lines, and the way in type. Cornell University has pro- which the experiments are carried posed building a small prototype to out, will be very similar to those at address such issues for the commu- storage ring sources. The extra few nity at large. orders of magnitude improvement in beam characteristics will, however, RLS AND XFELS are both have signiﬁcant consequences for the based on linac technology, experiments that can be performed. Eboth are limited by photoin- The more brilliant beams will allow jectors, both will optimally utilize smaller and weaker scattering sam- long undulators and both will deliver ples to be examined. For instance, the short bunches. How do these two practical limits of photon correlation synchrotron radiation sources differ? spectroscopy experiments will be From a physics point of view, the pushed into faster time regimes. biggest difference is that the Inelastic scattering experiments may well be extended to the sub-meV regimes, thereby greatly expanding The essential distinction between an ERL the kinds of experiments that can be done. The round source size will and a storage ring is that the electrons are extend the limits of what can be done with microbeams. With improved X-ray optics, ERLs may recycled in a storage ring whereas the electron make feasible 10 to 30 nm diameter hard X-ray beams. Thus, ERLs auto- energy is recycled in an ERL. matically have a huge natural con- stituency of advocates, namely, almost all users of existing storage self-ampliﬁed spontaneous emission rings when these users begin to (SASE) process to generate synchro- demand more and better synchrotron tron radiation is fundamentally dif- radiation resources. Of course there ferent from the spontaneous syn- will also be applications that take chrotron radiation process so far advantage of the distinctive charac- considered for the ERL. In order teristics of ERL synchrotron radiation to achieve SASE in the hard X ray, beams, such as short pulses, as dis- higher energy linacs, bunches with cussed earlier. higher peak current and far longer By contrast, XFEL beams will be undulators will be required. so intense that single-pulse specimen From a user point of view, how- and X-ray optics damage become ever, ERLs of the Cornell or BNL over-riding issues. As opposed to the designs and an XFEL of the LCLS or ERL, few experiments can be trans- TESLA designs couldn’t be more dif- ferred from storage rings to the XFEL ferent. The first difference is that without big changes in the way almost all the applications now in which the experiments are being performed at storage rings performed. The community will

30 SUMMER 2002 have to develop new ways of doing bigger and better storage rings; practically every kind of syn- now the ultimate storage ring is in chrotron radiation experiment. The sight (see article by Pascal Elleaume peak intensity of the XFEL pulses in this issue). ERLs and XFELs are is so high for many experiments that emerging as alternative synchrotron a new sample, or a new illuminated radiation sources with interestingly area of sample, will be required different properties. The first three for each pulse. Yet the number of generation sources were primarily photons available per pulse (~1012 incremental improvements of a sin- for a single sub-ps pulse from the gle technology, namely storage rings. SLAC LCLS, about equivalent to The future of next generation sources 0.1 to 1.0 seconds of monochro- is likely to be much richer than in matic beam at the APS) will limit the the past. ERLs and XFEL are both information available from a more appropriately termed alterna- given sample. Thus, the empha- tive technology synchrotron radia- sis at an XFEL will shift to devel- tion sources, since they utilize oping techniques to average and linacs rather than storage rings. merge data from different samples. ERLs, XFELs, and, no doubt, syn- The extreme properties of the X-ray chrotron radiation machines not yet beams (for example, enormous invented, are the creative fruits of an peak electric ﬁelds, full transverse emerging new generation of acceler- coherence) will enable unique areas ator physicists. Culturally, accelera- of investigation, which is very tor physics has been the sibling of exciting. But at the same time, it is high energy physics activity, because safe to predict that the learning curve new high energy physics experiments in utilization of XFELs will be long. required new accelerators. Yet accel- One of the most interesting cultural erators are but one tool towards the changes in synchrotron radiation end of understanding the funda- usage in recent years is the grow- mental constituents of Nature. The ing number of users who are not trends in recent years are leading sci- X-ray experts. The use of XFELs by ence in new directions. High energy these communities will likely come physicists are increasingly looking years after XFELs ﬁrst turn on. to the skies to understand the fun- Finally, we can’t resist pointing damental constituents. And accel- out that a high duty cycle XFEL will erator physicists are increasingly consume an enormous amount of finding other science areas chal- electrical energy, unless, of course, lenging and attractive. In a very real it is also an ERL. The potential ben- sense, both ERLs and XFELs are con- efits of merging ERL and FEL tech- sequences of a maturation of accel- nology, already demonstrated in the erator physics designed specifically Jefferson Lab IRFEL, are likely to from the very start to expand what become compelling. can be done with synchrotron radiation. We look forward to the next OR MORE THAN 30 years, decade when ERLs and XFELs will the focus of the synchrotron take the frontiers of synchrotron Fradiation community has been radiation to new vistas.

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