Analytic RF design of a linear accelerator with a SLED-Ⅰ type RF pulse compressor

This study presents the RF design of a linear accelerator(linac)operated in single-bunch mode.The accelerator is powered by a compressed RF pulse produced from a SLED-I type RF pulse compressor.The compressed RF pulse has an unflattened shape with a gradient distribution which varies over the structure cells.An analytical study to optimize the accelerating structure together with the RF pulse compressor is performed.The optimization aims to maximize the efficiency by minimizing the required RF power from the generator for a given average accelerating gradient.The study shows that,owing to the compressed RF pulse shape,the constant-impedance structure has a similar efficiency to the optimal structure using varying iris apertures.The constant-impedance structure is easily fabricated and is favorable for the design of a linac with a pulse compressor.We utilize these findings to optimize the RF design of a X-band linac using the constant-impedance accelerating structure for the Tsinghua Thomson X-ray source facility.

Optimal Matching of Magnetic Pulse Compressor

Energy transmission efficiency in the magnetic pulse generators varies with saturated time of magnetic switch. An optimal matching time exists and depends on the compression ratio, under which, the energy transmission efficiency can reach approximate 100%. The equation of required magnetic core volume is obtained by taken into account the optimal matching mode. It indicates that a great reduction on the volume is feasible under the optimal matching mode. The circuit simulation code-PSPICE is also introduced to simulate a 3-stage magnetic pulse compressor, and the results are in accordance with those of equivalent circuit analyses.

RF study of a C-band barrel open cavity pulse compressor

This paper focuses on the RF study of a C-band barrel open cavity （BOC） pulse compressor. The operating principle of BOC is presented and the technical specifications are determined. The main parts of BOC such as the cavity, the matching waveguide, the coupling slots and the tuning rings were numerically sinmlated by 3-D codes software HFSS and CST Microwave Studio （MWS）. The ＂whispering gallery＂ mode TM6,1,1 with an unload Q of 100000 was chosen to oscillate in the cavity. An energy multiplication factor of 1.99 and a peak power gain of 6.34 was achieved theoretically.

Design,fabrication,and cold-test results of C-band spherical RF pulse compressor prototype

Purpose Pulse compressor is a useful and common RF structure for Free Electron Laser and Linear Collider Facilities, whichcan improve the input power of accelerating structures significantly.Methods In this study, we designed, fabricated and tested a prototype of a compact C-band spherical pulse compressor.This pulse compressor utilizes one high Q0 spherical RF resonant cavity that works with two degenerated TE1,1,3modes anda dual-mode polarized coupler. The cavity and coupler were designed and machined separately and then brazed together.The prototype was tested carefully using vector network analyzer, and a cold-test system was set up to check the workingconditions of the spherical pulse compressor with inversed input powerResults Associated with the C-band high-gradient accelerating structures, the peak power gain and energy multiply factorcan be as high as 6.1 and 1.85, respectively, in theory. The average power gain is 3.8 in the first 0.372 μs after the phasereverse, which is the accelerator filling time. During the cold test, a peak power multiply factor of 6.04 and an energy multiplyfactor of 1.83 were achieved.Conclusions The test results agreed with the design well. The prototype was successfully made as a pre-research for thefuture energy multiplying scheme of SXFEL.

Design and simulation of a C-band pulse compressor

The design and optimization procedure of a pulse compressor is presented. A C-band （5712 MHz） pulse compressor using a TE0,1,15 mode cylindrical cavity with dual side-wall coupling irises has been designed. Also the coupling coefficient, position of the short plane and size of the bottom groove have been optimized by using HFSS.

RF design of an S-band spherical cavity pulse compressor

The pulse compressor plays an important role in accelerators by enhancing the RF peak power.An S-band(2856 MHz)spherical cavity pulse compressor consisting of a special 3 dB coupler and a single spherical energy storage cavity has been developed in Institute of High Energy Physics.The special 3 dB coupler converts the TE10 mode in rectangular waveguide to two polarization degenerated TE11 modes in cylindrical waveguide.It is also a circular polarizer and its general scattering matrix is given for the first time.The magnitude and the phase difference of the two TE11 modes are−3.016,−3.005 dB and 90.02◦in HFSS,while it is−3.01,−3.01 dB and 89.79◦in CST.The simulated S parameters show agreement with the general scattering matrix.The TE114 mode with an unloaded Q factor of 190,000 has been chosen to resonate in the spherical cavity.An energy multiplication factor of 1.75 and a peak power gain of 5.75 were obtained for the whole pulse compressor,which agrees with the design requirements.A good pulse compression performance is also proved by the CST time-domain simulation.In addition,the scheme for tuning and detuning is designed and given in detail.Compared to the traditional SLED type,the spherical cavity pulse compressor uses only one spherical cavity to realize the pulse compression,which makes it compact as a whole.

Analysis and simulation of a coupled-cavity pulse compressor

An equivalent circuit model is built for a coupled-resonator pulse compressor. Based on the circuit, the general second order differential equation is derived and converted into the first order equation to save computing time. In order to analyze the transient response and optimize parameters for the pulse compressor, we have developed a simulation code. In addition, we have also designed a three-cavity pulse compressor to get the maximum energy multiplication factor. The size of the cavities and coupling apertures is determined by HFSS.

A 100-PW compressor based on single-pass single-grating pair

A multistep pulse compressor(MPC)based on a single-pass single-grating pair(SSGP)is proposed to simplify the entire multi-petawatt(PW)compressor.Only one grating pair with relatively long perpendicular distance is used to generate the same amount of spectral chirp compared with a four-grating main compressor.As SSGP compressor induces the largest spatial chirp,it can introduce the best beam-smoothing effect to the laser beam on the last grating.When considering the diffraction loss of only two gratings,the total compression efficiency of the SSGP compressor is even larger than that of a four-grating main compressor.Furthermore,the wavefront aberration induced by the SSGP compressor can be better compensated by using deformable mirrors;however,it is difficult or complicated to be well compensated in a four-grating compressor.Approximately 50-100 PW laser pulses can be obtained using this SSGP-based multistage-smoothing MPC with a single laser beam.

Error and jitter effect studies on the SLED for the BEPC Ⅱ-linac

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 BEPCⅡ 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/Q 0 /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 BEPCⅡ linac, and the error and jitter effects on SLED performance are also investigated.

Design and simulation of a C-band SLED with mode converter

The transient response analysis of the SLED based on the equivalent circuit is described. Then, a C-band SLED using TE0,1,15 mode cylindrical cavity with TE10-TE01 mode converter has been designed. According to the main RF parameters of the accelerator, the coupling coefficient is optimized to obtain the maximum multiplication factor. The key components of the pulse compressor include a 3 dB directional coupler, a TE10-TE01 mode converter, and a cylindrical cavity, which are simulated and optimized using 3D electromagnetic field simulation software. In addition, the function defining the relation between the coupling factor and aperture size is derived by a mathematical fitting method.

Switching speed effect of phase shift keying in SLED for generating high power microwaves

SLAC energy doubler （SLED） type radio-frequency pulse compressors are widely used in large-scale particle accelerators for converting long-duration moderate-power input pulses into short-duration high-power output pulses. Phase shift keying （PSK） is one of the key components in SLED pulse compression systems. Performance of the PSK will influence the output characteristics of the SLED, such as the rise-time of the output pulse, maximal peak power gain, and energy efficiency. In this paper, a high power microwave source based on power combining and pulse compression of conventional klystrons is introduced. The effects of nonideal PSK with slow switching speed and PSK without power output during the switching process are investigated, and the experimental results with nonideal PSK agree well with the analytical results.

Design and cold test of an S-band waveguide dual circular polarizer

Purpose An S-band dual circular polarizer has been designed and tested with low power in the paper,which will be applied in a spherical cavity pulse compressor.It converts the TE10 mode in rectangular waveguide into two polarization degenerated TE11 modes in cylindrical waveguide.Methods The general scattering matrix of the dual circular polarizer has been deduced based on the properties of the passive microwave network,which is suitable for the usual 3 dB coupler,as well as the left/right hand circular polarizer.Based on the 3D code software CST,the dual circular polarizer was numerically simulated.Then,two prototypes have been manufactured and tested with the vector network analyzer(VNA).Results The simulation results show that the magnitudes and phase difference of the two TE11 modes are dB and 89.79 degrees,respectively.The cold test results show good agreement with CST simulation.−3.0103/−3.0104 Conclusion The method of analyzing the relationship between the voltage standing wave ratio(VSWR)and the errors of the magnitudes and phase difference of the two TE11 modes was given,which can be used to qualitatively analyze the properties of the circular polarizer.The S-band waveguide dual circular polarizer can be applied in the pulse compressor,phase shifter or other frequency bands.