The NSRRC photoinjector generates ultrashort electron beams for the production of superradiant radiation in the 100–500 μm wavelength range using a gap tunable U100 planar undulator. The accelerator consists of an S band, laser driven photocathode RF gun equipped with a compensation solenoid, followed by a 5 m long linear accelerator. Sub picosecond electron bunches are achieved through velocity bunching in the linac. Under specific operating conditions, electrons emitted from the cathode surface may be accelerated to high energies independently of the main beam, which continues to gain energy in the linac. These stray electrons can impact the vacuum chamber, producing unwanted radiation that poses risks to both accelerator components and radiation safety. To suppress dark current, a collimator was installed between the photocathode gun and the linac to intercept these electrons prior to downstream transport. Experimental measurements and numerical simulations are presented, demonstrating the effectiveness of the collimator system in reducing dark current.

The NSRRC Taiwan Photon Source (TPS) LINAC system consists of a DC thermionic electron gun, a Sub-Harmonic Pre-Buncher, a Primary Buncher, a Final Buncher, three S-band LINAC sections, and three 35-MW S-band klystrons. The TPS LINAC was designed by Research Instruments (RI). In the original design, the S-band klystrons were Thales TH2100A tubes. Due to unstable production quality in recent years and the resulting reduction in tube lifetime, NSRRC initiated a klystron upgrade program to replace the TH2100A with the E37310A klystron manufactured by CETD. The primary objective of this work is to analyze the RI-designed LINAC system and complete the necessary modification design so that it can operate with the CETD E37310A klystron and supply the required RF power. Leveraging the technology and experience gained from the in-house development of the THz FEL pulse RF system, NSRRC successfully upgraded the Taiwan Light Source LINAC pulse RF system in August 2023. After six months of stable operation, planning for upgrading the three TPS LINAC pulse RF systems began in 2024. Compared with the TLS system, the RI turnkey LINAC is more complex and requires additional study to support an in-house upgrade. This paper presents the upgrade work and results of the first and second TPS LINAC pulse RF systems completed in September 2025 and January 2026, including system installation, modulator modification design, testing results, and current operational status.