Photocathode Behavior During High Current Running in the Cornell ERL Photoinjector

TL;DR

This paper reports on the high-current operation of a photocathode in the Cornell ERL, analyzing substrate damage mechanisms and surface changes after prolonged use at 20 mA.

Contribution

It provides detailed surface characterization and a damage model for photocathodes used in high-current electron sources, highlighting ion back-bombardment effects.

Findings

Significant substrate damage observed after high-current operation.

Surface analysis techniques reveal morphological and crystallographic changes.

A damage model due to ion back-bombardment aligns qualitatively with observed damage.

Abstract

The Cornell University Energy Recovery Linac (ERL) photoinjector has recently demonstrated operation at 20 mA for approximately 8 hours, utilizing a multialkali photocathode deposited on a Si substrate. We describe the recipe for photocathode deposition, and will detail the parameters of the run. Post-run analysis of the photocathode indicates the presence of significant damage to the substrate, perhaps due to ion back-bombardment from the residual beamline gas. While the exact cause of the substrate damage remains unknown, we describe multiple surface characterization techniques (X-ray fluorescence spectroscopy, X-ray diffraction, atomic force and scanning electron microscopy) used to study the interesting morphological and crystallographic features of the photocathode surface after its use for high current beam production. Finally, we present a simple model of crystal damage due to ion back-bombardment, which agrees qualitatively with the distribution of damage on the substrate surface.