Study of electrostatic septum design and its high-voltage discharge protection
In this paper, we introduce the design of electrostatic septum (ESS) for the accelerator of Shanghai Advanced Proton Therapy (SAPT), and discuss its mechanical structure and the material selection of the electrode. The beam loss on the septum is studied, and the calculation results are given by the particle simulation and by the formula which is related to the placement angle and the divergence angle in the horizontal direction. Considering the thermal effect of the beam loss on the head of the septum, the equilibrium temperature at work is calculated. In addition, the distribution of the electric field and the trajectory of the particles are also simulated. The phenomenon of vacuum discharge in the working ESS is analyzed in detail on the relationship between the working current and the enhancement factor of the electric field on the electrode surface. Based on the discharge mechanism, the importance of degassing and cleaning of ESS is analyzed. We also analyzed the effect of external series resistance from circuit and discharge mechanism. It is considered that series resistance within a certain range can increase the breakdown voltage between vacuum electrodes and reduce the irreversible damage to the surface of the electrode system caused by the discharge process. The formula for the value of the resistance is given.