The Steady-State Microbunching (SSMB) mechanism, which combines the benefits of high repetition rate of a storage ring and coherent radiation, has the potential to produce high average power short wavelength light. In order to generate kilowatt level radiation, the electron injector should have the ability to provide a 1 A average current, 100 ns long DC beam, with the requirements of small emittance (<1~mm$\cdot$ mrad), and very small energy spread (<$5\times 10^{-4}$) for the SSMB storage ring. This paper presents an overview of the physical design of the electron gun, linac, and stretching ring components of the injector, as well as the beam loading compensation methods employed in the electron gun and linear accelerator.

We presented a novel concept of longitudinal bunch train compression capable of manipulating relativistic electron beam in range of hundreds of meters. It has potential to compress electron beam with high ratio, and raise its power to ultrahigh level within compressed duration of nanoseconds. Electron’s spiral motion in uniform magnetic field is utilized to fold the hundreds of meters long trajectories into a compact setting. Helix angle of bunches’ spiral track are adjusted by a local time-varying magnetic field. Spiral pitch of each bunch gets gradually increased from the leading edge toward trailing edge of the train. After the spiral procedure, interval between bunches is redefined and the compression is realized. The method is explored both analytically and numerically. Compared to microbunching or chicane modulation, this method could compress bunches at distinct larger scales, opening up new possibilities for generation of beam with ultra-large power storage.

High power input often leads to frequency deviation that cannot meet the high-precision frequency control requirements of keV Ultrafast Electron Diffraction (UED) compression cavities. In this paper, we propose new solu-tions for reducing heat generation and frequency devia-tion based on modifications to the cavity design and power input method, building upon the design of the orig-inal elliptical cavity. These solutions have been verified through simulation calculations. In pulsed input mode, the cavity temperature rise is within 2℃, and in continu-ous wave mode, the new cavity design can withstand temperature rises of up to 20℃, both of which meet the requirements of practical engineering.

With the high accelerating gradient, radiofrequency (rf) gun has a significant feature of suppressing the growth of transverse emittance caused by space charge. Field emission cathodes were first used in vacuum electronic devices, which do not require the high electron beam intensity, but the cathode size and integrality. A new X-band (11.424 GHz) rf electron gun has been proposed with the highlight of four-feed coupler, which can eliminate the quadrupole field component observed and analyzed from the imagine experiment, which have affected the resolution of the imaging system to some content.

In intraoperative radiation therapy (IORT), accelerators typically consist of two or more tubes to achieve adjusta-ble electron energy. To simplify the accelerator structure and meet the demand for convenient adjustment of elec-tron energy, we propose an X-band electron linear accel-erator for IORT, composed of 102 cavities. This accelera-tor can adjust the output electron energy over a large range solely by varying the input power, providing elec-trons with energy exceeding 13MeV at maximum and approximately 5.5MeV at minimum, which satisfies the requirements of electron IORT. We also measured the field distribution and S-parameters at low power, and the ener-gy spectrum distribution also was measured at different input powers. This accelerator design provides a feasible and simple solution for IORT-specific accelerators.

High brightness photoinjectors demand low thermal emittance and high electric field to deliver brighter electron beams for modern accelerator-based scientific instruments. High quantum efficiency, low thermal emittance photocathodes, mainly semiconductors, easily degrade in poor vacuum conditions and could not operate with an extended lifetime. Therefore, an ultrahigh vacuum electron gun is necessary to accommodate advanced photocathodes for high performance and reliable operation. In this paper, we report on the development of an ultrahigh vacuum, high gradient S-band gun at Tsinghua University. The gun geometry is redesigned to reach more than one order of magnitude improvement of the vacuum level at the photocathode. Preliminary commissioning results of the new gun will be presented. The new gun and beamline will partially serve as a test facility for advanced semiconductor photocathodes. We will also report on the design and commissioning results of an alkali antimonide photocathode deposition system.

An ultrahigh dose rate (UHDR) MV-level X-ray radiation platform for FLASH radiotherapy (RT) research based on a normal conducting linear accelerator is presented in this work. A S-band backward traveling-wave linear accelerator powered by a commercial klystron produces electron beams with 11 MeV energy, 300 mA pulse current, and 2.6 mA mean current at 0.88% beam duty ratio. The radiation platform generates X-ray by bremsstrahlung. Flattening filters and collimators are included to produce a 4 cm × 3 cm field with flatted profile dose distribution. The measured dose rate was 129 Gy/s and the flatness was 14% after flattening. The UHDR X-ray platform now is used for FLASH preclinical animal research.

Transverse deflecting cavity (TDC) providing time-dependent kick with fixed polarization is an important tool for beam diagnostics and manipulation. Recently, several types of novel TDC with variable polarization have been developed to fulfill the requirements of multi-dimensional phase space measurement of high-quality electron beam as well as fast scanning in proton therapy. Based on the parallel feeding technology, we propose a new design with alternating racetrack cells where the two chains are fed by waveguide networks independently. Each chain provides fixed polarization in either horizontal or vertical plane and variable polarization can be achieved by adjusting the amplitude and phase of the input power to the networks. The structure has several advantages, such as compactness, tunability, high shunt impedance, etc. In this manuscript, physical and mechanical design of this TDC will be presented in detail.