Heating of Micro-protrusions in Accelerating Structures

TL;DR

This paper uses numerical simulations to analyze how micro-protrusions on metal surfaces heat up under RF fields, finding Nottingham heating dominates and space charge prevents melting, informing high-gradient accelerator design.

Contribution

It introduces a unified numerical framework for thermal and field emission, providing new insights into heating mechanisms and the effects of space charge on micro-protrusions.

Findings

Heating is primarily due to the Nottingham effect.

Thermal runaway scenarios are unlikely.

High RF frequency results in smaller temperature swings.

Abstract

The thermal and field emission of electrons from protrusions on metal surfaces is a possible limiting factor on the performance and operation of high-gradient room temperature accelerator structures. We present here the results of extensive numerical simulations of electrical and thermal behavior of protrusions. We unify the thermal and field emission in the same numerical framework, describe bounds for the emission current and geometric enhancement, then we calculate the Nottingham and Joule heating terms and solve the heat equation to characterize the thermal evolution of emitters under RF electric field. Our findings suggest that, heating is entirely due to the Nottingham effect, that thermal runaway scenarios are not likely, and that high RF frequency causes smaller swings in temperature and cooler tips. We build a phenomenological model to account for the effect of space charge and show that space charge eliminates the possibility of tip melting, although near melting temperatures reached.