Stationary CT is a novel CT technology to significantly improve scanning speed, by using distributed multiple ray sources instead of conventional helical rotation with single source. This work presents an S-band multi-beam accelerator as a multiple MV-level X-ray source for industrial stationary CT application. This accelerator consists of 7 parallel-distributed acceleration cavity and 6 coupling cavity, operating in pi/2 standing-wave mode with a centre frequency of 2998MHz. This structure can generate 0.7 MeV electrons with 100 mA peak current at each beamline according to the imaging requirement. The novel multiple high-energy X-ray source will fill in the blank of source requirements in industrial stationary CT application.

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.

Distributed-coupling structures has been proposed as an advanced type of high-gradient accelerators, RF power flow independently into each cavity.This method has few advantages such as high shunt impedance, superior power efficiency, and low costs. And the most distributed-coupling structures typically set 0° or 180° as the phase advance which can simplify the design.In this study we introduces a new-designed distributed-coupling structures with phase advance greater than 180°. This choice of angle will significantly reduce costs without affecting the shunt impedance.

Stationary CT is a novel CT technology to significantly improve scanning speed, by using distributed multiple ray sources instead of conventional helical rotation with single source. This work presents an S-band multi-beam accelerator as a multiple MV-level X-ray source for industrial stationary CT application. This accelerator consists of 7 parallel-distributed acceleration cavity and 6 coupling cavity, operating in pi/2 standing-wave mode with a centre frequency of 2998MHz. This structure can generate 0.7 MeV electrons with 100 mA peak current at each beamline according to the imaging requirement. The novel multiple high-energy X-ray source will fill in the blank of source requirements in industrial stationary CT application.

A new X-band mode converter for the Very Compact Inverse Compton Scattering Gamma-ray Source (VIGAS) program in Tsinghua University has been fabricated and conducted low-power testing. S11 is under -30 dB with -0.05 dB of S21 at the operating frequency of 11.424GHz according to the low-power test using the vector network analyzer, which is consistent with simulation results.

A new X-band phase shifter for the Very Compact Inverse Compton Scattering Gamma-ray Source (VIGAS) program in Tsinghua University has been fabricated and conducted high-gradient testing. After 10 h of conditioning in the Tsinghua X-band high-power test stand (TPOT), the phase shifter reached a peak power of 72 MW at 230 ns pulse width, and peak power of 82 MW at 130 ns pulse width.

One of the main methods to generate X-rays is to bombard metal targets with electron beams. However, this process introduces uncertainty in the electron transport, which leads to uncertainty in the position and momentum of the secondary X-rays. As a result, the focal spot of the X-rays is larger than the electron beam. In this paper, we use the Monte Carlo software Geant4 to investigate the conditions for minimizing the X-ray focal spot size. We assign different weights to the X-rays according to their energy components, based on the actual application parameters, and calculate the focal spot size for three target materials: lead, copper, and tungsten, finding that when the incident electron energy is in the MeV range and the electron source radius is 1 um, the mass thickness of the target of 1.935×10e-3 g/cm^2 is the limit for achieving the smallest equivalent focal spot size.

We developed a magnetic compression method for relativistic electron beam macro-pulses. Our device, with a significantly larger transfer function R56 compared to the classical chicane structure, enables nanosecond-scale compression of relativistic electron pulses using a compact apparatus measuring just a few meters. This paper introduces the principles of this compression method and presents the results of dynamic simulations.

A new S-band photocathode RF gun proposed for ultrafast electron diffraction (UED) has been designed and optimized. The electron gun works at pi mode and the operating frequency is 2.998GHz. The pulsed RF power loss is 3.2MW and the final kinetic energy of the electron beam is 3.5MeV. The RF gun works at high duty factor of 0.2% and the average power loss reaches 6kW. We have used ASTRA, a space charge tracking algorithm to simulate the beam dynamics and improve the bunch properties. By comparing the simulation results under different conditions, we found that the electron beam has good properties both transversely and longitudinally under some conditions. The simulation of bunch properties helps improve spatial-temporal resolution of UED.

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.

Compact accelerator systems are assuming an increasingly significant role within the domain of radiotherapy. As processing technology continues to mature, X-band accelerators have garnered extensive utilization. This study introduces a design for a side-coupled traveling-wave Ku-band accelerator tube, leveraging established processing methodologies. The envisaged particle output energy stands at 2.5 MeV, with a microwave power source requiring a 300 kW input sourced from a klystron. The microwave design outcomes, derived using ANSYS HFSS, are delineated herein, alongside considerations pertaining to dynamic output and engineering design. Subsequent stages will subject this accelerator tube to processing tests, with the overarching objective of effectively supplanting the natural radiation source Co60 within the realm of radiotherapy.

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.

To develop a high-power, high-efficiency electron irradiation accelerator system with an adjustable electron beam, a novel constant-gradient backward-travelling-wave (BTW) accelerating structure has been designed. This accelerator tube implements a backward-travelling-wave design, which offers the advantages of short filling time and low power reflection, which are characteristic of traveling-wave acceleration structures, and can incorporate a nosecone design to achieve high shunt impedance. The constant-gradient concept is adopted to further enhance the electron beam power and beam efficiency. This paper presents the design of the BTW accelerating structure, encompassing parameter estimation and comprehensive three-dimensional simulations to validate the concept.

In an electron linear accelerator, the continuous beam emitted by an electron gun will become an equally spaced beam train after passing through the bunching section. If the current of the beam is large, its expansion may be more intense than the case where only a single bunch is considered, resulting from the space charge forces between different bunches. In this article, using an algorithm capable of calculating the space charge effects of a beam train containing infinitely many bunches with uniform spacing, we compare bunch trains with different parameters to find the pattern of their space charge effects.

In contemporary radiotherapy, most accelerators employ the scatter technique to achieve a relatively uniform dose distribution of electron beams. However, this method often results in the loss of a substantial number of particles, leading to suboptimal efficiency. This paper proposes a method utilizing permanent magnet components to homogenize the beam, achieving both beam spreading and uniformity within a short distance without particle loss. The proposed homogenization beamline comprises two quadrupole magnets and two octupole magnets, ultimately yielding a square field with a side length of approximately 20 cm. The manuscript includes theoretical derivations and simulation validations, with the physical prototype currently under fabrication. Experimental results will be provided in future work.

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.

A compact pulse compressor with dielectric assistance structure is proposed and simulated. The novel pulse compressor adopts a spherical resonant cavity design with dual-mode polarization mode. A dielectric sphere added at the centre of the spherical cavity can reduce the volume and weight of the pulse compressor and improve the unloaded quality factor of the cavity. A C-band compact storage cavity model is designed and simulated on ANSYS HFSS working on 5.712 GHz. The dielectric permittivity of the dielectric sphere is 9, and the dielectric tangent loss is 0.00001. The simulation of the dielectric-assist resonant cavity with an inner diameter of 34 mm indicates an unloaded quality factor about 72000.