Ion Back-Bombardment in RF Guns

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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 - PowerPoint PPT Presentation

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  • Ion Back-Bombardment in RF Guns Eduard Pozdeyev BNL

    with contributions from D. Kayran, V. Litvinenko, I. Ben-Zvi

    E. Pozdeyev

  • RF Photoguns with NEA CathodeNEA GaAs photocahtodes:High QE (unpolarized)Polarization (lower QE)

    RF Photoguns:Good beam quality at high charge/bunchPossibly, high average intensity (SRF)

    Linac/ERL based applications:eRHIC and other Linac/ERL based collidersElectron coolers, conventional high(er) energy and coherentLight Sources and FELsRequired beam currents > 100 mA! Polarization!

    E. Pozdeyev

  • Ion Bombardment in DC photoguns Achieved operational current and life time- DC, unpolarized: ~10 mA, ~500 C- DC, polarized: ~500 A, ~500-1000 C

    Ion back-bombardment causes QE degradation of GaAs photocathodesA large portion of ions comes from the first few mms of the beam path.This problem is hard to overcome.

    E. Pozdeyev

  • 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

  • Ion in RF fieldProposed by Kapitza (1951), Landau+Lifshitz (Mechanics, 1957), A. V. Gaponov and M. A. Miller (1958) applied to EM FieldsL ~ a few cms, ~ 10-100 m 1)2)a fast oscillating termMethod is applicable if Magnetic field is of the order 3)

    E. Pozdeyev

  • Effective potential energyFast oscillations (0th order in |a|/L):4)5)Plugging 4) into 1) and average with respect to time yields:Ponderomotiveforce

    E. Pozdeyev

  • Initial Velocity and Kinetic EnergyAssume: Ions produced by the beam only,Ions originate with zero energy and velocity Now the problem can be solved trivially. Important! x0 depends on the RF phase when ionization happens.

    E. Pozdeyev

  • Axially symmetric geometry:on-axis motion describes areas accessible to ionsSolution of describes ion motionElectron acceleration forceInitial effective ion velocity-/2
  • Ions originating close to cathode Ions and electrons have charges of opposite signsIons accelerated towards cathode after ionizationIons originating close to cathode can reach cathode on the first cycleIf not, they drift away (if phase is right)The distance is smaller than double amplitude of fast oscil. zt0Ion

    E. Pozdeyev

  • Off-axis ion motionElectric field on the gun axis: Ea=Ez(z,r=0) Field off axis:Effective potential energy off-axis:Equation of motion:

    E. Pozdeyev

  • Off-axis ion motion: Contd If r
  • BNL 1/2-cell SRF Gun fRF = 703.75 MHzEmax = 30 MeV/m (on axis)Energy = 2 - 2.5 MeVIav = 7-50 mA (0.5 A)qb = 0.7-5 nCfb = 10 MHz (up to 700 MHz)=0

    E. Pozdeyev

  • BNL Gun: On-axis motionBeam RF phaseBeam phasewas calculated using PARMELA

    E. Pozdeyev

  • BNL Gun: off-axis motionvs. ionization coordinate

    E. Pozdeyev

  • Comparison to a DC gun Common: p=510-12 Torr

    BNL -cell Gun:E=2 MeV, Ions come from z

  • ConclusionsIon bombardment is possible in RF gunsIons 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 -cell gun by a proper phasing. Ions from the very close vicinity (~50 m) 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 100s V 1 kV.Ions cannot penetrate from outside. No biased electrodes needed.

    E. Pozdeyev