WHAT LEADS TO HIGH GRADIENT BREAKDOWN?

Various applications rely on maintaining extreme electric fields in vacuum between metallic surfaces. However, what limits the maximal attainable field in vacuum remains a mystery. While it is well known that late stage development of breakdown involves plasma evolution, the mechanism leading to its nucleation within the vacuum is unknown. As part of the development of the next generation of accelerators it is crucial to understand what leads to breakdown under certain fields. The presentation covers recent experimental and theoretical efforts to establish the possibility that breakdown nucleation happens through a critical transition in the dislocation population driven by stresses as a result of the applied field. A stochastic mean field model was calibrated to a range of available data on breakdown rate variation with applied field. Field and temperature dependencies measured in SLAC and CERN using accelerating structures were used to calibrate and then validate the model. Microscopy work, including SEM and TEM on post mortem samples taken from accelerating structures demonstrate that while breakdown is a violent event involving melting and splashing of the electrode surface, we can identify pre-breakdown sub surface evolution which may lead to subsequent conditioning (stabilization of an increased field). Finally, I will discuss prospects for advancement in mechanism identification using dedicated experiments, some of which are currently underway.