An update on high peak power (HPP) RF processing of 3 GHz nine-cell niobium accelerator cavities

Two 3 GHz, nine-cell niobium accelerator structures have been fabricated and tested multiple times. An unambiguous improvement in cavity performance can be shown due to high peak power (HPP) RF processing of the cavities. The average achieved accelerating gradient prior to HPP processing was E_acc=12 MV/m, (standard deviation=3 MV/m). The average maximum accelerating gradient following all HPP processing was E_acc=17 MV/m, (standard deviation=2 MV/m). Gains in cavity performance can be directly correlated with magnitude of field reached during pulsed HPP processing. Durability of processing gains has been tested by exposing processed cavities to filtered air, at room temperature, and unfiltered air, under both room temperature and cryogenic conditions. Filtered air had no discernable effect on cavity performance. Unfiltered air degraded cavity performance, through increased emission, however much of the cavity performance could be regained through further RF processing