High-brightness photoinjectors generate low emittance, ultrashort electron beams that are capable of tracking dynamical states of matter with atomic-scale spatio-temporal resolutions via ultrafast electron scattering, as well as providing precisely-shaped electron beams for advanced acceleration research and large-scale facilities such as free-electron laser and inverse Compton scattering. In this paper, we report on the status of the newly constructed FORTRESS (Facility Of Relativistic Time-Resolved Electron Source and Scattering) beamline at Tsinghua University, which will be dedicated for studies of advanced electron sources and photocathodes, new electron beam manipulation and characterization methods, and ultrafast electron scattering applications. The layout, beam dynamics simulation, initial beam measurement results, as well as main hardware components will be discussed in detail.

A light source project named Very Compact Inverse Compton Gamma-ray Source (VIGAS) is under development at Tsinghua University. VIGAS aims to generate monochromatic high-energy gamma rays by colliding a 350 MeV electron beam with a 400-nm laser. To produce a high-energy electron beam in a compact accelerator with a length shorter than 12 meters, the system consists of an S-band high-brightness injector and six X-band high-gradient accelerating structures. The X-band structure’s frequency is 11.424 GHz, and it adopts a constant gradient traveling wave approach; thus, the iris from the first cell to the end cell is tapered. The total cell number is 72, so we named it XT72. In the last two years, we conducted the design, fabrication, and tuning of the first prototype of XT72. Recently, we finished the high-power test, and the result demonstrates that it has the ability to work at an 80 MV/m gradient. In this paper, we present the latest update on this structure.

High-brightness photoinjectors generate low emittance, ultrashort electron beams that are capable of tracking dynamical states of matter with atomic-scale spatio-temporal resolutions via ultrafast electron scattering, as well as providing precisely-shaped electron beams for advanced acceleration research and large-scale facilities such as free-electron laser and inverse Compton scattering. In this paper, we report on the status of the newly constructed FORTRESS (Facility Of Relativistic Time-Resolved Electron Source and Scattering) beamline at Tsinghua University, which will be dedicated for studies of advanced electron sources and photocathodes, new electron beam manipulation and characterization methods, and ultrafast electron scattering applications. The layout, beam dynamics simulation, initial beam measurement results, as well as main hardware components will be discussed in detail.

A 217 MHz VHF gun operating in CW mode is being developed at Tsinghua University, which will be served as the beam source of the high repetition XFEL facilities and high repetition MeV UED. The cavity profile has been optimized to minimize input power, peak surface electric field, peak wall power density, and multipacting. The fabrication of the gun has been completed, and the frequency and quality factor measured in cold test are in good agreement with simulation expectations. During high power conditioning, 75 kW cw RF power was successfully fed into the gun, corresponding to a cathode gradient of 27  MV/m and a gun voltage of 780 keV.Under this condition, the maximum dark current collected by the Faraday cup at the gun exit was 376 nA. To measure and optimize the beam quality, a test beamline was constructed. After preliminary optimization, the 95% projected transverse emittance was 0.161 μrad for 10 pC, 0.429 μrad for 50 pC, and 0.853 μrad for 100 pC. Now one of the guns has been delivered to Shanghai and installed in the SHINE tunnel. Recently, it was operated in CW mode with ~70 kW input power and generated the first beam successfully.

High brightness electron source is a key component for accelerator based scientific instruments. The beam brightness can be further improved with the combination of high electric field and low thermal emittance cathode. We develop an ultrahigh vacuum S-band electron gun to accommodate advanced semiconductor photocathodes, which are easily degraded in poor vacuum condition. The high power test of ultrahigh vacuum S-band gun has finished and the axial electric field has reached over 100 MV/m with dark current around 500 pC. The vacuum under operation is 2.08e-8 Pa, which is one of the best vacuum condition with regards to such high gradient operation. The gun has been operated with Cs2Te cathode for 3 months without quantum efficiency (QE) degradation. Advanced photocathode deposition system has been setup in a clean room and the deposited photocathodes are transferred through suitcase to load lock system in ultrahigh vacuum environment. We have deposited K2CsSb cathode in the deposition system. The QE is between 1% and 4.5% with driven laser wavelength of 532 nm. The QE and thermal emittance of the K2CsSb have been measured under high gradient in the gun and results are presented in the paper.

In this work, MAX FLASH system (Multi-Angle ultrahigh dose rate megavolt-level X-ray radiation system for FLASH radiotherapy) is presented. This system consists of a rapid RF power distribution network and five linacs vertically installed at different coplanar angles. The distribution network can switch all power to one terminal linac between pulses. Electron beams are accelerated to 10 MeV with more than 400 mA peak currents in the high-performance linac and then convert into X-ray at a compact rotating target. The system aims for a compact FLASH radiotherapy clinical facility with a gantry 3 meter in diameter and 2.5 meter in length, which can be installed in most of hospital radiotherapy treatment rooms. There is reserved space in the gantry for a coplanar CBCT to implement for image guidance. The gantry can rotate to an optimized angle for a better conformality before radiation while the system remains stationary and switches the operating linac during radiation. Construction of the first system prototype, with 40 Gy/s dose rate at 80 cm source-axis-distance, is supposed to be finished in the summer of 2024.