Avoiding vacuum arcs in high gradient normal conducting RF structures

Abstract

In order to build the Compact LInear Collider (CLIC), accelerating structures reaching extremely high accelerating gradients are needed. Such structures have been built and tested using normal-conducting copper, powered by X-band RF power and reaching gradients of 100 MV/m and above. One phenomenon that must be avoided in order to reliably reach such gradients, is vacuum arcs or “breakdowns”. This can be accomplished by carefully designing the structure geometry such that high surface fields and large local power flows are avoided. The research presented in this thesis presents a method for optimizing the geometry of accelerating structures so that these breakdowns are made less likely, allowing the structure to operate reliably at high gradients. This was done primarilly based on a phenomenological scaling model, which predicted the maximum gradient as a function of the break down rate, pulse length, and field distribution in the structure. The model is written in such a way that it allows direct comparison of different criteria, i.e. the peak electric field, the peak local power flow or “modified Poynting vector” Sc, and the global power flow per iris circumference. Using this method, a set of highly optimized accelerating cells were created, as was a C++ library capable of estimating the performance of an RF structure based on these accelerating cells. This library was used in the rebaselinging of the CLIC machine. In addition to this, the thesis also presents a particle in cell simulation of the initial stages of a vacuum arc. This model follows the development of the arc from a small field emitter into a multi-ampere discharge, tracking the evolution of the plasma and the circuit, as well as the interaction between the plasma and the surface. From this model, our understanding of the importance of the ion bombardment under the plasma sheath was improved, especially the ion energy distribution and its effect on sputtering vs. the effective sputtering yields required for the arc to sustain itself and grow.