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.
Highlights
Beam brightness, defined by current over emittance, is one of the most important properties of photoinjectors
The beam brightness of a photoinjector heavily depends on the photocathode, because its thermal emittance sets the lower boundary of beam emittance and its quantum efficiency (QE) determines the current with a certain incident laser
Most cathode studies work on the average thermal emittance and QE of large areas [18,19,20,21,22,23,24]; several groups have begun to develop mapping technologies to look into the detailed distributions of these key properties over the cathode [25,26,27,28,29,30,31,32,33,34]
Summary
Beam brightness, defined by current over emittance, is one of the most important properties of photoinjectors. Most cathode studies work on the average thermal emittance and QE of large areas [18,19,20,21,22,23,24]; several groups have begun to develop mapping technologies to look into the detailed distributions of these key properties over the cathode [25,26,27,28,29,30,31,32,33,34]. Several QE mapping technologies have been developed and applied in photoinjectors, and nonuniform QE distribution has been observed [30,31,32,33] These variations could be caused by localized surface conditions, such as physical and chemical roughness, material defects, and contaminants, etc., [27,30]. According to the correlation between QE and thermal emittance [9,35,36,37,38], these conditions would lead to localized variations in thermal emittance
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