E. Pozdeyev1 Ion Back-Bombardment in RF Guns Eduard Pozdeyev BNL with contributions from D. Kayran, V. Litvinenko, I. Ben-Zvi

E. Pozdeyev2 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. Pozdeyev3 Ion Bombardment in DC photoguns Achieved operational current and life time - DC, unpolarized: ~10 mA, ~500 C - DC, polarized: ~500 μA, ~ C Ion back-bombardment causes QE degradation of GaAs photocathodes cathode Ionized residual gas strikes photocathode anode A large portion of ions comes from the first few mm’s of the beam path. This problem is hard to overcome.

E. Pozdeyev4 Simulation of ion bombardment in RF guns Lewellen, PRST-AB 5, (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. Pozdeyev5 Ion in RF field Proposed by Kapitza (1951), Landau+Lifshitz (Mechanics, 1957), A. V. Gaponov and M. A. Miller (1958) – applied to EM Fields L ~ a few cm’s,  ~ μm 1) 2) a – fast oscillating term Method is applicable if Magnetic field is of the order 3) Fast, 0 th – order in |a|/LSlow and Fast, 1 th – order in |a|/L

E. Pozdeyev6 Effective potential energy Fast oscillations (0 th – order in |a|/L): 4) 5) Plugging 4) into 1) and average with respect to time yields: Ponderomotive force

E. Pozdeyev7 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. Pozdeyev8 Axially symmetric geometry: on-axis motion describes areas accessible to ions Solution ofdescribes ion motion 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. - ion motion is 1D

E. Pozdeyev9 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. z t 0 Ion

E. Pozdeyev10 Off-axis ion motion Electric field on the gun axis: E a = E z (z,r=0) Field off axis: Effective potential energy off-axis: Equation of motion:

E. Pozdeyev11 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. Pozdeyev12 BNL 1/2-cell SRF Gun f RF = MHz E max = 30 MeV/m (on axis) Energy = MeV I av = 7-50 mA (0.5 A) q b = nC f b = 10 MHz (up to 700 MHz)  =0

E. Pozdeyev13 BNL Gun: On-axis motion Beam RF phase Beam phase was calculated using PARMELA

E. Pozdeyev14 BNL Gun: off-axis motion vs. ionization coordinate

E. Pozdeyev15 Comparison to a DC gun Common: p=5· 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. Pozdeyev16 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 V acc 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. Conclusions