Speaker
Description
Various modern applications rely on maintaining high electric fields in miniaturized environments surrounded by metallic electrodes. High fields trigger the breakdown and arc that may produce (instantaneously or accumulatively) serious physical damage and put long term operation of a high-field device at risk. There is a particular and critical need in accelerator R&D to understand vacuum breakdown and arc. Looking into 2023-2028, the normal conducting rf roadmap lists the achievement of the accelerating gradient in excess of 300 MV/m as a top priority. C- to X-band systems, planned to utilize those high accelerating gradients, are in development for future colliders, high brightness X-ray sources, or therapy systems. Breakdown and arc are a critical issue in that it hinders the progress of increasing the operating accelerating gradient in high power rf systems.
Studying breakdown directly inside an accelerating or high field structure/environment is the most difficult but most effective way to study fundamental processes underlaying the breakdown phenomenon. Because breakdown takes place on micrometer length scale in macroscopic rf cavities, microscopy and spectroscopy with high lateral resolution and sensitivity have been applied in dc and rf environments to study breakdown in situ, in real time that measured dark current and light emission and arc expansion accompanying/triggering breakdown.
Despite profound experimental and computational work, a basic argument still remains whether final/terminating stage of the breakdown and arc is thermally driven or not. To study and refine this argument and extend prior in situ microscopy findings, we designed a test cavity setup of the field emitter at the X-band frequency. The rf configuration includes an X-band cavity operating at the TM02 mode. The field emitter material is plated with various materials that are suitable to enhance the Nottingham heating channel and thus to attempt to find its relation to breakdown. The gun has two viewing ports to conduct microscopy and spectroscopy at the location of the emitting tip, and a Faraday cup to measure the output current as a function of the gradient. Part of the back plate as well as the emitter rod are demountable from the cavity, allowing for exchange of the field emitter. The TM02 mode was chosen such that the design of the demountable back plate does not induce field enhancement at the installation gap. The field in the gap is also negligibly small. We optimized the depth of the insertion rod and the cavity gap to achieve a macroscopic gradient well above 100 MV/m and a maximum energy gain of the emitted electrons. We will present the rf and mechanical design of the cavity and results of beam dynamics associated with its operation.
Work at MSU was supported by DOE under Award No. DE-SC0020429 and under Cooperative Agreement Award No. DE-SC0018362. Work at SLAC was supported by DOE under contract No. DE-AC02-76SF00515.
