The Jefferson Lab 200 kv Inverted Gun: Lifetime Measurements Using Strained Superlattice GaAs and K 2 CsSb Photocathodes

Transcription

1 The Jeffersn Lab 200 kv Inverted Gun: Lifetime Measurements Using Strained Superlattice GaAs and K 2 CsSb Phtcathdes R. Suleiman, P. Adderley, M. BastaniNejad, J. Clark, S. Cvert, J. Grames, J. Hansknecht, R. Mammei, J. McCarter, M. Pelker, M. Stutzman, K. Surles- Law,T. Ra, J. Smedley KEK, Japan, Octber 20, 2011

2 Jeffersn Lab Site: FEL Arcs 2 SRF Linacs (600 MeV) Injectr Test Cave CEBAF Injectr Arcs 3 Experimental Halls

3 Benefit f Higher Gun Bias Vltage Reduce space-charge-induced emittance grwth, maintain small transverse beam prfile and shrt bunch-length. In ther wrds, make a stiff beam right frm the gun - particularly imprtant fr high bunch charge beam CEBAF guns have always perated at 100 kv (β = 0.55) Achieved better transmissin in CEBAF Injectr at high beam current with 130 kv (β = 0.60) Later, we envisin an imprved CEBAF pht-injectr with a 200 kv gun and SRF capture sectin (β = 0.70) Indentify what it takes t reach 350 kv bias vltage r higher (β= 0.8+). Fr ILC, CLIC, EIC, etc. Biggest bstacle: Field emissin and HV breakdwn which lead t Phtcathde death

4 Rad t Higher Gun Vltage Old Gun Old Ceramic Expsed t field emissin Large area Expensive ($50k) Lts f metal at HV New Inverted Gun Medical x-ray technlgy e New Ceramic Cmpact $5k Less metal at HV N SF 6 f N cm Mved away frm cnventinal insulatr expensive, mnths t build, prne t damage frm field emissin. High gradient lcatins nt related t beam ptics, lts f metal t plish. Less flanges material, less utgassing, better vacuum: Outgassing rate: Trr. L/s. cm 2 Extractr Gauge: Trr (raw value)

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6 First Inverted CEBAF I. 100 kv: Lifetime ~ 100 C at 180 μa with transmissin f 85% II. 130 kv: Lifetime ~ 100 C at 180 μa with transmissin f 95% Spring, 2009 Built first inverted gun (stainless steel cathde electrde) July, 2009 Installed at CEBAF HV Pwer Supply: Glassman 150 kv, 12 ma Ran CEBAF 100 kv September, 2010 Cnditined t 150 kv successfully Since Operating at 130 kv (limited by present injectr design)

7 Secnd Inverted Injectr Test Cave Large grain Nibium (Nb) t 225 kv Large grain Nibium (Nb) cathde electrde HV Pwer Supply: Spellman 225 kv, 30 ma Why a Nibium electrde? I. Buffered-chemical plishing (BCP) fr ~ 1 hur, n need fr hand-plishing (takes weeks) II. N field emissin at high field gradients

8 HV Cnditining and Field Emissin Prblematic field emissin at 200 kv Cnditined t 225 kv helpful (limited by HVPS) Flating Ande cnnected t Pic-ammeter Macr glass-ceramic spacer 100 MΩ Cnditining Resistr UNIVOLT Transfrmer Oil Tank A Shrt fr Beam Delivery

9 Measure x-rays Ande may nt capture all field emissin current In Chamber Detectr: Mdel IP 100 Energy threshld f 50 kev Readut: CANBERRA ADM 616

10 Re-plished Large Grain Nb Electrde Lnger buffer-chemical plish (BCP) was successful N field emissin up t 225 kv x-ray stayed at backgrund level (8 E-3 mr/h)

11 Injectr Test Cave SUITCASE WIRE SCANNER

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13 High Plarizatin 4 ma Parameter Gun Bias Vltage Laser Rep Rate Laser Pulselength Laser Wavelength Laser Spt Size Phtcathde Beam Current Bunch Charge Duratin Extracted Charge Lifetime Value 200 kv 1500 MHz 50 ps 780 nm 350 µm (FWHM) GaAs/GaAsP 4 ma 2.7 pc 1.4 hr 20 C 85 C High initial QE ~ 1.5% Higher 200 kv vltage superseding 1 ma dem (PAC 07) Push technlgy in supprt f Electrn In Clliders > 50 ma QE(q) = QE 0 * e (q / 85)

14 K 2 CsSb Phtcathde Lifetime Phtcathde made at Brkhaven Natinal Lab Transferred t UHV suitcase and driven dwn t Jeffersn Lab K 2 CsSb exhibits very lng dark lifetime Measured lifetime at different beam currents and laser wavelengths See Pster Sessin fr mre details Parameter Gun Bias Vltage Laser Laser Spt Size Beam Current Duratin Extracted Charge Lifetime Value 200 kv 532 nm, dc 350 µm (FWHM) 5 ma 26 hr 465 C N Change in QE

15 K 2 CsSb Phtcathde Emittance Measured the Emittance after lifetime measurements x Used a slenid scan technique and a wire scanner t measure beam size Measured at tw Gun HV: 100 kv and 200 kv with 3 µa beam current Measured at tw 532 nm laser spt sizes: 0.35 mm and 0.70 mm (FWHM)

16 Summary and Outlk I. Inverted Gun #1 Stainless steel electrde perating at 130 kv (cnditined t 150 kv) at CEBAF fr precisin parity vilatin experiments requiring high beam current (180 µa) II. Inverted Gun #2 Large grain Nibium electrde perating at 200 kv (cnditined t 225 kv) at Injectr Test Cave fr studies f Phtcathdes perating at milliampere current III. Future research. next

17 Will Higher HV Imprve SSL Lifetime? ande (+) electrn beam At lwer Gun HV, the crss sectin is larger ver lnger distance cathde (-) H 2 In energy Lifetime is limited by in back-bmbardment At 200 kv, nly 60% f ins are created cmpared t 100 kv, Lnger lifetime? Any lifetime dependence n in energy?

18 Build 350 kv Inverted Gun I. Use FEL spare HV Pwer Supply: Glassman 500 kv, 5 ma II. Use a cable frm HVPS Gun III. Pssible design: HV Cable 10 psi SF 6 Pressure vessel filled with 45 psi cled SF 6 Cnditining Resistr

19 Backup Slides

20 Emittance and Brightness Nrmalized Emittance frm GaAs: q Bunch Charge (= 0.4 pc, 200 μa and 499 MHz) E s T eff k B T eff n, x, y q k BT 4 E m c Electric Field at GaAs surface Effective Temperature f GaAs (= K, 780 nm) Thermal Energy (= 34 mev) Gun HV (kv) E s (MV/m) ε n (μm) s e eff 2 Nrmalized Brightness: B 1 n 2 E s Brightness is prprtinal t Gun HV

21 Bunchlength and Transmissin At higher gun HV, bunchlength is shrter when Chpper Slit is fully pen, it passes 111 ps - and space charge emittance blw-up is suppressed Transmissin thrugh Injectr, imprves at higher gun vltage Better transmissin prlngs lifetime: Less beam lss that degrades vacuum Lwer current frm gun creates less ins and thus slws NEA damage Better transmissin reduces current fluctuatins and thus imprves Hall measurement f helicity-crrelated charge asymmetry Electrn Bunchlength (ps) Transmissin (%) Electrn Bunchlength vs. Gun Vltage Ave. Gun Current (μa) Transmissin vs. Gun Vltage Ave. Gun Current (μa) 115kV 100kV 85kV 70kV 115kV 100kV 85kV 70kV

22 Space Charge Limit at CEBAF Maximum current density that can be transprted acrss cathde-ande gap is (fr an infinite charge plane): Child s Law (1D): 6 3 / V d j1 10 Child s Law (2D) (PRL 87, ) : j 2 j ~ [A/cm 2 ] Fr electrn emissin frm a finite circular spt n the cathde: d r Fr CEBAF electrn beam (499 MHz): / X X CL CL Shrt Pulse (PRL 98, ): jscl j, X CL t b 3, CL 2 CL Single electrn

23 V D Gun Vltage Cathde-ande Gap (6.3 cm) R Laser Spt Size (0.5 mm = 2r) T b τ CL Bunchlength (50 ps) Gap Transit Time (0.96 ns at 100 kv)