Simulation of a thermionic electron gun including control grid effects

For a thermionic electron gun working at 250 kV and producing bunches up to 85 pC, the measurement of the transverse emittance shows an increase with bunch charge. In order to understand this effect numerical simulations have been performed. The influences of the electrode geometry, space charge and of the control mesh in front of the cathode are included in the simulation. The results show that the growth of the transverse emittance for higher bunch charges is mainly caused by the control mesh. The strong acceleration field penetrates through the mesh holes and for the electrons each hole acts as a small lens. The portion of electrons which are deflected increases with electron current. In the numerical simulation the field and electron trajector calculation was performed with the EGUN code [1]. For the electron transmission throught the potential barrier of the mesh the free electron gas approximation with a local penetration probability, a Maxwell energy distribution and an angular distribution according to Lambert´s law was applied.

1. W.B. Herrmannsfeldt, Report SLAC-331-UC-28, Stanford University, 1988