SYNCHROTRON RADIATION AT THE CAMBRIDGE ELECTRON ACCELERATOR"Herman Winick

Cambridge Electron AcceleratorHarvard University and Massachusetts Institute of Technology

Cambridge, Mass.

Introduction

The Cambridge Electron Accelerator (CEA)has operated from 1962 to 1970 as an electronsynchrotron, accelerating 20 mA of electronsto 6 GeV at 60 Hz and producing externalbremsstrahlung and electron beams for a broadhigh energy physics program. It has now beenconverted to an e+e storage ring collidingbeam machinel and has recently completed anexperiment2 at Ecm = 2 + 2 GeV. A final ex-periment at Ecm = 2.5 + 2.5 GeV is in progress,after which the colliding beam physics programwill be terminated because higher luminosityis available from the storage ring SPEAR.

In parallel with the high energy physicsprogram, a parasitic program using synchrotronradiation has been pursued. A special roomhas been constructed by Harvard University,and a fully-instrumented beam run was com-pleted in April 1972. The beam run was de-signed initially to meet the requirements of aunique scanning x-ray microscope 3 which Is nowin routine operation. Five experimentalgroups share the three ports on this run andmany more proposals have been received inanticipation of an expanded facility.

The CEA is a most potent source of syn-chrotron radiation. Large stored electroncurrents (55 mA has been achieved and 100 mAis expected) and high stored beam energy (3.5GeV has been achieved and 5.0 GeV is possiblewith minor improvements) produce a large fluxof ultra-violet and x-radiation.

A proposal to operate the CEA as aNational Laboratory dedicated to the use ofsynchrotron radiation for research in physics,chemistry, biology, and medical diagnostics,is now under consideration by the NSF. Thisproposal projects the installation of many ad-ditional beam runs and "wiggler" magnets Inthe target area, a 35-ft x 130-ft fully-equipped experimental hall. Flux densities onthe experimenters' target are high becausebeam runs are short (8 ft to 30 ft). Shield-ing will be installed to permit safe occupancynear experimental equipment during storageconditions and very likely during injection.

This paper discusses the basic featuresof synchrotron radiation (with particularreference to the CEA), its enhancement by theuse of wiggler magnets, the present perform-ance and future capability of the CEA instorage and cycling modes of operation,special pulsing and modulation techniques,the features of the present beam run, andplans for future beam runs and shielding topermit occupancy of the target area.

Features of Synchrotron RadiationAn electron of energy E = ymec2 moving

with a radius of curvature R in a magneticfield B, emits synchrotron radiation. Themean angular spread of high-energy photonemission with respect to the electron direc-tion is X 1l/y. The power radiated by oneelectron with energy E, travelling with v t cperpendicular to a magnetic field B, is: (all

equations in MKS units unless otherwise stated)

p2 re2c 3

3 mec 21 e2

e 4TrE0 mec2 (1)

Multiplying equation (1) by R/c, where R=E/eBcgives the energy loss per radian of arc as

AE/radian = 2 r ecBy33

For a current ia = e dN/dt, the power radiatedper radian is dN/dt x AE/radian:

2 3P/radian = T reciaBy3

=B FE3 iA kW

o 24kG) GeVJ radian

(2)

This energy is radiated as a continuum (seeFig. 1) characterized by the critical energy

3 e2y 3 3 e3c By2c 2 aR 2 amec2

= 0.0670( |B keV (3)

where a is the fine structure constant e2/tc.The number of photons emitted per second,

per IO% energy bandwidth, per milliradian ofangle in the plane of the orbit, integratedover the narrow range of angles out of thehorizontal plane, is given by:

= 5.28 x 10 " 9(

where g(c/c.) is a function which has beentabulated and plotted.4 It has a broad maxi-mum with a value of X 0.4 at c/ec z 0.4 anddecreases as (c/Ec) VI below that energy.

Photon Energy (KeV) -

Spectral photon distribution of synchrotron radiation from CEA. Curves (ID®) ®) indicatopresent capabilities with existing damping mognets. Curoes (3)) show capabilities that wIllresult when a new wiggler magnet is built and installed. Curves (®)®) show copabliltiesthat wi li resunt when a 50 CeV beam Is stored.

Figure 1.

Because of the very small natural emis-sion angle of the photons, the horizontalangular divergence of a synchrotron radiationbeam is usually fixed by the arc subtended bythe defining slit. The vertical angular widthis usual ly determined by the vertical angular

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divergence of the electron beam itself and/orthe natural emission angle. The "source size"is given by the projected size of the electronbeam where the radiation originates.

The radiation is linearly polarized withits electric vector in the orbital plane forphotons emitted in this plane and slightly el-liptically polarized for photons leaving theplane.

Wiggler Magnets

As experimenters require shorter wave-length and higher-intensity synchrotron radi-ation beams the use of local regions of inde-pendently controlled high magnetic fields togenerate the photon beams becomes highly de-sirable. The local magnetic field, Bw, canbe made much larger than the ring magneticfield, Ba, and n such local regions or sec-tors, alternating in polarity (so as to pro-duce no net displacement or deviation), eachof length lw, can be put in series to form awiggler as sketched in Fig. 2 where the side-wise deflection is exaggerated for clarity.

WAOW= fW/RW -B1WTW/Y

length in units of tw

Electron path through a wiggler magnet

Figure 2.

How does a photon beam produced in such awiggler compare to that produced in a standardring magnet? From equations (2) and (3) inthe previous section we obtain the following:

(1) The y of the electrons remains, of course,determined by the ring magnetic field Ba;(2) The critical energy (Sc% By2) is increasedthrough the use of Bw by a factor Bw/Ba;(3) The total power radiated per milltradian(P% iaBy3 per sector) is increased through theuse of n sectors of Bw by a factor nBw/Ba.

In addition to its enhancement of thespectrum of synchrotron radiation, a wigglermay be used to produce beams with specialemittance characteristics. For instance, thevery small vertical angle of emittance (,, l/y)of synchrotron radiation can be increased bya factor of X 10 or more by a wiggler withhorizontal magnetic field. Independently con-trolled vertical and horizontal fields, alter-nating in the same wiggler, can be used toilluminate flexibly a rectangular area withdimensions independently variable from r 0.2in.to X 10 in. and located say 100 ft from thesource. This may be particularly useful formedical purposes.

Since wigglers may be designed to produceno significant disturbance to the beam (otherthan the extraction of energy) several wig-glers may be used in the same ring, each pro-viding independent control over the spectraland emittance properties of the synchrotronradiation beam for a particular user.

Thus an ideal synchrotron radiation facil-ity would employ such wigglers to generate In-tense photon beams at specific useful loca-tions, while minimizing rf and magnet powerrequirements in the ring itself. The power

that goes into synchrotron radiation from thering magnets varies as iay4/R2, and the ringmagnet power requirement varies as Y2/Ra.Since large values of ia and y also maximizewiggler-produced synchrotron radiation, theonly parameter which can be adjusted to reducethe ring magnet and ring rf power requirementsis Ra. Clearly the largest value of Ra mini-mizes the power requirement. Thus, although acompact installation of small Ra possessesobvious construction cost advantages, it hasrelatively higher power requirements, and anincreased RQ may reduce the overall cost. Inaddition, minimization of the synchrotron radi-ation produced, other than at chosen locationsfor wiggler magnets, reduces outgassing ef-fects and helps to keep a low gas pressure anda long beam lifetime.

Because of the substantial benefits ofwigglers, as described above, we plan to makeimmediate use of operating wiggler magnetswhile designing magnets of even stronger fieldand larger number of poles. The existing wig-gler magnets are the CEA damping magnets withn = 4, Bw = 7 kG and 1w = 3 hin. These mag-nets are used to redistribute synchrotron radi-ation damping among the three modes of oscil-lation (synchrotron oscillations, horizontaland vertical betatron oscillations) such thatall modes are damped (in the normal alternat-ing gradient structure of the CEA the horizon-tal betatron oscillations are radiation anti-damped). Since the damping strength is deter-mined by the product B x aB/ar (whereas wig-gler strength is determined by Bw and thenumber of poles, n), damping magnets have non-uniform fields, which introduce changes in Vvalues and chromaticity. If not compensatedthis can result in resonances, beam size en-largement, and reduced lifetime. Compensationis now provided by special quadrupole and sex-tupole magnets, with the result that the damp-ing magnets are in routine use and cause noproblems.

As is shown in Fig. 1, the photon beamfrom the damping magnets has a critical energygreater than that of the photon beam from thering magnets by a factor of 7 kG / 4.4 kG =1.58, and a radiated power/mrad greater by afactor of 4 x 7/ 4.4 = 6.32.

By comparison, a new design of wigglermagnet (not needed for damping), in which thefield is designed to be as high and uniformas possible, is a simple device and the in-sertion of these into the CEA lattice isstraightforward. Because the bending radiusof the CEA is large (86 ft), the guide fieldis low (4.4 kG at 3.5 GeV) and the enhancementprovided by a wiggler is particularly large.

The mechanical tolerances to which a wig-gler must be built, in order that orbit dis-tortions, changes in tune and chromaticity,etc. are negligible or easily correctable,have been evaluated5 and are readily met. De-signs have been made5 for a powered wigglerwith n = 8, Bw = 18 kG, lw = 1.5 in., and alsoa novel permanent magnet wiggler with n = 8,Bw = 9 kG, 1w = 1.5 in. Both have a gap heightof 0.8 in.

Mode of Operation - Capability and Performance

At present CEA can operate in two modes:

(1) Storage Mode: 100 mA of electrons at ener-gies up to 3.5 GeV with lifetime of 1 hour or

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more. Thus far the maximum current stored hasbeen 55 mA, filling 55% of the orbit, at 2.65GeV (limited by present restrictions due tocolliding beam requirements).(2) Cycling Mode: 20 mA of electrons withenergies up to 5.5 GeV with 60 Hz repetitionrate (has been achieved).

See Fig. 1 for spectral distributions.The main parameters of storage and cyclingmodes are given in Tables 1 and 2.

TABLE I

Beam Parameters in Storaqe Mode

Energy 1.0 - 3.5 GeV*Current

Rf LimittInstability LimitAchieved

Li fet imeBeam Size at 3.5 GeV

Location

"Odd" Junction"Even" JunctionStraight Section

X' 300 mA at 3.5 GeV> 100 mA55 mA with 55% orbIt fill> 1 hour

(full width at l/e height)Vertical Horizontal0.06 mm 5.0 mm0.14 2.10.10 3.5

Critical Energy at 3.5 GeV

Source

Ring Magnets (4.43 kG)Damping Magnet (7.0 kG)Proposed Wiggler Magnet (18.6 kG)

3.6 keV5.7 keV

15.1 keV*All systems except the damping system arecapable of operating in storage mode at ener-gies up to 5.0 GeV. Additional damping mag-nets are required for 5.0-GeV operation.

tRf limits are calculated for our present trans-mitter (210 kW) and 16 rf cavities (Rs= 108 Q).The rf current limit in storage at 5.0 GeV isX 30 mA.

TABLE 2

Beam Parameters in Cycling Mode

Peak Energy 1.0 - 5.5 GeV

Minimum Energy 120 - 280 MeV

Current < 5.0 GeV 5.5 GeV

Rf Limit > 200 mA X 30 mAInstability Limit > 100 mA > 100 mAAchieved 30 mA 20 mA

Peak Critical Energy at 5.5 GeV Operation

Source

Ring Magnets (7.0 kG) 14.0 keVDamping Magnets (12.0 kG) 24.0 keVProposed Wiggler Magnet (18.6 kG) 37.2 keV

We expect that the main demand of usersof synchrotron radiation will be for operationin storage mode. In this mode the synchrotronradiation has constant critical energy and itsintensity decays smoothly with a lifetime of> 1 hour. In the cycling mode both criticalenergy and photon intensity vary continuouslyduring each period of 1/60 second. Additionalintensity variations are minimized by keepingalmost all of the beam throughout the accel-eration and deceleration portions of the cycleand maintaining a fairly constant average cur-rent with multicycle injection. The only ad-vantage of cycling mode is the e istence of aband of very high energy x-rays (Fig. 1).

Storage Mode

The CEA has a unique system of beam stor-age. See Fig. 3 for the operation cycle.

e- MulticycleInjection

- 30sec.

Transition to D.C.

Adjust tofinal energy

10sec.

Experiment

-I hour

/-I /-i / -.Time

Operation cycle

Figure 3.

While the ring magnets cycle at 60 Hzbetween field values corresponding to 240 MeVand 2.1 GeV, 260-MeV electrons are injected atthe appropriate times. Radiation damping atthe top of the cycle reduces the phase spaceof the radial betatron motion and permits off-axis injection of additional current at thenext minimum. The electrons originate in a5-stage Varian linac. When the desired circu-lating beam intensity is reached (the maximumvalue is determined by single-bunch phase in-stability), the ac component of the magneticfield is turned off slowly in such a way thatthe peak energy remains at approximately 2.1GeV. The dc field is then slowly raised (orlowered) to the desired final value. Duringthis whole cycle, currents in the damping mag-nets and sextupole coils are programmed to in-sure stability of the electron current. Theentire process of filling and changing themagnet excitation from ac to dc at the desiredlevel takes X, 1 minute. The multicycle injec-tion scheme, by adding up many linac pulses,results in a uniform, reliable filling of thering, largely independent of the linac outputpulse ampl itude.

The position of the stored electron beamis exceedingly stable. In an alternatinggradient machine such as the CEA, the beamposition is dependent only on the frequency ofthe accelerating voltage which is stable to 1part in 108, giving a theoretical radial posi-tion stability of < 10-4 cm. In confirmationof this, the users of the CEA synchrotron radi-ation have not detected any shifts in beamshape or position.

Cycling Mode

This mode differs from the conventionalsynchrotron operation only in that the beam isno longer extracted or steered onto a targetat the peak energy. Instead, electrons remainin orbit indefinitely, their energy varyingsinusoidally from 240 MeV to the top energy,which can be varied from 1 to 5.5 GeV.

The synchrotron radiation spectrum thusvaries at 60 Hz. However, the electron beamspends 20% of its time in a magnetic fieldB > 0.9 Bmax. The beam size (and thus the syn-chrotron radiation source size) varies through-out the cycle.

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Wiggler magnets can be used just as wellin the cycling mode as in the storage mode bypulsing the wiggler at the top of the cycle orby moving the beam at the bottom of the cycleso that the high field of the wiggler magnetdoes not perturb the injection process and thelow energy beam.

Special Pulsing and Modulation TechniquesBecause of the unusually high frequency

(475 MHz) of the accelerating voltage of theCEA, the rf bunch structure is quite sharp.Thus the synchrotron radiation is produced inbursts, 0.1 ns or less in duration, separatedby 2 ns. Any consecutive number of thesebunches from 1 to 360 may now be filled andto first order the total beam intensity isproportional to the number of bunches filled.Installing a gridded gun in the linac injec-tion will make it possible to fill any or allof these bunches in any pattern desired by anexperimenter. Theorbitalperiodis760nsec.

Also many sets of high voltage electro-static plates exist inside the CEA vacuum sys-tem. By connecting pairs of these plates,separated by 180° in vertical betatron phaseangle, to existing high voltage pulsers, in-dividual synchrotron radiation beams can bedirected towards or away from collimator open-ings. Thus for example, a particular usercould direct synchrotron radiation into hisapparatus for say 50 ns with a repetition rateof say 10 Hz, while other users select a dif-ferent program or steady radiation.

Finally, a fast-ejection system is in-stalled in the CEA that can be used to deflectthe beam into an external channel within oneorbital period. This external beam could bepassed through a long superconducting wigglerwhich could extract much of the total beamenergy (0 150 J) in an intense burst of syn-chrotron radiation lasting from 0.1 to 600 nsdepending on how many rf bunches were filled.

Beam Runs

Present Beam Run

The existing beam run is shown in Fig. 4.Planning for this run began in 1968 with theexpectation that experiments would be para-sitic on colliding beam experiments. Hencethe run is long (80 ft) and leads into a sepa-rate room, well-shielded from the ring tunnel.

0 20FT 32SP .0T BOFT

LEAD S mELOINO FlED

-ST-CUS WL- - N OLE

_s NBr III-OFgr 4

MANUALA)

LV MM) ALVO(

NO0LISEB,SEE S300LINEBVSEE SA1LITEMISEL ION PUYPB103 PUMAP ION PUMP lON POMP

EXISTING BEAMl RUN STh

Figure ~4.

This run is a fully-instrumented, interlocked,bakeable, high-vacuum run complete with iongauges, pumps, a shutter, an automatic valve(interlocked to close if a pressure rise isdetected by gauges or pumps), leak-detectorand fore pump connections, a remotely-movablefocusing mirror, a flip-up mirror, and a beam

splitter mirror with a central hole. Thus therun can accommodate 3 sets of experimental ap-paratus (one x-ray and two UV) two of whichcan operate simultaneously.

The focusing mirror is a totally exter-nally-reflecting ellipsoidal quartz mirror3which collects radiation from about 10 inchesof curved orbit of a ring-bending magnet (orabout 10 mrad), reflects it upwards by 10, andfocuses it to a spot about 80 ft from thesource and 7 in. above the median plane. Thisdisplacement from the median plane permits theinstallation of lead shielding to block highenergy radiation from the machine and makesoccupancy in the experimental area safe duringall phases of synchrotron operation.

The size of the focused spot is about thesame as that of the circulating beam (< 1 mmhigh, 2 - 5 mm wide) thus producing about a500-fold increase in x-ray flux density overan unfocused beam. The resulting x-ray beamis probably the most intense soft x-ray beamever made. It is limited to energies of ~ 4keV because reflection from the focusing mir-ror at 1/2° grazing angle of incidence fallsrapidly at higher energies. At 3.5 GeV with100 mA stored, this beam would contain X 1016UV and soft x-radiation photons/second.

The beam is readily visible as blue airfluorescence (Fig. 5) after emerging through a0.001" Be window even at lower stored beam cur-rent and energy.

Figure 5

Visible Air Fluorescence causedby intense x-ray beam emergingfrom .001-inch Be window.

Additional Beam Runs

We plan to install up to 15 beam runs inan enlarged section of our ring tunnel (35 ftwide, 150 ft long, 12 ft high) part of whichis shown in Fig. 6.

POCUSNGMlREO

WIGGLER

BEAM S

TYPICAL LAYOUT OF FUTUIFE ADJACENT DEAM RUNS

Figure 6.5 f*

987

I --1. .1 LEADI,---

E ----E

This section of the ring tunnel is already afully-equipped experimental hall complete withoverhead crane, and electrical, vacuum, water,and compressed-air services.

If these new runs are kept short, thecomplexity and cost of the beam run hardwareare small, and if the experiment uses photonenergies so high (greater than X 5 keV) thatfocusing is difficult or impossible, the gainin radiant flux per square millimeter of userapparatus is large. Our standard runs will be18 ft to 30 ft from the point of origin of theradiation to the user apparatus, with specialruns being as short as 8 ft.

If the apparatus is close-in to the ac-celerator, it is important that the experi-menters be allowed to work at these close-inlocations, under storage conditions, to elimi-nate the need for remote controls.

Radiation measurements at the CEA6 showthat a 4-in-thick circumferential lead shieldplaced in the median plane attenuates radia-tion doses by about a factor of 1000, thuspermitting safe occupancy under stored beamconditions even in the unlikely event of a"worst-case" accident, i.e. the local abruptloss of 100 mA of electrons stored at 3.5 GeV.With such a shield (Fig. 7) the highest doselevel in the above "worst-case" accident wouldbe X 375 mrem (sum of neutron plus beta-gammadoses) over X 20 square inches of a person'sbody and the average whole-body dose would beone to two orders of magnitude lower. Obser-vations made during normal injection and fill-ing of the storage ring showed that dose ratesof X 50 mr/hr were present. Thus it should bepossible to allow occupancy even during brief(% 30-second) controlled injection periods,since injection occurs only about once an hour.

Figure 7. Cross-Section of Ring MagnetShowing Placement of Lead Shield Curtain.

Acknow ledgemen ts

It is a pleasure to acknowledge the con-tributions of William Paul and Karl Strauch inwriting the proposal to the NSF which is thebasis for this paper.

Many members of the CEA staff contributedto the work described in this paper, particu-larly R. J. Averill, J. Hagopian, A. Hansen,R. D. Hay, A. Hofmann, B. J. Maddox, H. Mieras,G. Nicholls, and W. A. Shurcliff.

Paul Horowitz and John Howell are largelyresponsible for the present beam run. Theirwork on the scanning x-ray microscope3 andalso the work of Dean Eastman and John Freeoufon electron energy levels 7 have shown the greatpotential of the CEA as a source of synchro-tron radiation.

References

*Work supported by the U.S. Atomic Energy Com-mission under Contract No. AT(11-1)-3063.R. Averill et aZ., "Colliding Electron andPositron Beams in the CEA Bypass", Proceed-ings 8th International Conference on HighEnergy Accelerators, Geneva, Switzerland,1971, p. 140. See also R. Averi lIl et aZl.,"Performance of the CEA as an e+e- StorageRing", Proceedings of this conference, andreferences in both of these papers.

R. Madaras et al., "Electron-Posi tron ElasticScatte ring at a Center-of-Mass Energy of 4GeV", accepted for publication in Phys. Rev.Letters. See also R. Averill et al , "InitialResults on e+e- Interactions at Ecm = 4 GeVat the CEA", Proceedings XVI InternationalConference on High Energy Physics, Chicago,Illinois, 1972. (CEAL-3063-02)

P. Horowitz et al., "Scanning X-Ray Micro-scope Using Synchrotron Radiation", Science178, 608 (1972).

4R. Mack, "Spectral and Angular Distributionsof Synchrotron Radiation", CEAL-1027 (1966).R. Averill et al., "Wiggler Magnet Design",CEAL-TM-199 (1973).

6 B. J. Maddox et aZ., "Demonstration ofAdequacy of Circumferential 4-1nch-Thick LeadShield to Protect Persons Close to a 100-mA-Avg 3.5-GeV Stored Electron Beam Even in theEvent of a Worst-Case Beam Dump", CEAL-TM-198(1973).

7D. Eastman et al., "XUV Photoemission Spec-troscopy Using the CEA 2.5-GeV Storage Ring:Valence Band and Core Level Studies of InSb",to be published in Phys. Rev. Letters.

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