High-brightness megahertz-rate electron beams are highly desired for cutting-edge applications in many areas of science. Photocathode electron guns capable of generating such beams with low dark current remain to be a challenging field. In this paper, we report breakthroughs with a hybrid gun combining a direct-current gap and a superconducting radio-frequency (SRF) cavity. The gun, employing K 2 CsSb photocathodes driven by a green laser, delivers a few MeV electron beam at 1 and 81.25 MHz rates with an average current up to 3 mA and a dark current several orders lower than current normal conducting continuous-wave guns. Emittance compensation and multipole field corrections have been applied to achieve a high-quality beam. The achievement will inspire the development of high-brightness SRF guns and promote megahertz-rate beam applications including coherent linac light sources and ultrafast electron diffraction/microscopy. Published by the American Physical Society 2024

Temperature control is important in second harmonic generation (SHG) based on non-critical phase matching, which is widely used in the accelerator field to generate drive lasers. To further improve the stability of the drive laser for the DC-SRF photocathode electron gun at Peking University, a high-precision temperature control oven for lithium borate (LBO) crystals was developed. The oven’s structure was designed to minimize heat exchange with the external environment. The temperature control circuit uses a thermoelectric cooler to ensure the temperature stability of the sampling circuit. The program utilizes a cascaded proportional-integral-derivative and an anti-saturation integral algorithm to achieve high-precision temperature control. Experiments showed that fluctuation at the working temperature of the LBO crystal in this oven was within $\pm 0.009$ °C, corresponding to a root mean square (RMS) jitter of 0.003 °C, and the long-term power fluctuation of the 13.7 W green laser generated with SHG was less than 1%.

A liquid helium-free cryostat for radio frequency (RF) test of the superconducting cavity is designed and constructed. Gifford-Mcmahon (G-M) cryocoolers are used to provide cooling capacity, and the heat leakage at 4 K is less than 0.02 W. Vertical and horizontal tests of two Nb₃Sn cavities are carried out in the cryostat with different surface treatments outside the cavities. Both of the cavities achieve stable continuous wave (CW) operation. A novel treatment, which cold-sprayed a 3.5 mm thick Cu layer onto the outside of the cavity, enables the maintenance of an average temperature of 5.5 K in the cavity at a RF loss of 10 W, implying that the thermal stability and uniformity of the cavity has been significantly improved. Through the synergistic control of four metal film resistors, a cooling rate of 0.06 K/min near 18 K is realized, and the cavity temperature gradient is reduced to 0.17 K/m, which effectively improves the RF performance of the cavity. The maximum $E_{acc}$ of the cavity reaches 3.42 MV/m, and the $Q_0$ is 1.1 × 10⁹. An electromagnetic–thermal coupling simulation model for the superconducting cavity is established and is in good agreement with the experimental results. The simulation results show that the cavity with a Cu-spraying treatment and the thermal links of 5N Al can satisfy the $E_{acc}$ of 10 MV/m under conduction cooling.

A laser-microwave synchronization system using an all-fiber optical-microwave phase detector (AFOM-PD) has been developed and experimentally validated at the Institute of High Energy Physics (IHEP). A Yb-doped fiber amplifier (YDFA) was employed to suppress the relative intensity noise (RIN) of the reference laser. And the root-mean-square (RMS) integrated RIN was reduced from 7.78 × 10^-4 before suppression to 6 × 10^-5 after suppression in the interval of 1 Hz to 1 MHz. The performance of this system has been demonstrated using a mode-locked fiber laser with a repetition rate of 29.1 MHz, in conjunction with L-band and S-band microwave signals. Residual phase noise levels of -118 dBc/Hz and -116 dBc/Hz at the 1-Hz offset frequency have been achieved for the L-band and S-band microwave signals, respectively. The integrated RMS timing jitters measured within the frequency range of 1 Hz to 1 MHz during the same experiments are 18.6 fs for the L-band signals and 6.0 fs for the S-band signals. Tests conducted over a 6-hour period have shown that this system can achieve a long-term RMS timing drift of less than 16 fs. This system has potential applications in the plasma wakefield accelerator currently being developed at IHEP.