A rapid method for prediction of the non-resonant ultra-fast multipactor regime in high gradient RF accelerating structures

The purpose of this work is to present an analytical method that allows to estimate in an approximate and fast way the presence of the non-resonant and ultra-fast multipactor effect in RF accelerating structures in the presence of high gradient electromagnetic fields. This single-surface multipactor regime, which has been little studied in the scientific literature, is characterised by appearing only under conditions of very strong RF electric fields (of the order of tens or hundreds of MV/m), where it is predominant over other types of single- or dual-surface resonance described in classical multipactor theory. This type of multipactor causes a rapid growth of the electron population and poses a serious drawback in the operation of RF accelerator components operating under high gradient conditions. Specifically, in dielectric-assist accelerating structures (DAA) it has been experimentally found that the presence of multipactor limits the maximum operating gradient of these components due to a significant increase in the reflected power due to the discharge, being this phenomenon the main problem to overcome. In a previous work, we found and described in detail by means of numerical simulations the presence of this non-resonant and ultra-fast multipactor regime in a DAA structure design for hadrontherapy. Here we aim to present a simple and fast method to predict the presence of this non-resonant and ultra-fast multipactor regime in RF accelerator structures with cylindrical revolution symmetry around the acceleration axis. This method is especially useful in the design stages of accelerating structures as it provides much faster results than numerical simulations of the multipactor, with quite good accuracy in a wide range of cases as shown in this paper.