[en] The FLASH L-band (1.3 GHz) superconducting accelerator facility at DESY has a Low Level RF (LLRF) system that is similar to that envisioned for ILC. This system has extensive monitoring capability and was used to gather performance data relevant to ILC. Recently, waveform data were recorded with both beam on and off for three, 8-cavity cryomodules to evaluate the input RF and cavity gradient stability and study the rf overhead required to achieve constant gradient during the 800 (micro)s pulses. In this paper, we present the recent experimental results and discuss the pulse-to-pulse input rf and cavity gradient stability for both the beam on and off cases. In addition, a model of the gradient variation observed in the beam off case will be described.

[en] Generally, a standard bunching system is composed by an SW pre-buncher, a TW buncher and a standard accelerating section. However, there is one way to simplify the whole system to some extent by using the hybrid buncher, which is a combined structure of the SW pre-buncher and the TW buncher. Here the beam dynamics studies on an S-band bunching system with the hybrid buncher is presented, and simulation results show that similar beam performance can be obtained at the linac exit by using this kind of bunching system rather than the standard one. In the meantime, the structure design of the hybrid buncher is also described. Furthermore, the standard accelerating section can also be integrated with the hybrid buncher, which can further simplify the usual bunching system and lower the construction cost. (authors)

[en] To improve the beam performance of the two-bunch acceleration (two macro beam pulses in one macro microwave pulse), a new sub-harmonic bunching system has been designed and optimized for BEPCII Linac. Due to the difference between the new bunching system and the present one, the physical tolerances to the key components must be reconsidered. In this paper, the most important jitter effects in the new bunching system are studied systematically, which include the gun timing, high voltage jitters, the phase and power jitters of the two sub-harmonic bunchers and the phase jitters of the 4-cell traveling wave buncher and A0(accelerating section in the bunching system) accelerating tube. A multi-particle simulation program PARMELA is employed, and the physical tolerances of the jitters have been decided. (authors)

[en] Polarized positrons can be generated through the electron-positron pair creation from circularly polarized gamma-rays hitting a conversion target. Laser-Compton scattering is an efficient method to generate circularly polarized gamma-rays. Simulation studies on these two processes have been done with the Monte Carlo codes, CAIN and GEANT4. Using CAIN to simulate the Laser-Compton scattering process, the energy spectrum of the generated polarized photons could be obtained. GEANT4 was used to study the yield, energy spectrum and the mean polarization of the positrons emanating from the conversion target. To increase the yield of the generated positrons, an optimization study on the thickness of conversion target was also performed. (authors)

[en] The linear accelerator for C-ADS includes two injectors with the output energy of 10 MeV and the beam cur- rent of 10 mA. In order to safeguard the stability, reliability and availability of the linac, as one injector is in operation, the other is at hot stand-by spare. The injectors consist of a electron cyclotron resonance (ECR) proton ion source with the output energy of 35 keV, a low energy transport line (LEBT), a radio frequency quadrupole (RFQ) accelerator and a superconducting accelerator with the output energy of 10 MeV. In order to research and develop the physics and technology of the injector, the working frequencies for injector-1 and injector-2 are chosen to 325 MHz and 162.5 MHz, respectively. The structures for the superconducting accelerator are also chosen differently as the spoke cavity and the half wave resonator, respectively. Here, only the physics design on C-ADS injector-1 RFQ is presented in detail. (authors)

[en] An RF pulse compressor is a device used to convert a long RF pulse to a short one with a much higher peak RF magnitude. SLED can be regarded as the earliest RF pulse compressor to be used in large-scale linear accelerators. It has been widely studied around the world and applied in the BEPC and BEPC Ⅱ linac for many years. During routine operation, error and jitter effects will deteriorate the performance of SLED, either on the output electromagnetic wave amplitude or phase. The error effects mainly include the frequency drift induced by cooling water temperature variation and the frequency/Q₀/β unbalances between the two energy storage cavities caused by mechanical fabrication or microwave tuning. The jitter effects refer to the PSK switching phase and time jitters. In this paper, we re-derive the generalized formulae for the conventional SLED used in the BEPC Ⅱ linac, and the error and jitter effects on SLED performance are also investigated. (authors)

[en] According to the SLED (SLAC energy doubler) working principle, the error effect of the 3 dB coupler on the SLED performance has been studied theoretically. In the meantime, the effect of the 3 dB coupler on the SLED used in the BEPC and BEPC Ⅱ linac is investigated. These analyses provide theoretical reference for judging the waveguide distortion of the 3 dB directional coupler after final assembly and welding. Both power imbalance error and phase difference deviation will cause reflection of the RF power back to the klystron. The larger the power imbalance error and the phase difference deviation are, the more the power multiplication factor decreases. (authors)

[en] In order to double the injection rate of positron beam from the linac to the storage ring of BEPC II, a two-pulse generation and acceleration scheme has been proposed. The two-pulse simulation by programs including LIAR, PARMELA, EGUN and TRANSPORT is described first and the method is applied in the beam dynamics studies of BEPC II linac. The experiment of two-pulse acceleration was performed in BEPC II linac and some preliminary results are obtained, which provides a good reference for further upgrading of BEPC II injector linac. (authors)

[en] The BEPC II future pre-injector consists of a thermionic gun followed by two subharmonic bunchers (SHB), a travelling wave prebuncher and a travelling wave buncher. All components downstream of the gun are immersed in a solenoid field for transverse focusing. Beam dynamics simulation and optimization have been carried out with programs PARMELA and EGUN. SHBs' bunching voltage and bunching drift distance, prebuncher and buncher's phase and acceleration gradient, and solenoid field profile have been studied. The bunch charge limitation for 10 ps bunch length at the buncher exit is also investigated

[en] The structure optimization of 142.8 MHz sub-harmonic buncher has been carried out with two-dimensional Finite Difference Method software Superfish and three-dimensional Finite Integration Method software MAFIA. The method of optimizing the cavity structure is presented in this paper, and the optimized structure of the cavity is also decided. The simulation results of Superfish and MAFIA are analyzed and compared, and they are consistent with the design data. In addition, the application of methods of Superfish and MAFIA in the cavity design are also investigated