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 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.

Applications such as observing transient phenomena in materials and real-time terahertz (THz) spectroscopic imaging of molecules have highlighted the importance of high repetition rate, high power THz radiation sources with a broad frequency tuning range. Nevertheless, the development of such sources remains a bottleneck, hindering the advancement of these applications. We have recently developed a high repetition rate, picosecond-pulsed THz parametric amplifier system driven by a passively mode-locked femtosecond fiber laser, which can be synchronized to an external reference signal with sub-picosecond timing jitter. The system operates stably with a frequency tuning range from 1 to 6 THz. It achieves a maximum peak power of 4.3 kW at 5.7 THz when operating at 10 kHz and a maximum average power of 470 μW at the same frequency under 100 kHz operation. This THz source can not only serve as a seed for THz free-electron laser amplifiers, but also provide precise timing control for various THz applications.

In a high-brightness photocathode gun, the non-uniformity in both the transverse distribution of the photocathode drive laser and the quantum efficiency (QE) map can lead to degradation in electron beam quality. This study explores an adaptive transverse laser shaping technique and its application to improve the uniformity of the electrons emitted from the photocathode. Utilizing a spatial light modulator (SLM) with a feedback algorithm, we effectively control the drive laser distribution and achieve single-shot QE map measurement and QE compensation. The experiments conducted on the DC-SRF-II gun demonstrate a rapid and efficient QE measurement process, as well as a notable improvement in electron beam uniformity. Our study offers a promising approach for improving beam quality in accelerator applications that demand high-brightness electron sources.

Continuous-wave photocathode guns are capable of delivering electron beams with high repetition rates and high average currents. However, they are limited by relatively low accelerating gradients and cathode fields. This limitation leads to a larger beam size at the gun exit due to space charge forces, making the beam more sensitive to optical aberrations and posing challenges for low-emittance electron beam transmission. This paper focuses on the aberrations induced by multipole fields. We analyze the effects of quadrupole, sextupole, and higher-order fields on beam profile and transverse emittance and propose a beam-based method for the rapid characterization and correction of these multipole fields. Furthermore, we present an experimental study conducted on the DC-SRF-II photocathode gun equipped with dedicated correctors. Both simulation and experimental results demonstrate that our approach effectively mitigates beam aberrations caused by multipole fields, resulting in improved beam symmetry and reduced emittance.