  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.
