A new polarized electron gun for use on the SLC at SLAC has been built and tested. It is a diode gun with a laser driven GaAs photocathode. It is designed to provide short (2ns) pulses of 10 A at 160 kV at 120 Hz. The design features of the gun and results from a testing program on a new and dedicated beam line are presented. Early results from operation on the SLC will also be shown

Clendenin, J.

Frisch, J.

Hoyt, E.

Klaisner, L.

Schultz, D.C.

Woods, M.

Wright, D.

Zolotorev, M.

Schultz, D. C.

UNT Digital Library

SLAC-FOB--5768_ThePolarized Electron Gun for the SLC 012410D. C. Schultz, J. Clendenin, J. Frisch,E. Hoyt, L. Klaisner, M. Woods, D. Wright, M. ZolotorevStanford Linear Accelerator Center, Stanford University, Stanford CA 94309_bstract ......................,_ _==._,,. \...\_.,o_po,,,_| _--__'_"_'_"=/ A new polarized electron gun for use on the SLC at |/" SLAC has been built and tested. It is a diode gun with a [ _'" _/ laser driven GaAs photocathode. It is designed to provide , "_ __'__._, short (2ns) pulses of 10 A at 160 kV at 120 Hz, The design /: features of the gun and results from a testing program on a /i new and dedicated beam line are presented. Early results [t from operation on the SLC will also be shown. )IlhrrRODU CTFIONThe SLC Polarized Electron Source is based on a GaAs(or related III-W semiconductor) photocathode. This type.of electron source is not new and sources of this type have _,0_, _d "---- _""= _ | /--'= .h'T'=been used at SLAC [1] and elsewhere. CsF is deposited on c-, r.,,_,,,_,,,.-- i__ V=cuumthe GaAs (100) crystal to achieve a negative electron Polarized Gu. II °mim' ..affinity (NEA) surface. The interior of the gun must be ""held in a ultra-high vacuum condition to maintain the Figure 1. Elevation of the SLC polarized gun. Laser lightNEA of the photocathode. A spin polarized beam is enters, and electrons extt on the right. Negative high voltageis connected to the leE-end corona dome and held off frombrought off the photocathode by exciting electrons from thegrounded anode and gun support by an alumina ceramic.only the top valence state into the conduction band withcircularly polarized light. The electrons diffuse to the anode. The cesiator is mounted on a bellows allowing itphotocathode NEA surface, enter the vacuum and are to be retracted when not in use, or inserted onto theaccelerated by an applied electric field, gun axis so that cesium can be directed through the hole inThe SLC source differs from previous sources in the the anode and deposi_,ed on the photocathode. For high-accelerator's requirements for high peak current. The source voltage operations the ceramic insulator and high-voltagemust put out 10"_1electrons in the central 2ns portion of each co:'ona dome are placed in 1 atm of SF6.pulse, a peak current of 10A. lb accomplish this the Th_.:,eguns of this design have now been produced.gun must run at a voltage of 160 kV. Maintaining the quan- Vacuum parts are made from 304L vacuum arc remeltedrum efficiency of the photocathode while operating at high stainless steel with low carbon content and fine grain struc-voltageis t/,e central problemfor the design of the SLCgun. ture. The electrodes are machined from 317L vacuum arcremelted stainless steel wifl_ a very low carbon content andGUN DESIGN few inclusions. The end of the photocathode support tubeThe design of the SLC gun is shown in Fig. 1, with the is arc cast high purity molybdemun for uniform heating ofhigh-voltage end of the gun on the left of the drawing and the photocathode during heat cleaning. The molybdenumthe electron beam exiting to the right. The cathode elec- is brazed to a short piece of Kovar that matches thetrode is held on a long support tube that is cantilevered molybdenum's coefficient of thermal expansion. The Kovarfrom flanges on the high-voltage end through an alumina is t!.0n welded to a 304L stainless tube. All vacuum parts(AL-300 or AD--94) ceramic insulin.or. The photocathode, were outgassed in vacuum at 450°C pric: to assembly. 'Ihemounted on the end of its support tube, is held behind a guns were assembled in a class 1000 clean room to mini-14-mm diameter hole in the cathode. The photocathode mize contamination in the ultra-high vacuum volume.supt,_rt tube is mounted on a bellows (shown inside thecorona dome) so that it can be retracted 10 _ from the ELECTRODE CONFIGURATIONScathode electrode during bakeout and photocathode heatdeaning. The anode electrode, which is at ground poten- The high current required to come from the SLC sourcetial, is mounted to the anode support that is attached to the places requirements on the perveance of the gun. Operat-main gun support flange. The vacuum chamber surrounds ing the source near the space charge limit is desirable to:' the anode and anode housing, and supports the vacuum minimize beam intensity jitter. The surface fields on thepumps (not shown). The effusion cell cesium dispenser cathode electrode must also be low to minimize electron(not shown) is located down beam (to the right) of the field emission and breakdown, which cause gases to beevolved and can irreversibly contaminate the photocath-*Worksupported by Department of Energy contract ode. The problem of field emission is made worse by theDE-AC0-3276SF00515.adsorption of cesium on the cathode surface during orPresented at the 3rd European Particle Accelerator Coilfcrencc, l]crlin, (]ermany, March 24-28, 1992t- 2,8 x 10-11i co! zLLI__ 4 1,94X10 -12i, .t ]8 kV/mm 9,4'0 I ' I ' ' I ' ' 1(a)0 12 16 28 44r MASS 7118A2m _ ...... 3.92Figure 3. Residual Gas Analyser spectrum showing160kV (b) the partial pressure in the gun of H2 (2.8x10 "11mm,off scale), of CO (1.9x10"12)and of CO2 (lx1012).13 kV/mmnoble gases. The ion pump is paired with an SAES non-evaporable getter (ST707) pump [5] rated at 2001/s andoperated at room temperature. The NEG pump has verygood pumping characteristics at low pressures, holding thepumping speed at pressures below 10"11mm and pumpingCO and CO2. lt does not pump methane, which can be det-'"_'_ rimental to the photocathode lifetime, nor' noble gases. TheFigure 2. The lower half, (b), shows the design of the orig- NEG pump also pump:_ well in the range at 250°C, n_kinginal electrode set. The photocathode sits at the centerline it an effective pump during vacuum bakeout.behind the cathode electrode. The upper half, (a), shows To monitor the vacuum levels in the gun, a Residualthe new 'low field' electrode set. Light enters from the top Gas Analyzer (RGA) [6] is attached. A typical RGAright to illuminate the photocathode during activation. sl:xx:trum of gases in the gun is shown in Fig. 3. The hydro-after photocathode activation. These requirements on gen partial pressure is shown off scale and equal tothe cathode field strengths have led to two electrode 2.8x10 11 mm. The RGA spectrum shows small CO andstructure designs. CO2 peaks at masses 28 and 44 respectively. The s_trumThe first desigTl is shown in Fig. 2b (and in Fig. 1). The shown in Fig. 3 is very stable, and minute (1013nwn)design was based on the electrodes used in the SLC therrni- changes can be seen in the partial pressures, but theabsolute value of the pressures is only accurate to within aonic gun [2] which routinely gives the needecl beamcurrent. The electrcKte structure was modeled using the factor of two. The peaks at nr_ss 16 and 19 are attributed toprogram EGUN [3] which calculates the surface fields and surface effects in the analyzer head.the space cl_rge limit of the structure. The perveance ofthis electrode configuration is 0.16 #pcrvs, giving a space HIGH-VOLTAGE PERFORMANCEcharge limit of 10 A at 160 kV. The maximum field is 'The initial high-voltage t_xMonnance given with either13 kV/mm on the outer txtge of the cathode electrode, electrode set was good. The guns were processed toWhile fields of this strength can be held on stainless steel 180 kV and were able to hold 160 kV for extended period,,;surfaces in vacuum, the proximity of the photocathode without breakdown. _rl_elow-field electrode set processc\tsuggests that cesium will be deposited in the high up to voltage much more easily than did the originalfield area. design, which rcxluirLKt processing with nitrogen to machA second electrode set was design(xi to low'er the voltage. After proo:._ssing there was no evidence of darksurface fields on the cathode electrode. The low-field current drawn by the guns at 160 kV, and no dark currentelectrode design, first suggestc, d by W. Herrmannsfeldt, at 180 kV for the gun with the low-field electrcKtes.is shown in Fig. 2a. The perveance of this ehxrtrode After high.- vol tage processing the photocathode isconfig-uratior_ is approximately the &'m-_e as the first, l.Xfisoned and must be replaced. This rc_quires venting the"[he nvaximum field is 9.4 kV/mm on the outer edge of the gun vacuum volume with dry nitrogen and opening it tocathode electrode. The _cond, low field, electrode design air. The gun is open with a N 2 purge for the ten minutes(not shown in Fig. 1) is now installed in the third gun, and rcxtuired to change the photocathode. The gur_ is thenthe first two guns are being retrofitted with the new design, vacuum baked at 250°C for I(X) hours. After this prck-vdurethe high-voltage processing is partially lost, and the gunVACUUM PERFORMANCE shows high-,.,c_ltab_.,,and vacuum activity as low as 140 kV.Tl"_emaintenance of the ultra-high vacuum condition is lt is this t.x.,havior which sets the value of the operatingperformed by two pumps. The first is a Perkin Elmer dif- voltage at 12(.)kV (space charge limit 6.5 A).ferential ion pump [4], rated at 120 l/s. A D-I pump is After reD_atc_ use of the guI_,_ (several photocathodechosen for its effectiveness with pumping argon al_d other changes and system bakeouts) the high-w)ltage tx2haviordegrades significantly. The degradation seems to be CONCLUSIONdependant on photocathode cesiations. Eventually, one A new polarized electron gun for use on the SLC atstandard cesiation will degrade the performance such that SLAC has been designed and three guns have been fabri-the high voltage cannot be brought up to the operating cared. Considerable attention was paid to the vacuumlevel; high voltage and vacuum activity is seen at 80 kV or aspects of the design, to the choice and sek.-vtion of rnateri-lower. The high-voltage performance can be regained by als used, and to the fabrication process. The guns all exhibitprocessing, but the processing destroys the photocathode, very good vac'uum characteristics, both when new and afterAt this point the gtm is no longer operational. We have one repeatcx:t cathode char, ges and bakeouts. The high-voltagesuch gun which is being disassembled, cleaned and rebuilt perforn'tance of the gun, however, is problen_tic, While ato remove the cesium found throughout this gun. gun's initial high-voltage performance appears satisfactory,it has proven impossible to reach full voltage with a freshPHOTOCATHODE PERFORMANCE photocathode. High-voltage performance after repeatedTesting with the new guns has been performed with photocathode activations is further degraded until the gunGaAs photocathodes, zinc doped to 1-2 x 1019/cc. The is unusable. We are pursuing two improvements to the18 mm diameter photocathodes are cut from a 3-inch high-voltage performance; an insulator with a resistivewafer. The bandgap of GaAs is not well matched to the titanium chromium oxide coating to reduce static chargedye used in the SLC polarized source laser (715 nra.), buildup on the insulator, and a segmented insulator whichbut the GaAs material should give --,30% polarization, is shielded from the high-voltage cathode support tube.Matching the III-IV semiconductor bandgap to the It is not clear whether either of these improvements willlaser wavelength should yield 40-45% polarization, alleviate the effects of excess cesium in the gun.GaAs(.87)P(.13) has been acquired for this purpose and The quantum efficiency of the photocathodeactivated to good quantum efficiency, but not yet tested in might also be improved. In a separate cathode testingthe guns. system, photocathodes are introduced into the vacuumIn the gun the GaAs photocathode is activated in the volume through a load lock. In this system GaAs andfollowing mannen The photocathode is heat treated to GaAs(.87)P(.13) photocathodes have bee.n activated to 14%620°C for one hour to remove oxides from the surface. The quantum efficiency at 750 xm't. lt is suspec'ted that subject-temperature of the GaAs surface has been measured with ing photocathodes to high temperature bakeouts degradesan infrared pyrometer active at 900nm [7] and that their perfom-_ance. A higl_ initial quantum efficiency wouldtemperature referenced to a thermocouple behind the decrease the mean time between cesium depositions,photocathode. "[he cesium effusion cell is heated bringing thus prolonging the lifetime of the gun. High quantumthe cesium reservoir to 100°C, The effusion cell is insertcx:! efficiencies might also alleviate the current density limit,to face the photocathode and the photocurrent, excited b_ving increased beam currents. We are working on a loadwith a white lamp, is monitored. When the photocurrent lock system for use on the gun in which ali activationspeaks, the cesiator is retracted and NF 3 is let into the would take place in a separate chamber outside the gunsystem through a variable leak valve to a pressure of vacuum volume. This will minimize the amount of cesium10.8 mm. Cesium and NF 3 are deposited in cycles in introduccuff into the gun, will allow the photocathode to bethe so-called 'Yo-Yo' technique. With this technique we changed after high-voltage processing without venting, andhave been able to consistently achieve quantum efficiencies should lead to high quantum efficiencies and long gunof 1-3%, measured with a 750 nm diode laser, lifetimes.The quantum efficiency decreases with time, and canbe characterized by an exponential with an increasing char- ACKNOWLEDGEMENTSacteristic lifetime, A range of lifetimes from a few hours to We wish to thank the large number of SLAC per.,Km-hundreds of hours has been obserwxt, with 10 to 40 hours nel who have nwde si__ificant contributions to the gunbeing typical at room temperature (--20°C) when the effort, particularly J, Langton, Tim Montagne, Mikequantum efficiency is .5-3%. When the quantum efficiency Palrang, and Kathleen Ratcliffe. John Edgecumbe (consult-has dropped below a useful level, depositing a small ant), Ed Garwin, Takashi Maruyan_, Roger Miller andamount of cesium (only) oi, the photocathode is suffident Charles Sinclair (CEBAF) have provided imt.x_rtant advice.to restore most of the original quantum effidency. We arenew establishing ways in which quantum efficiency can be REFERENCESstabilized. [11 c. K. Sinclair et al., Proc. Argonne Symp., 1976, p. 424, andRelated to the quantum efficiency, there appears to be SLAC-PUB-. :109.a limit on the current density which can be brought off the 121 le,.F Koontz :t al., Proc, Particle Accel. Conf., 1981, p. 2212photocathode. We have observed a 'saturation' in [3] W B. Hvrrmannsfeldt, SLAC-331,1988.photocurrent as laser intensity is increased. This limit has 141 lk.tkin Elmer Physical Electronics Division, 6509 Flyingbeen observed when the photocathode is illumix_tc_d with Cloud Drive, Eden Prairie, MN 55M4 USA.715 and 760 nm lasers, near the bandgap threshold, but not [51 SAES Getters S.p,A., Via Gallarate 215, 20151 M',lano, Italy.when a 533 nm laser is used. This current limit increases (_,l VG Quadrupoles Ltd., Aston Way, Middlewich, Cheshireas quantum efficiency increases, but even at quantum CW1001-1SEngland.effidcncies of 2--3% the limit is less ttum the space charge [7l IRCON Radiation Thermometer modline V _2ric_, Niles, I1.,limit of the gun. This effect is not yet understood. 6o648 LISA.

The polarized electron gun for the SLC

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The polarized electron gun for the SLC

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