Determine the cause of anomalous MgF₂ window damage

Determine why the MgF₂ vacuum window in the 257.5 nm femtosecond photoinjector experienced damage at substantially lower laser fluence during operation than in the instantaneous laboratory damage-threshold experiments.

Background

The photoinjector experiments used 257.5 nm laser pulses with sub-picosecond duration and a 1000 Hz repetition rate. Although MgF₂ exhibited comparatively high transmission retention in short-duration damage tests, its transmissivity decreased during operation at a much lower fluence, accompanied by an observed burn mark. The authors explicitly state that the mechanism responsible for this discrepancy is unresolved. They suggest that ionizing radiation from the electron beam may reduce the laser-induced damage threshold, or that damage may accumulate over a longer period than the instantaneous tests could measure.