Measurements of copper and cesium telluride cathodes in a radio-frequency photoinjector

Radio-frequency (rf) photoinjectors are commonly used to generate intense bright electron beams for a wide range of applications, most notably as drivers for X-ray Free-Electron Lasers. The photocathode, mounted inside an rf gun and illuminated by a suitable laser, thereby plays a crucial role as the source of the electrons. The intrinsic emittance and the quantum efficiency of the electron source are determined by the properties of the photocathode’s surface material. We present measurements of the intrinsic emittance and the quantum efficiency performed with copper and cesium telluride cathodes in the same rf photoinjector, thus comparing, for the first time, the performance of metal and semiconductor cathodes under the same conditions. Our results are consistent with theoretical expectations and show that the difference in intrinsic emittance for the two types of material is not significant in view of accelerator applications. We conclude that cesium telluride photocathodes provide a much higher quantum efficiency at essentially negligible degradation in beam emittance.

Figure 1

SwissFEL cathode plugs parked in the carousel holder of the loadlock chamber. A disc of 10 mm diameter of ${\mathrm{Cs}}_2\mathrm{Te}$ has been deposited on the front surface.

Figure 2

Quantum efficiency evolution of the five photocathodes tested at the SwissFEL Injector Test Facility. Time zero indicates the beginning of operation for each cathode individually.

Figure 3

Measured emittance along the bunch length (slice index) for different bunch charges $Q$ (upper plot). Core slice emittance as a function of the bunch charge (lower plot). For charges equal or below 1.5 pC the slice emittance is not affected by space charge.

Figure 4

Examples of normalized intrinsic emittance measurements for two of the cathodes. In the ${\mathrm{Cs}}_2\mathrm{Te}$-17 case accelerator conditions were more stable than for Cu-19, resulting in a smaller error on the normalized intrinsic emittance.

Figure 5

Measured normalized intrinsic emittances for the five cathodes studied at the SwissFEL Injector Test Facility.