Mini-Workshop for ERL under the collaboration meeting between CLASSE and KEK Development of an electron gun for the ERL project in Japan R. Hajima, T. Nishitani, H. Iijima, R. Nagai, N. Nishimori ERL Development Group Japan Atomic Energy Agency March 12, 2007. ● Performance of ERL-LS relies on its electron gun. ● High-average current (~100mA) and small emittance (~0.1mm-mrad) are essential. ● Further improvement from the existing technologies is required (JLAB-FEL=10mA and XFEL=1mm-mrad). ● “NEA cathode + DC gun” is only the practical solution. ● many R&D issues exist.
emittance vs laser wavelength N. Yamamoto et al. Proc. Acc. Meeting in Japan (2006) ~0.1mm-mrad is available at small current improvement is required 0.1mm-mrad and 100mA is not proven. 3
re-cesiation recovers Q.E. → damage is only at the surface o collision, deposition of residual gases o dark current (and its enhancement) o ion back bombardment existing guns CEBAF polarized gun (100kV, 0.1mA) life ~ 2 x 105 C/cm² JLAB-ERL gun (350kV, 9mA) life  ~ 2 x 10³ C/cm² improvement is required ERL-LS life ~ 106 C/cm² 100mA / φ 2mm、100 hours
cathode holder UHV chamber and laser Optimization of cathode material for the better QE and life. photocathode prepared at Nagoya Univ(1) GaAs, Al¸Ga¹¸¸As x=0.17, 0.28 (1) Venture Business Laboratory, M. Tabuchi, Y. Takeda et al.
AIGaAs shows higher QE than GaAs larger band gap = higher DOS → more efficient excitation of electrons Quantum Efficiency
Cathode Lifetime
AlGaAs shows longer life than GaAs electron affinity GaAs = 4.1 eV Alo.28Gao.72As = 3.8 eV
damage on the Cs layer → a rise of vacuum potential cathode material with smaller χ is preferable for keeping the NEA state (Xeff <0)
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From the measurement of GaAs, AlGaAs, we have confirmed that QE and life can be improved by optimizing band structure. proposal of superlattice cathodes
A DC gun is under development. H. lijima et al., Proc. Acc. Meeting in Japan (2006) all the chambers are made of Ti alloy. vacuum = 2-3 x 10-10 Pa 9
solenoid magnet bucking coil to compensate B, at the cathode surface minimum emittance and drift length 77pC → ε n ~ 0.62 mm-mrad emittance of a 250-keV electron bunch (PARMELA) 0.62 mm-mrad  for 77 pC 0.14 mm-mrad  for 7.7 pC (initially, Gaussian in longitudinal and uniform in transverse) 10 ERL Action Plan of Laser Development for ERL KURIKI Masao (KEK)
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1.3 GHz, 700-900 nm (tunable) fund bidding - MEXT-KAKENHI, 2007-2010, ~1M USD for the laser. 20 W @ 700-900nm → 100 mA, ~1 mm-mrad 1.5W @ 700-900mm, 3D shaping → 10 mA, ~0.1 mm-mrad NOPA = non-collinear optical parametric amplifier 3D shaping = deformable mirror (transverse) + pulse stacker (longitudinal) gain-switched LD will be replaced by Yb solid-state oscillator in due time. related description is found in ERL-REPORT-003 (Aug. 17, 2006) http://pfwww.kek.jp/ERLoffice/info/index.html 13
*Non-collinear Parametric Converter ▶Parametric amplification with non-collinear condition make a wave length tune-ability. – For example, 515nm (Driver) -> 800nm(signal) + 1500nm (Idler). ▶It extends our selection range for laser system. –Yb:YAG + Yb fiber for ILC/ERL driver. Phase Matching n,w,+n,w,=n,w,
depend on R&D progress 15 ● we have initiated R&Ds for a photocathode DC gun. ● QE and life have been measured for GaAs and AlGaAs. ● AlGaAs shows better QE and life as predicted by semiconductor theory.  (QE ~ 2x,  life ~ 10x ) ● 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 summer this year. ● a superlattice cathode is also under development. ● design of a drive laser is under way in cooperation with a virtual laboratory, LAAA (Laser- aided Accelerator Association).
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