Possible role of rf melted microparticles on the operation of high-gradient accelerating structures

Abstract

High-gradient accelerating structures should operate reliably for a long time. Therefore studies of various processes which may lead to disruption of such an operation are so important. In the present paper, the dissipation of rf electromagnetic energy in metallic microparticles is analyzed accounting for the temperature dependence of the skin depth. Such particles may appear in structures, for example, due to mechanical fracture of irises in strong rf electric fields. It is shown that such microparticles with dimensions on the order of the skin depth, being immersed in the region of strong rf magnetic field, can absorb enough energy in long-pulse operation to be melted. Then, the melted clumps can impinge on the surface of a structure and create nonuniformities leading to field enhancement and corresponding emission of dark current. Results are given for several geometries and materials of microparticles.

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

“…We suppose that these defects occur as a result of the deep fatigue growth which stimulates breakdown. However, the obtained results do not allow one to give full interpretation of the specific mechanisms of breakdown described in [13,14], which were mentioned in the Introduction. Possible reasons of the breakdown could be numerous melted particles observed in the experiment as well as the field enhancement at the sharp edges of the cracks.…”

“…Two mechanisms could be important. 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.…”

Electrostatic-elastoplastic simulations of copper surface under high electric fields

“…In other studies [27] the influence of Joule heating and Nottingham effect on heating of a microprotrusion was analyzed. In [28], the role of micrometer size molten particles was investigated in high gradient systems. However, in these works, preexisting protrusion or surface defect was assumed.…”

Molecular dynamics simulations of near-surface Fe precipitates in Cu under high electric fields

“…In [22], heating of a microprotrusion, caused by Joule heating and the Nottingham effect, was studied. In other works, the influence of microscale molten particles in a high gradient system was investigated [23]. The finite element method was applied in [24,25] to investigate the field enhancement arising at the edges of the microcracks that can appear due to fatigue in the material under repeated loading.…”