Researches on Thermal Emittance of Metal Cathode in Photoinjectors

Theoretical analysis of thermal emittance of photoinjectors is presented in this paper.Thermal emittance of metal photo-cathode mainly consists of two parts.One is due to thermal emittance of ideal flat metal cathode,and the other is due to surface roughness.To analyze the former part of the thermal emittance,a new model for the electron distribution emitted from the photocathode is presented,and a formula of the thermal emittance of an idea flat cathode is derived according to this model,which is e n,rms,flat=R2·hυ-3m 0c2.As to the influence on the thermal emittance from the roughness of the cathode,we also give the analy- tical results with a simplified model.Surface profile measurement of Copper and Magnesium after diamond cutting shows normalized RMS thermal emittance due to surface roughness is about 0.4mm·mrad.Thermal emittance expressions analytical given in this paper can explain some of the existing experimental results very well.