Characterization of photocathode damage during high current operation of the cornellerl photoinjector

The Cornell ERL Photoinjector prototype has recently demonstrated successful operationat 20 mA for 8 hours using a bi-alkali photocathode grown on a Si substrate. Thephotocathode film was grown off center, and remained relatively undamaged; however, uponremoval from the gun, the substrate at the gun electrostatic center displayed significantvisible damage. Here we will describe not only the parameters of that particular highcurrent run, but a suite of post-operation surface morphology and crystallographicmeasurements, including X-ray fluorescence, Xray diffraction, and contact profilometry, performed about the damage site and photocathode film. The data indicate violenttopological changes to the substrate surface, as well as significant inducedcrystallographic strain. Ion back-bombardment is proposed as a possible mechanism fordamage, and a simple model for induced crystal strain is proposed (as opposed to ioninduced sputtering), and is shown to have good qualitative agreement with the spatialdistribution of damage.