The hot cathode of an electron gun is replaced by a metallic photocathode driven by an excimer laser. The current, current density, and emittance of the 500-kV electron beam produced by the photoelectron source are presented. In addition, the temperature of the photocathode is varied to study the possibility of a hybrid source

Builta, L. A.

Crutcher, J. K.

Elliott, J. C.

Kauppila, T. J.

Moir, D. C.

UNT Digital Library

A major purpose of theTechnical Information Center is toprovide the broadest disseminationpossible of information contained inDOE’S Research and DevelopmentReports to business, industry, theacademic community, and federal,state and local governments.Although portions of this reportare not reproducible, it is beingmade available in microfiche tofacilitate the availability of thoseparts of the document which arelegible.‘34.LA- UR-87-81O LA-uR--87-81ODE87 007499Tm.e: A PULSED ELECTRON INJECTOR USING A METAL PHOTOCATHOC)E IRRADIATED BY ANEXCIMER LASERAUTFtO~~x T. J. Kauppila, L, A. Bullta, J. K. Crutchei’, J. C. Elliott,and D, C. MoirWSMITTWTO: IEEE ~article Accelerator ConferenceWashington, DCMarch 16-19, 1997DIS(’LAIlklFiR‘l’his rcp)rl WIIS prcpllrcd II% :In :itwunl uf work $~msorcd hy un ugcncy t)f ihc IJnllcd SIUICS(iovcrnmcnl, Neither Ihc (Jnitcd SIUICS (jt)vcrnmcnl nor any #flcncy Ihcrcd, nor uny d’ [heircIIIplt Iyccs, n):~kcs :my wtirrunty. express or implied, or UWUIIICS uny lqul Iiuhility or rcsp)nsi.hIIIt V f~)r !hc wcuruu), complttcncss, ~n urmfulncw or UIIy in[olmuli{ml uppurtltu$, product, orpnxxss dIscloscd, (Jr Icprcscn[s Ihut ils usc would nd infrirl~c privu!cly uwncd rigllls, Kcfcr.IJIICChcrcill I() nny specific c~)mn]crciill prtxluc(, pItMws, or scrvicc hy {rmlc nuItIc, trudcmllrk,m~lnufudurcr, or (Ilhtrwisc dom MII ncccwurily conq(itulr or imply its cnd~)r~cmcn(, Irctml.mcndutitm, (II Iltvo ring hv lhc (Inilcd SIUICS (hwcrnmcrri or uny ltMcncy tlmrc{)l. I’IIc vIcw~MASTER 01! ‘II!IIIIIIINUIllllSUUIJUMLNlSUNLIMl:OJAuarmosLos Alamos National LaboratorLos Alamos,New Mexico 87’54!4.--, . . . . . . . .,A PULSED ELECTRON INJECTOR USING A METAL PHOTOCATHODEIRRADIATED BY AN EXCINER LASER*T. J. i(auppila,L. A. 9ullta, J. K. Crutch.r,J. C. cllictt,andD. c. HOlrLOSAlamos National LaboratoryLos AIuoS, l?OW MOXiCO 87545Tho hot cathode of an ●loctron gun 1s roplacod bya motallie photocmthodo drlvm by an ●xclmor laser.Th@ curr9nt, currtmt dmslty, and euittanco of the500-kV ●loctron beam producoa by the photooloctronSuurco are presented. In addition, the temperature ofthe photocathode 1s varied to study the possibility ofa hybrid source.Laboratory ●xperiment have been ~rformed toassess the feasibility of an ●xcimar Lsse:-drivenphotocathodo as a high-cUrrOnt electron source. 1-3This paper describes ●xperlmonts using an 800-MJ, 20-nspulso width, KrF ●xcimer laser to generate an ●lectronbeam in a fulk-scale ●lectron udard PNER2fEX hot cathode scurce U’th:oh::n :s :p::::-cal Pierce geometry, The 10G-mm-diam dispenser cathodeiG replaced with ● simple metal cathode that is illu-minated at a 4S0 angle. Because th cathode can beheated, ● hybrid (hot/phococathode) source was alsostudied. The ●leceron beam produced by tho photo-cathode has the temporal shape of the laser pulse and1* lndependont of the v~ltage source. The eventualphoto-oathode ●pplication is ●n injector for PNM.NEXthat can produce ● train of electron beam pul~os,synchronizedwith the 50-KNz rf ●ccelerating fields,TtIis new ihjector could minimize ●fiorgy dispersion*tween micropulses, maxf,mize beam t:ansport, ●ndsimplify pulse power requirements for multi~~lsegoneraticm (i.e., ●lectron bursts separated by greaterthan the characteristic cavity ●ccelerating cycl~ of2G ns).The ●xperiuanta ~pparatus included ● standardiP2iNXlrUX ●ioctron gun ‘- modified to ●ccapt laser illu-mination ?f the ca*hode. changes for laser beaB ●ntryincluded ● 25-me-diam, Oup:aSil window, ●ntrance portlil the VaCUUS Walt ●nd a 2S-u-dlam hole in the ●node,Corning 7940 +W-m focal lsngth uv grade opcicc wereinstalled batwemn the ●ntrance port ●nd anode to ●xpendchm beam aaross the cathode mo nhown in ris. 1. Theiasor used for iilum:nationof the cathode N*S ● LubdePhysik ENO-)50CST, injection-locked, unstable reoona-tor. The loo-ma-dlam cathode had ● r dius af curvatureiaf 22S u ●nd total area of S1OO mm . The ●xpwimentsrqorcf..d here were with ●n aLu8inu8 (fI061 T) phOtO-Catrlorh.Cathode Voitace was produced by ● WtCOr 200-nS,500-kV, rna;x [?nerator. voltage was Wn!tored with a0.1-0, $OO-Q, Voltase divider in parallel with the gun.A sehemxtlc of the ●xperimental ●rrangement kc shown in71O. ‘2. Current eenerated from the photocathode wasaonitorod with ● self-lnte ratlnt,~masnetic-sense,B-dot loop in the drift space.A BMC Mode: 7102 delay ~enerator in the lasertrlggw line was used to synchronize tho itner pUl?Owith the center of the Yoltago pulse. !!mt~run voltareand uurmtt were WnitOrOd with Toktronlx Wodel 7103,L-orla-bandwidth oscilloscope@. Voltago on tne gun wasvaried by Changin$ kho charge Wltago on tho Karx.Laser intensity was selectively %arted by tnsert~en ofcalibrated beam tpiitters into the laser path ●nd@dJustSent to the laser dimcharb. voltage. An open-sbuttor ca~eta was used eo photograph the t imo-. —.—‘Work performee Wtder khe auspiceu of the U.S. Depart-ment of Energy by LOU Alamo- )IatiWtai LO’boratorY underw-1409-KWo-tb!“%%ww”IWT7MCS nAu-t&==--—=””-’ ‘LAun WmAwcsPCm b&mmY&Ixwlmmal.sms Sn?awMoos IFig. 1. Standard PHl?EXE%gun modified for pnotocathode●xperiment.Schemtk Electron Gun Expodmental Arrmgmontstandardloonwnoul~+i$izE3L---- .. —-- --” b%------ ‘YQL . Load fOO!BlW(500ft) e~,,,=OfItt ID~COO-dot bOCFlcv. curmlltVIsvflngmlslor(o !00) OCT ● AltO, hloCtOf,. . Oh FOC61 Lonam Wlct EM . Enxunco Maskrig. 2. Schematic ●iectron sun ●x~rimentai ●rrange-ment.integrated Mam pattern transmitted :hrou~h the●mittmnce mask. Emittance was then measured as inRef. [4].The illuminationarea of the photocathode deter-mines the perveance of the ●iectron gun and thereforetot!~ ●lectron beam curtent. r~lu.e.,lation of the●ntlro cathsde area would produce a perveance of 1 AIPowith the ●vailable optics this wan not po@sibie.2 TheiilUBinatiOn area of th@ cathode was 19?4 mm orapproximately one-fourth the total cathode ●rea, Thisconstraint limlt~d th~ peak current but did not ●ffectthe current densitYt The laser ●llergY (on the cathodewas llmited to a maximum of 130 SJ becau?e of th~~porture on tho laser ●ntrance port.Waveformrn rcpresentutive of tho vi*ctron sunvolt-co, the ●lectron beam cu:rent pulta, ●nd th~ L.is@rpUISO aro shown in ri~. 1, doaonotrat,ng that the* beam currmt tracked the laser and was lndepsndent ofthe voltage pulse on tho electron gun. Current rise-tlmes of 7,0 ns were measured but may have bO@n l@5Sdue to cable inductance.The peak photoelectron current density vs voltage1s ShOWn in Fig. 4 for various laser powors. For allof the laser powers and gun peak voltages S420 kV, thecurrent d@nslty 1s space-charge limited. The data fitthe function1/2J = (0,21 2 0.02) V , (1)whore J = I/A, I is the tOtal current measured by theself-integrated B-dot loop , and A (the illuminationarea) equals 1974 mm2. At the lowest laser power, thObeam 1s ●mission limit@d at 420 kV; at tho highestpower the ●mission limlt increases to 600 kV. Themaximum urrent density achieved in the ●xperiment9Was7.6 A/Cm .The ●mlsslon-llmited data can be used to derive aquantum ●fficiency as was dono in Ref. [3]. For thea~hmlnum photoc@thode, the quantum ●fficiency was foundto be 7.s x 10-’ ●3.ectrons/photon.This nunber is fivetimes the value obtained in Ref. [3] f’x aluminum:however, there was a vast dlfferonce in surface prepa-ration. That may ●xp:lal)l the discrepaficy. Heating thecathode to approximately 400” -f produced a rise inquantum ●f!iclency to 1.S x 10 ●lectrons/photon. TheW=lA I4If! 1 J__L~---J100 200 300 400l.- t (na) JFIK. 3. Uaveforms of the ●lectron voltaie, laserpulse, ●nd ●lectron beam current, --— ——●.,● ✌●,,“,,0.,.11 ..., --------- .,. , 0!,,%) 1(,I m, 4wJ4aowJMou M WI wcause was likely a natural cleaning of surfac* contami-nates bOCaUSO the quantum efficiency did not changeafter the cathode was retooled. As was Observed InROf. [3], the quantum efficiency was roughly inde-pendent of laser power.Figure 5 :s a photograph Of the Space-chsrgelimited beam distribution beyond the emittance nas~.The time-integrated emittance taken fram Fig. 5 isplotted in Fig. 6. For 6misslon-llrIrlted flow, therewas evidence of nonuniform (hot spot) ercisslon,as 1stypically seen in the conditioning of dispenser (hot)cathodes. The FUN OMlttanCO was found to 5e50 r mm-rad for a gun voltage of 355 kv, a tot .( beam!current of 45 A. a current density of 2.25 A/cm , and apulse width of 20 ns. The normalized emittance ofc = 413-1 is found to be 68 K mrs-rad. Normalized time-i}tegrated effilttance value obtained for a standardelectron gun with hot cathode is u = 160 t?mm-radfOr a gun voltage of 440 kV3 ~ A;cm$, current ‘f 28G A, ~current density of . and a pulse width of200 ns. The primary dlfferonce between the ●mittancemeasurements 1s the energy dispasion due to the 30 nstis~ and fall time in the hot cathode measurement.The normalized ●mittance of ●lectrohs produced b<an excimer-Laser-driven photocathode for spac~-chargellmltea flow has been determined. The ●mlttance :sless than the valuo obtained for ● hot cathode. How-ever, because of the temporal differsvi=os of tha Mamcurrent pulsas, an absolute comparison wouLd requir* atime-resolved measureme~: Uf hot cathode ●mittence overa corresponding 20-n? Per:’.ldof the laser,Extrapelatlon of tnesa results to ● 5-KV, S-kAplanar cathode injector with a 39-CM, anode-cathodf? gapand an ●luminum photocathode indicate th&t ● laser●nergy of 3.3 J would be required to obtain space-charte limited flow. According to Ref [3], use of a Pbphotocathede would :*duce the laser ●nergy requirementsto less than 1 s, well wl’.hln thG ●nerm ra~s* of com-mercially available ●xclmer lasers.o●●9●●9●.,— -.—-.. . ...__._.rig, 4. i’eak photoelectron current density vs voltage, rig! 5! Typical ●mlctarco record,./’-’,,.80- -— ——— —---—-- . ..— ——. .+0 20 0 alRarilus(mllllmeteml40Fig. 64[1] Y. Kawamur6, K.Of R@latlvistlcEmlttance plot.~OyOd~. ar!dH.PhotooloctronsKawal, “GenerationInducsd by Exclmer[2][3][4]is]i6][7]Laser irradiation,” APP1. PhYs. Lett. Q, p. 307,1984.s. U. Downey’, L. A. Builta, and D. C. PfOir,“simple Laser-Driven, Fletal-PhotocathodeElectronsource.” Phys. Lett. fi, p. 911, 1986.J. D. Saunders, T. J. Rlngler, L. A. Builta,T. J. Kauppilao D. c. Heir, and S. W. Downey,“simple Laser-Driven, l@tal-Photocathodes ascold, High-current Electron Sources,” Proc. o:IEEE Part. AcC. COnf., Washington, DC, March 16-19, i98?, to be published in this issue.J. C. Eliiott, R. J. faehl, H. J. Fullbrlght,D. C. 1401r, and C. E. Swannak, “Intense Low-Emittance Electron Oun Development at PERK?.?.,”IEEE Trans. Nut. Sci. ~, P. 2699, 1981.L. A. Builta, J. c. Elliott, D. C. MOir,T. P. Starke, and C. A. Vecere, “PHER14EXElectronGun Development,” IEEE Trans. WC. Sci. u, p.2728, 1983.R. L. Carlscn, R. N. Ridlon, and L. E. Stout,“14ulclgiiahertzBeam current and Positron Monitorfor Relativistic Electron Beams,” Rev. Sci.Instrum. ~, 2471 (1986).J. R. Pierce, lh9QXlL_aI@~.L_UIULQn-, Van Xostrand Co., Inc., p. 183 !1954).

A pulsed electron injector using a metal photocathode irradiated by an excimer laser

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A pulsed electron injector using a metal photocathode irradiated by an excimer laser

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