Experiment on pulse heating and surface degradation of a copper cavity powered by powerful 30 GHz free electron maser

N S Ginzburg,A K Kaminsky,A A Vikharev,I I Golubev,N I Zaitsev,S N Sedykh,E A Perelstein,S V Kuzikov,A S Sergeev,A P Sergeev,N Yu Peskov,A P Kozlov,M I Petelin

doi:10.1103/physrevstab.14.041002

Abstract

Experiments to investigate copper surface fatigue caused by pulsed rf radiation were carried out using the 30 GHz free electron maser. The copper surface of a special test cavity was exposed to $15--20\text{ }\text{ }\mathrm{MW}/150--200\text{ }\text{ }\mathrm{ns}$ rf pulses with a repetition rate of 1 Hz, providing a temperature rise of up to $250\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in each pulse. An electron microscope was used to study the copper surface both before and after exposure to ${10}^{4}--{10}^{5}$ rf pulses. An examination of the copper microstructure and cracks which developed during the experiment was made. Dramatic degradation of the copper surface and causes of very frequent breakdown were observed when the total number of rf pulses reaches $6\ifmmode\times\else\texttimes\fi{}{10}^{4}$.

Highlights

The Compact Linear Collider (CLIC) project is currently undergoing intensive development
The fatigue effect was already known as parasitic phenomena arisen in high power, high repetition rate electron devices where the beam collector surface is heated by spent electrons [1]
The physical cause of this effect is fast heating of the Ohmic skin layer caused by high-power rf radiation leading to thermal expansion of the hot upper metal layer which is resisted by the internal cold layers

Summary

INTRODUCTION

The Compact Linear Collider (CLIC) project is currently undergoing intensive development This collider is designed to achieve an acceleration gradient of $100 MV=m using 12 GHz=200 ns pulses at a repetition rate of $50 Hz. When operating at the design parameters, the temperature rise in the accelerating structures during the pulse would be $50–60C. The physical cause of this effect is fast heating of the Ohmic skin layer caused by high-power rf radiation leading to thermal expansion of the hot upper metal layer which is resisted by the internal cold layers. The first mechanism is based on the fact that local temperature rise at microinhomogeneities of the surface is essentially higher than the average temperature rise of the rest surface [13] This explains an appearance and growth of the melted particles which due to metal evaporation could introduce trigger sources of the breakdown.

EXPERIMENTAL SETUP

DEMONSTRATION OF BRAGG FEM OPERATION INTO A HIGH-Q LOAD

RESULTS

SUMMARY