Role of rf electric and magnetic fields in heating of micro-protrusions in accelerating structures

TL;DR

This paper investigates how rf electric and magnetic fields contribute to heating micro-protrusions in accelerating structures, affecting their breakdown and surface deterioration, by deriving power density expressions and analyzing their dominance.

Contribution

It introduces a quantitative analysis of microwave heating effects from rf magnetic and electric fields on micro-protrusions, providing a criterion to evaluate their relative importance.

Findings

Derived expressions for power densities of magnetic and electric field heating.

Formulated a criterion to determine dominant heating mechanism.

Applied analysis to parameters from SLAC experiments.

Abstract

It is known that high-gradient operation in metallic accelerating structures causes significant deterioration of structure surfaces that, in turn, greatly increases the probability of microwave breakdown. At the same time, the physical reason for this deterioration so far is not well understood. In the present paper, the role of two effects is analyzed, viz. (a) the microwave heating caused by penetration of the rf magnetic field into microprotrusion of a radius on the order of the skin depth and (b) the Joule heating caused by the field emitted current, i.e. the effect of the rf electric field magnified by a sharp protrusion. Corresponding expressions for the power densities of both effects are derived and the criterion for evaluating the dominance of one of these two is formulated. This criterion is analyzed and illustrated by the discussion of an example with parameters typical for recent experiments at the Stanford Linear Accelerator Center (SLAC) National Accelerator Laboratory.