# Rapid thermal emittance and quantum efficiency mapping of a cesium telluride cathode in an rf photoinjector using multiple laser beamlets

## TL;DR

This paper introduces a rapid, single-scan method using multiple laser beamlets to map thermal emittance and quantum efficiency of cesium telluride cathodes in RF photoinjectors, significantly reducing measurement time.

## Contribution

A novel technique employing multiple beamlets for simultaneous thermal emittance and QE mapping with only one solenoid current scan, validated through simulation and experiment.

## Key findings

- Successful demonstration with seven beamlets showing emittance from 0.93 to 1.14 μm/mm
- QE varied from 4.6% to 8.7% across beamlets
- Method reduces measurement time compared to traditional scanning approaches

## Abstract

Thermal emittance and quantum efficiency (QE) are key figures of merit of photocathodes, and their uniformity is critical to high-performance photoinjectors. Several QE mapping technologies have been successfully developed; however, there is still a dearth of information on thermal emittance maps. This is because of the extremely time-consuming procedure to gather measurements by scanning a small beam across the cathode with fine steps. To simplify the mapping procedure, and to reduce the time required to take measurements, we propose a new method that requires only a single scan of the solenoid current to simultaneously obtain thermal emittance and QE distribution by using a pattern beam with multiple beamlets. In this paper, its feasibility has been confirmed by both beam dynamics simulation and theoretical analysis. The method has been successfully demonstrated in a proof-of-principle experiment using an L-band radiofrequency photoinjector with a cesium telluride cathode. In the experiment, seven beamlets were generated from a microlens array system and their corresponding thermal emittance and QE varied from 0.93 to 1.14 $\mu$m/mm and from 4.6 to 8.7%, respectively. We also discuss the limitations and future improvements of the method in this paper.