Development of an electron gun for the ERL light source in Japan

1. ERL07 WG1Session 1 DC Guns Development of an electron gun for the ERL light source in Japan N. Nishimori, R. Hajima, H. Iijima, R. Nagai Japan Atomic Energy Agency(JAEA) ERL Development Group T. Nishitani RIKEN May 22, 2007.

2. JAEA,KEK,ISSP Collaboration for an ERL Light Source As a first step of the collaboration, we started to develop key technologies for the ERL, and are planning to construct together an ERL test facility at KEK site. Tentative parameters of the ERL Test facility

3. An EUV ERL SASE FEL for semiconductor lithography 580MeV energy recovery loop dump (10MeV, 120kW) Bunch compressor SCA 1 SCA 2 undulator EUV FEL RF 80m Single pass SASE FEL Acceleration 10MeV Injector R. Hajima et al., EUV source workshop, Barcelona Spain October 2006. Deceleration beam parameters FEL parameters

4. Outline • status of 250 kV 50 mA gun development • simulations of beam envelopes and planned diagnostics • a photocathode test bench • summary

5. Development of a 250 kV-50 mA DC gun A DC gun is under development. high voltage test without beam loading main chamber load-lock chamber preparation chamber H. Iijima et al., Proc. Acc. Meeting in Japan (2006) solenoid beam diagnosis e-beam R. Nagai et al., Proc. Acc. Meeting in Japan (2006)

6. Gun chambers . e-beam load-lock system load-lock chamber preparation chamber main chamber

7. Gun vacuum . vacuum = 3 x 10-10 Pa (design value) all the chambers are made of Titanium. Ion 500l/s NEG 2000l/s Ion 200l/s NEG 2000l/s e-beam Turbo 300l/s Cryo 1700l/s Ion 500l/s NEG 2000l/s

8. Gun electrode all the electrodes are made of Titanium. 40 mm 40 f cathode holder 1 mm e-beam cathode laser 120 f 8 f anode cathode holder anode 180 f Anodization has not been applied. The electric field is 5 MV/m.

9. Design of emittance compensation solenoid solenoid magnet minimum emittance and drift length 77pC  en ~ 0.59 mm-mrad 2.5 1.4 1.2 mrad] 2.0 main coil [m] 1.0 ･ [mm 1.5 0.8 drift length 0.6 1.0 emittance 0.4 0.5 0.2 Bucking coil to compensate Bz at the cathode surface 0.0 0.0 220 230 240 250 260 270 280 solenoid field [Gauss] emittance of a 250-keV electron bunch (PARMELA) 0.59 mm-mrad for 77 pC 0.11 mm-mrad for 7.7 pC (initially, Gaussian in longitudinal and uniform in transverse) z (cm)

10. Simulated transversal envelope for 250keV 77pC beam emittance 1 mm mrad 1 m 2 m sz= 60 ps Beam size 1 mm slit scan tool cathode solenoid

11. Space charge and emittance dominant beamlet rms beam envelope equation for a relativistic beam in a drift space en2 I sx’’= + g2b2sx3 g3b3I0(sx+sy) where I is the peak beam current, I0=ec/re is the characteristic current en is the normalized rms emittance Id2 2 ・ 250keV (g= 1.49, b=0.74) ・ 77pC, 25ps (2s), 3A ・ en=0.3 mm-mrad ・ I0=17,000 A (re=2.82x10-15m) ・ d=20mm ・ Rb=0.33 Rb= gbI0 en2 3p 25 mm slit is already available at SPring8 Compact SASE Source. K. Togawa et al., PRST AB 10, 020703 (2007).

12. Bunch length measurement using a kicker cavity ・ 500MHz TM010 cavity ・ shunt impedance: 2.5MW ・ transit time factor: 0.93 ・ 3.7kW RF power is needed 250keV +20keV -20keV 100ps 8kW 500MHz RF power supply 500MHz kicker cavity f 72

13. emittance of NEA photocathode CB emittance is a function of laser wavelength and laser spot size. 100keV-10mA vacuum B.M. Dunham et al., PAC-1995, 1030. VB 120keV-10nA ~0.1mm-mrad is available at small current Longer wavelength is better for low emittance emittance vs laser wavelength N. Yamamoto et al. Proc. Acc. Meeting in Japan (2006)

14. a photocathode test bench at JAEA cathode holder Optimization of cathode material for the better QE and life. Cs holder heater cathode holder UHV chamber and laser photocathode prepared at Nagoya Univ(1). bulk-GaAs, bulk-AlxGa1-xAs x=0.17, 0.28 (1) Venture Business Laboratory, M. Tabuchi, Y. Takeda et al.

15. Performance of bulk-GaAs and bulk-AlGaAs Quantum Efficiency Cathode Lifetime gun voltage =200V, current =100nA, photon energy= band gap + 0.2eV • Al0.28Ga0.72As • GaAs twice Q.E. >10-times longer life • Al0.28Ga0.72As • GaAs Bulk-AlGaAs shows higher QE than bulk-GaAs Bulk-AlGaAs shows longer life than bulk-GaAs Larger band gap and higher JDOS  more efficient excitation of electrons electron affinity GaAs = 4.1 eV Al0.28Ga0.72As = 3.8 eV

16. How to design a long-life cathode vacuum without Cs with Cs effect of Cs χeff χeff φD χ vacuum CB CB φBB Eg Eg activation of NEA VB VB effective electron affinity effective electron affinity damage on the Cs layer  a rise of vacuum potential cathode material with smaller c is preferable for keeping the NEA state (ceff <0) c =4.1 eV (GaAs), c =3.8 eV (Al0.28Ga0.72As)

17. band gap energy of AlxGa1-xAs Band gap is variable from 1.42 to 2.17 eV. Egap QE direct transition Yb 2w photon energy indirect transition NOPA thermal emittance (thin cathode) Ti:sap. fabricated at JAEA/Nagoya-U. ref: “GaAs and Related Materials” Sadao Adachi, World Scientific (1994) same for a thick cathode ?

18. Summary • We have initiated R&Ds for a photocathode DC gun. • a 250kV-50mA DC gun is under development. • normalized emittance is expected to be ~0.6mm-mrad for 77pC, ~0.1mm-mrad for 7.7pC at 250-keV. • the first beam from the gun will be this summer. • QE and life have been measured for bulk-GaAs and bulk-AlGaAs. • Bulk-AlGaAs shows better QE and life as predicted by semiconductor theory. (QE ~ 2x, life ~ 10x )