SSRF (Shanghai Synchrotron Radiation Facili-ty)/SXFEL (Shanghai Soft X-ray FEL) Facility has de-veloped an advanced variable polarization transverse deflecting structure TTDS (two-mode transverse deflect-ing structure) to perform variable polarization based on the design of a dual-mode RF structure. The 15-cell prototype of the TTDS was fabricated at SSRF/SXFEL. Because the two modes operate in the same structure, any geometric change will affect both modes. A new RF design of the regular cell is proposed to improve rf per-formance. The two modes are coupled independently in two pairs of side coupling holes. The work presented in this paper is focused on the new design and the rf param-eters compared with the initial design.

To generate the very high brightness beams in light sources, injectors based on radiofrequency photo-guns with very high peak electric fields on the cathode are used. However, this very high surface electric field on the surface of a radio frequency cavity leads to the generation of dark current due to the field emission effect which can damage the instrumentation and radio-activate components. Consequently, it is important to reduce the emission of these electrons and evaluate the subsequent transportation. In this paper, the deflector has been innovatively positioned at the exit of the photo-gun so as to reduce the dark current as much as possible. The dark current emission and spectrum of the dark current of the C-band electron gun have been evaluated by Particle-In-Cell simulations. The dark current before the accelerating sections has been captured and observed both with and without the deflector.

Beam lengthening is an effective and commonly used method to improving the beamlife of storage rings. Based on the previously proposed design of a room temperature conducting bimodal RF cavity. we conducted relevant dynamic simulations. Tracking study on a simulated storage ring lattice with the beam energy of 2 GeV and the synchronous radiation energy of 357 KeV, the results show that, the bimodal RF cavity which contains an accelerating field and a third harmonic field can effectively lengthen beam length, the beam lengthening effect similar to the double RF system which consists of main RF cavity and third harmonic cavity.

In order to enhance the accelerating gradient of accelerators, cryogenic accelerating structures have been investigated. Based on material characteristics and technical conditions, a fundamental design has been accomplished. Photonic band-gap (PBG) structures employ a lattice of rods to impede the propagation of RF field through the lattice at specific frequencies while effectively damping higher order modes. The design of the single-cell PBG structure has been refined by altering the shape of the rods surrounding the defect region in order to miti-gate peak surface magnetic field within the structure. The combination of PBG cells and a bi-periodic accelerating structure has resulted in the design of a novel structure. This innovative configuration possesses the advantageous characteristics of a bi-periodic structure while incorporating the additional functionality of a PBG structure to effectively damping higher order modes.

Beam lengthening is an effective and commonly used method to improving the beamlife of storage rings. Based on the previously proposed design of a room temperature conducting bimodal RF cavity, we conducted relevant dynamic simulations. Tracking study on a simulated storage ring lattice with the beam energy of 2 GeV and the synchronous radiation energy of 357 KeV, the results show that, the bimodal RF cavity which contains an accelerating field and a third harmonic field can effectively lengthen beam length, the beam lengthening effect similar to the double RF system which consists of main RF cavity and third harmonic cavity.

SSRF (Shanghai Synchrotron Radiation Facili-ty)/SXFEL (Shanghai Soft X-ray FEL) Facility has de-veloped an advanced variable polarization transverse deflecting structure TTDS (two-mode transverse deflect-ing structure) to perform variable polarization based on the design of a dual-mode RF structure. The 15-cell prototype of the TTDS was fabricated at SSRF/SXFEL. Because the two modes operate in the same structure, any geometric change will affect both modes. A new RF design of the regular cell is proposed to improve rf per-formance. The two modes are coupled independently in two pairs of side coupling holes. The work presented in this paper is focused on the new design and the rf param-eters compared with the initial design.

To generate the very high brightness beams in light sources, injectors based on radiofrequency photo-guns with very high peak electric fields on the cathode are used. However, this very high surface electric field on the surface of a radio frequency cavity leads to the generation of dark current due to the field emission effect which can damage the instrumentation and radio-activate components. Consequently, it is important to reduce the emission of these electrons and evaluate the subsequent transportation. In this paper, the deflector has been innovatively positioned at the exit of the photo-gun so as to reduce the dark current as much as possible. The dark current emission and spectrum of the dark current of the C-band electron gun have been evaluated by Particle-In-Cell simulations. The dark current before the accelerating sections has been captured and observed both with and without the deflector.

In order to enhance the accelerating gradient of accelerators, cryogenic accelerating structures have been investigated. Based on material characteristics and technical conditions, a fundamental design has been accomplished. Photonic band-gap (PBG) structures employ a lattice of rods to impede the propagation of RF field through the lattice at specific frequencies while effectively damping higher order modes. The design of the single-cell PBG structure has been refined by altering the shape of the rods surrounding the defect region in order to miti-gate peak surface magnetic field within the structure. The combination of PBG cells and a bi-periodic accelerating structure has resulted in the design of a novel structure. This innovative configuration possesses the advantageous characteristics of a bi-periodic structure while incorporating the additional functionality of a PBG struc-ture to effectively damping higher order modes.