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

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 radio-frequency 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 $1.14\mu \mathrm{m}/\mathrm{mm}$ and from 4.6% to 8.7%, respectively. We also discuss the limitations and future improvements of the method in this paper.

Figure 1

Beam line setup in the simulation and experiment.

Figure 2

Definition of beamlet geometry parameters on the cathode (a) and on the screen after the solenoid (b). (1) and (2) in the figure indicate the two beamlets.

Figure 3

The simulated beamlet distribution on the screen as a function of $B_0$. In (a)–(f), $B_0$ is set to 0.1859, 0.1900, 0.1940, 0.1970, 0.1995, and 0.2220 T, respectively. (1) and (2) in the figure indicate the two beamlets.

Figure 4

Beam evolution as a function of $B_0$ when $B_0$ is higher than 0.1995 T. (a) Center-to-center distance. (b) Closest distance between the two beamlets. (c) The rms beam size. Red stars: astra simulation results. Solid blue lines: Matrix calculation results.

Figure 5

(a) $R_{11}$ and (b) $R_{12}$ as a function of $B_0$.

Figure 6

Laser transverse pattern observed on the virtual cathode. The brightness variation among the laser beamlets was caused by the incident laser, as well as nonideal conditions of the MLA system. The third beamlet was centered on the cathode.

Figure 7

Beam images as a function of solenoid strength. $B_0$ in (a)–(d) is 0.2045, 0.2170, 0.2294, and 0.2530 T, respectively. The electron beamlets are marked in (c) following the same laser beamlet index as in Fig. 6.

Figure 8

The rms beam sizes of the seven beamlets (marked by number) as a function of $B_0$. Blue dots: Experimental data. The error bar at each $B_0$ denotes the standard deviation of the measured beam size. Red lines: Fitting with $\overline{\sigma }$ according to Eq. (1).

Figure 9

Measured emittance of beamlet 1 as a function of the beamlet charge. Black circles with error bars: Experimental data. Dashed green line: Converged emittance.

Figure 10

Measured charge as a function of the laser energy for beamlet 1. Blue dots: Experimental data. The error bar denotes the standard deviation of the measured charge. Red line: Linear fitting.

Figure 11

Dependence of thermal emittance on QE. Black circles with error bars: Experiment data of the seven beamlets (marked by number). Green line: Least-square fitting.

Figure 12

(${ϵ}_n-{ϵ}_{\text{therm}})/{ϵ}_{\text{therm}}$ as a function of the beamlet offset on the cathode.

Figure 13

Dependence of (a) $D_{\min }$ and (b) $\sigma /{\sigma }_{\text{waist}}$ on $△x_c$ and $B_0$. The white areas indicate the beamlets cannot be distinguished. Black dotted line: Solenoid strength when the beam size reaches a waist.

Figure 14

Maximum $\sigma /{\sigma }_{\text{waist}}$ at the low $B_0$ end of the distinguishable range as a function of $△x_c$.