  Field emission is one of the key issues in superconducting RF for particle
accelerators. When present, it limits operating gradient directly or via
induced heat load at 2K. In order to minimize particulate contamination of and
thus field emission in the CEBAF SRF cavities during assembly, a cold ceramic
RF window was placed very close to the accelerating cavity proper. As an
unintended consequence of this, the window is charged by field-emitted
electrons, making it possible to monitor and model field emission in the CEBAF
cavities since in-tunnel operation began. From January 30, 1995, through
February 10, 2003, there were 64 instances of spontaneous onset or change in
cavity field emission with a drop in usable gradient averaging 1.4 (sigma 0.8)
MV/m at each event. Fractional loss averaged 0.18 (sigma 0.12) of pre-event
gradient. This event count corresponds to 2.4 events per century per cavity, or
8 per year in CEBAF. It is hypothesized that changes in field emission are due
to adsorbed gas accumulation. The possible implications of this and other
observations for the International Linear Collider (ILC) and other future
accelerators will be discussed.
