Beam tracking simulation of photocathode test bench

We study GaAs photo-cathode with NEA (Negative Electron Affinity) surface for polarized and high brightness electron source for International Linear Collider and Energy Recovery Linac. The NEA surface is made by artificial treatment, so that an ultra-low emittance and polarized electron beam can be extracted to vacuum by photo-electron effect with laser photon whose energy corresponds to the band gap. The surface is so sensitive to pollution of residual gas and impact to the cathode surface of ions generated by ionization of residual gas molecule with electron beam. The latter effect is called as Ion Back-Bombardment (IBB) and limits the effective operational lifetime of the cathode. The electrons and generated ions orbits in the photo-cathode test bench of Hiroshima University are simulated by GPT to qualify the effect of IBB. We also consider modifications of the electrode shape to control the effect of IBB in our test bench. (author)