RF Gun SuperKEKB review @ 14, Mar, 2018
High charge &low emittance is required for SuperKEKB.
4 nC 10 mm-mrad electron beam generated by RF gun. + 10mm-mrad emittance preservation is required. Quasi –travelling side couple RF-Gun with Yb-fiber and Nd/Yb solid laser system Preliminary test using DAW RF-Gun with Nd laser system => Yb-fiber & Nd:YAG due to earthquake C 1 2 3 4 5
• Space charge is dominant. — Longer pulse length : 20 - 30 ps • Stable operation is required. — Lower electric field : < 100MV/m • Focusing field must be required.| – Solenoid focus causes the emittance growth. — Electric field focus preserve the emittance. Epaxial coupled cavity    :   BNL Annular coupled cavity  :   Disk and washer / Side couple S-band RF-Gun development strategy for SuperKEKB • Cavity : Strong electric field focusing structure –  Disk And Washer (DAW) => 3-2 – Quasi Traveling Wave Side Couple=> A-1 — Cut Disk Structure => A-1 90 deg line and normal laser injection =>  Reduce beam divergence and projected emittance dilution • Cathode : Long term stable cathode — Middle QE (QE=10-4～10-³ @266nm) and long lifetime — Solid material (no thin film)  =>  Metal composite cathode =>  Started from LaBs (short life time) =>  Ir 5 Ce has very long life time with QE>10 -4 @266nm •   Laser :  Stable laser without/with temporal manipulation – LD pumped laser medium •  Nd doped solid laser => 3-2 •  Yb doped fiber and Nd/Yb solid hybrid laser   => A-1 ground/underground — Both side irradiation to QTW — Temporal manipulation =>  posteponded until Phase-III =>  Minimum energy spread Cathode : Quantum Efficiency Map Spatial dependence of QE on the Ir , Ce, cathode surface was measured. 【Results : Non-activation】 【Sample】 Photocathode in the QTWSC-RF gun using Spark Plasma Sintering (SPS) x (mm) x, y : Position of focus lens QE > 10 -4  in off-line measurement (10 -6 Pa). QE = 10 -5  in operation condition because of vacuum level (10 -5 pa).
Menlo oscillator (Commercial) λ :1015～1055nm Grating pair (Dispersion compensation) Band pass filter YDFA Self Phase Modulation (SPM)
. Nd laser system is used for SuperKEKB phase II (Both SuperKEKB ARC (R70) and RF gun review comment) . Two lasers synchronous injection, the second line is built. Two lasers injection → low emittance and high charge (3.6 nC achieved) . Mser with horizontal polarization ● Laser with vertical polarization,
Virtual cathode monitor was installed (SuperKEKB ARC) Laser Beam Profile (Current)
Irradiated by two lasers B-sector Wire Scanner  SP_A1_G 1.2nC 3-wire ABC : γεχ=23.314、γεγ=19.939 3-wire ABD : γεχ=27.558、γεγ=21.983 3-wire ACD : γεχ=22.752、γεγ=35.889 3-wire BCD : γεχ=25.387、γεγ=20.515 4-wire ABCD: γεχ=24.958、γεγ=18.640
Laser monitors for fiber laser part and every amplifier stage were installed. We can justify the error part immediately from these logs. (SuperKEKB ARC R71)
by fully remote control (mirrors, BBO, telescope, wave plates) .
Summery for Phase-II & Upgrade for Phase-III •  Stable operation using Yb:Fiber+Nd:YAG laser with many monitors and remote controls. •  Required charge & emittance for Phase-II were achieved   └  Phase-III •  Current maximum charge output is  3nC due to lower quantum efficiency of 10 -5 compare with off-line QE of over 10 -4 . =>  High quality IrCe cathode & Increase ion pump & Cathode cleaning system •  Laser system with flat top pulse shaping using pulse stacking method for both Nd and Yb.
【Advantage of Single crystalline Metal】 • Higher QE than that of multicrystal[1] • Better QE uniformity than that of multicrystal[2] Our group collaborated with H. Sugawara (Kobe university) to develop the large size single crystalline IrCe compound. SC Growth Method Czochralski Process 【Czochralski Process】 【Developed SC IrCe】 Succeeded in the crystal growth of 4mm diameter Larger-diameter crystal growth experiments are in progress. [1] D.A. Reis, at al., 'Transverse Emittance Measurements from a Photocathode RF Gun with Variable Laser Pulse Length', SLAC-PUB,8018 [2] P.R. Boltona, et, al., 'Photoinjector design for the LCLS', SLAC-PUB-8962
【Design of EB heating type cathode plug】 Iridium cerium EB heating is … a main cathode is bombarded by an
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【Experimental Setup & Results】
Heating a photocathode over 1000°C by EB heating method was succeeded.
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• In order to get low energy spread in a strong space charge effect, laser pulse with rectangular temporal shape is required Temporal shaping scheme by pulse stacking technology for Nd laser system Input laser pulse with temporal Pulse stacking setup made by a crystal Output laser pulse • . 30 ps and 40 ps rectangular laser pulses are achieved by one stage and two stages stacking system separately * Supported by Dr. Yosuke Honda Available to both Nd and Yb laser system Summery for Phase-II & Upgrade for Phase-III •  Stable operation using Yb:Fiber+Nd:YAG laser with many monitors and remote controls. •  Required charge & emittance for Phase-II were achieved   └  Phase-III •  Current maximum charge output is  3nC due to lower quantum efficiency of 10 -5 compare with off-line QE of over 10 -4 . =>  High quality IrCe cathode & Increase ion pump & Cathode cleaning system •  Laser system with flat top pulse shaping using pulse stacking method for both Nd and Yb. Backup Closed gap makes focus field Side coupled cavity is one candidate (or DAW / ACS / CDS ...) Concentrated field has focusing effect This structure has long drift space This structure has focusing field. Long drift space is problem.
Normal side couple structure Quasi traveling wave sidecouple structure accelerated gradient than DAW.
Quasi traveling wave side couple RF gun (100 MV/m, 6mm-mrad, 13.5 MeV)
No reflection to klystron cavity
Photocathode Efficiency vs. Lifetime The thermocathodes can also be used as photoemitters [13]. LaBs should be noted as a promising photoemitter [14], which has a quantum yield of about 10-3 at a laser wavelength of 266 nm and 4·10 4 at 532 nm for face (100). and enough QE (10 -4) at room temperature. Inactive in air Recover by heating or laser cleaning Best choice for SuperKEKB   5 nC long time operation Physica Scripta. Vol. T71, 39-45, 1997. Cathodes for Electron Guns G. I. Kuznetsov
Condition Continuous pulse laser 200μJ@266nm) Irradation  => 2.5 nC emission
Quantum efficiency improvement by Laser cleaning 3000 4000 2000 0 1000 Energy density of cleaning laser(μJ/mm²) X 20 Laser cleaning  --- - Non activation Max QE = 1.00 × 10 -4 0E = 5.00 × 10-6 5000 Condition HV = 16kV | Vacuum ; 5.8 × 10 -6 Pa | Cleaning time ; 10 min | No oxidization is observed [SUPER-KEKB e Linac] Laser Power ; 233μJ/pulse (λ=266nm) ! Target value ;  5nC