Dark current issues for high gradient C-band medical linac

Abstract

Simulations of dark current emission for C-band linear accelerating structure were performed taking into account the electric field distributions inside the cavities and the field enhancement factor. Matching calculated dark current energy spectra with the experimentally measured ones made it possible to understand the nature of the dark current and to reveal the areas of the accelerating structure requiring improved surface treatment. Microscopic investigation of the copper accelerating cell surface treated using different techniques showed that gas cluster ion beam irradiation results in the best surface smoothness. Furthermore, it removes the surface insulating layers and prevents from subsequent oxidizing. Both effects can significantly reduce the dark current emission.