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

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 $6\ifmmode\times\else\texttimes\fi{}{10}^{4}$.