Enhanced terahertz generation via plasma modulation on two bunches

Terahertz(THz)radiation finds important applications in various fields,making the study of THz sources significant.Among different approaches,electron accelerator-based THz sources hold notable advantages in generating THz radiation with narrow bandwidth,high brightness,high peak power,and high repetition rate.To further improve the THz radiation energy,the bunching factor of the free electron bunch train needs to be increased.We propose and numerically reveal that,by adding an additional short-pulse drive beam before the main beam as the excitation source of nonlinear plasma wake,the bunching factor of the main beam can be further increased to∼0.94,even though with a relatively low charge,low current,and relatively diffused electron beam.Two such electron beams with loose requirements can be easily generated using typical photoinjectors.Our work provides a way for a new THz source with enhanced radiation energy.

RF trapping and acceleration in CSNS/RCS

In this paper, two injection scenarios with different chopping rate are discussed. The waveforms of the RF voltage are studied and optimized, respectively. Some suggestions are made, concerning chopping and momentum painting of the injected beam. Furthermore, the momentum spread and transverse tune shift are calculated so that the beam aperture and the beam loss can be estimated. Finally, the beam loss with magnet field error is analyzed.

Enhanced high gain harmonic generation scheme with negative dispersion

The enhanced high gain harmonic generation （EHGHG） scheme has been proposed and shown to be able to significantly enhance the performance of HGHG FEL. In this paper we investigate the EHGHG scheme with negative dispersion. The bunching factor at the entrance of the radiator is analyzed, which indicates that the scheme with negative dispersion can further weaken the negative effect of the dispersive strength on the energy spread correction factor. The numerical results from GENESIS （3D-code） are presented, and are in good agreement with our analysis. Then we comparatively study the effects of the initial beam energy spread and the relative phase shift on the radiation power. The results show that the EHGHG scheme with negative dispersion has a larger tolerance on the initial beam energy spread and a nearly equal wide good region of the relative phase shift compared with the case of positive dispersion.

Theoretical study of a dual harmonic system and its application to the CSNS/RCS

Dual harmonic systems have been widely used in high intensity proton synchrotrons to suppress the space charge effect, as well as reduce the beam loss. To investigate the longitudinal beam dynamics in a dual rf system, the potential well, the sub-buckets in the bunch and the multi-solutions of the phase equation are studied theoretically in this paper. Based on these theoretical studies, optimization of bunching factor and rf voltage waveform are made for the dual harmonic rf system in the upgrade phase of the China Spallation Neutron Source Rapid Cycling Synchrotron （CSNS/RCS）. In the optimization process, the simulation with space charge effect is done using a newly developed code. C-SCSIM.

Parameter study for enhanced high gain harmonic generation scheme

An easy-to-implement scheme called Enhanced High Gain Harmonic Generation is expected to be able to significantly enhance the performance of HG-FEL. In this paper we investigate the effects of the system parameters in the new scheme, including the electron energy detuning, initial electron-beam energy spread, seeding laser power, dispersive field strength and amount of the phase shift, etc. The numerical results from GENESIS （3D-code） are presented and show that the new scheme has acceptable parameters tolerance requirements and is no more or even less sensitive to the system parameters than that of the existing scheme; With the electron energy above the resonance, the efficiency is enhanced for both the new scheme and the existing scheme compared with the resonant energy case.

Studies of dual-harmonic acceleration at CSNS-Ⅱ

The Rapid Cycling Synchrotron （RCS） of the China Spallation Neutron Source （CSNS） complex is designed to provide 1.56×10^13 protons per pulse （ppp） during the initial stage, and it is upgradeable to 3.12× 10^13 ppp during the second stage and 6.24× 10^13 ppp during the ultimate stage. The high beam intensity in the RCS requires alleviation of space charge effects to reduce beam losses, which is key in such high beam power accelerators. With higher intensities in the upgrading phases, a dual-harmonic RF system is planned to produce flat-topped bunches that are useful to reduce the space charge effects. We have studied different schemes to apply the dual-harmonic acceleration in CSNS-Ⅱ, and have calculated the main parameters of the RF svstems, which are presented in this Darter.

Optimizing the RF cavity parameters at CSNS-II with Particle Swarm Optimization

Objective This study is aimed at optimizing the parameters of a dual-harmonic RF systemwhich will be used in CSNS-II to have a larger bunching factor and less particle losing to decrease the space charge effect.Methods By studying the synchrotron equation of motion in a double RF system,some features of the bunch are quantized.The normalized bucket area should be constant,the injection condition should be satisfied,and larger bunching factor is preferred during the cycle period.With these ideas,three objective functions are proposed.Particle swarm optimization(PSO)method is employed to search the optimal solution.Results There is less particle loss in the simulation with the optimized parameters.The growth rate of the transverse emittance is smaller after the optimization.The bunch can keep a larger bunching factor which is very useful to decrease the space charge force.Conclusion The parameters of the dual-harmonic RF system are optimized with swarm optimization method.This study provides a general method to optimize the similar system,especially for an accelerator.