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Ion Back-Bombardment in RF Guns

Ion Back-Bombardment in RF Guns. Eduard Pozdeyev BNL with contributions from D. Kayran, V. Litvinenko, I. Ben-Zvi. RF Photoguns with NEA Cathode. NEA GaAs photocahtodes: High QE (unpolarized) Polarization (lower QE) RF Photoguns: Good beam quality at high charge/bunch

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Ion Back-Bombardment in RF Guns

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  1. Ion Back-Bombardment in RF Guns Eduard Pozdeyev BNL with contributions from D. Kayran, V. Litvinenko, I. Ben-Zvi E. Pozdeyev

  2. RF Photoguns with NEA Cathode • NEA GaAs photocahtodes: • High QE (unpolarized) • Polarization (lower QE) • RF Photoguns: • Good beam quality at high charge/bunch • Possibly, high average intensity (SRF) • Linac/ERL based applications: • eRHIC and other Linac/ERL based colliders • Electron coolers, conventional high(er) energy and coherent • Light Sources and FELs • Required beam currents > 100 mA! Polarization! E. Pozdeyev

  3. Ion Bombardment in DC photoguns anode cathode Ionized residual gas strikes photocathode Achieved operational current and life time - DC, unpolarized: ~10 mA, ~500 C - DC, polarized: ~500 μA, ~500-1000 C Ion back-bombardment causes QEdegradation of GaAs photocathodes A large portion of ions comes from the first few mm’s of the beam path. This problem is hard to overcome. E. Pozdeyev

  4. Simulation of ion bombardment in RF guns Lewellen, PRST-AB 5, 020101 (2002) Ion bombardment in RF gunsis possible. Results are hard to interpret and extrapolate to other guns. Analytical model is needed for better insight! E. Pozdeyev

  5. Ion in RF field Fast, 0th – order in |a|/L Slow and Fast, 1th – order in |a|/L Proposed by Kapitza (1951), Landau+Lifshitz (Mechanics, 1957), A. V. Gaponov and M. A. Miller (1958) – applied to EM Fields 1) a – fast oscillating term 2) Method is applicable if L ~ a few cm’s,  ~ 10-100 μm 3) Magnetic field is of the order E. Pozdeyev

  6. Effective potential energy 4) Fast oscillations (0th – order in |a|/L): 5) Plugging 4) into 1) and average with respect to time yields: Ponderomotive force E. Pozdeyev

  7. Initial Velocity and Kinetic Energy • Assume: • Ions produced by the beam only, • Ions originate with zero energy and velocity Now the problem can be solved trivially. Important! x’0 depends on the RF phase when ionization happens. E. Pozdeyev

  8. Axially symmetric geometry:on-axis motion - ion motion is 1D Solution of describes ion motion describes areas accessible to ions Electron acceleration force Initial effective ion velocity -/2 <t+ <0, F and x’0 point in opposite directions 0 <t+</2, F and x’0 point in the same direction This can be used to repel ions from ½-cell gun. In multi-cell guns, cathode biasing can be used. E. Pozdeyev

  9. Ions originating close to cathode • Ions and electrons have charges of opposite signs • Ions accelerated towards cathode after ionization • Ions originating close to cathode can reach cathode on the first cycle • If not, they drift away (if phase is right) • The distance is smaller than double amplitude of fast oscil. Ion t 0 z E. Pozdeyev

  10. Off-axis ion motion Electric field on the gun axis: Ea=Ez(z,r=0) Field off axis: Effective potential energy off-axis: Equation of motion: E. Pozdeyev

  11. Off-axis ion motion: Cont’d If r << λ , r and z are decoupled Solve z-motion first, r-motion next using x’z on-axis 1) Numerical solution 2) Solve by iterations E. Pozdeyev

  12. BNL 1/2-cell SRF Gun fRF = 703.75 MHz Emax = 30 MeV/m (on axis) Energy = 2 - 2.5 MeV Iav = 7-50 mA (0.5 A) qb = 0.7-5 nC fb = 10 MHz (up to 700 MHz) =0 E. Pozdeyev

  13. BNL Gun: On-axis motion Beam phase was calculated using PARMELA Beam RF phase E. Pozdeyev

  14. BNL Gun: off-axis motion vs. ionization coordinate E. Pozdeyev

  15. Comparison to a DC gun Common: p=5·10-12 Torr BNL ½-cell Gun: E=2 MeV, Ions come from z<3.36 (E~750 keV) HV DC Gun: Gap = 5 cm, V=650 kV The number of ions in the BNL gun can be reduced by a factor of 5 by (im)proper phasing of the gun (accelerate in phase range 0<t+</2) E. Pozdeyev

  16. Conclusions • Ion bombardment is possible in RF guns • Ions move in the effective potential field • RF phase of the beam defines the effective initial velocity and kinetic energy • Ions move towards the cathode if acc. voltage is growing and from the gun if Vacc is going down. => It is possible to repel most of ions from a ½ -cell gun by a proper phasing. • Ions from the very close vicinity (~50 μm) still will be able to bombard the cathode. This limits gain to DC guns to ~ 10. • Phasing will not work in multi-cell guns. Cathode can be biased to a 100’s V – 1 kV. • Ions cannot penetrate from outside. No biased electrodes needed. E. Pozdeyev

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