Pulse compressor is a useful and common RF structure for Free Electron Laser and Linear Collider Facilities, which can improve the input power of accelerating structures significantly. 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 TE1,1,3 modes and a 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 working conditions of the spherical pulse compressor with inversed input power Associated with the C-band high-gradient accelerating structures, the peak power gain and energy multiply factor can 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 phase reverse, which is the accelerator filling time. During the cold test, a peak power multiply factor of 6.04 and an energy multiply factor of 1.83 were achieved.
 The test results agreed with the design well. The prototype was successfully made as a pre-research for the future energy multiplying scheme of SXFEL.

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

A compact $X$-band two-stage rf pulse compression system has been successfully designed, fabricated, and tested at Tsinghua $X$-band high-power test stand. The pulse compression system consists of a correction cavity chain, a first-stage, and a second-stage storage cavity. The correction cavity chain adopts a new design whose transmission loss and length are reduced by half compared with the old one. A detuning device is applied to the second-stage storage cavity so that the system can work in one-stage or alternatively two-stage compression mode. In the one-stage compression mode, a 150-ns, 70-MW flattop output, with a standard deviation of 1.5% in amplitude and 1\ifmmode^\circ\else\textdegree\fi{} in phase, was generated with a gain factor of 3. In the two-stage compression mode, a first two-stage pulse compression experiment with correction cavities in the $X$ band was performed. A peak power of 320 MW was achieved with a gain factor of 9.7 and full-width at half-maximum pulse durations of 53 ns. 

X
-band two-stage rf pulse compression system with correction cavity chain

Novel open cavity design for rotating mode rf pulse compressors

In theory, a 180 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> hybrid bridge can be used as a variable power divider and combiner. This study focuses on applying the variable power divider and combiner to a high-vacuum and high-power environment, aiming at realizing future applications in particle accelerators. Beginning from theoretical derivation, this article reports the design of an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S$</tex-math> </inline-formula> -band variable waveguide power divider and combiner based on a Magic Tee. The simulation results are consistent with those predicted by theory. A prototype of the variable power divider was fabricated for the microwave cold test, and the measurement results verify the variable power division. The power division ratio curve is consistent with the theoretical and simulated curves. The variable power combiner was experimentally verified by combining the Magic Tee and coaxial components. The measurement results are consistent with those of numerical analysis. Simulations evaluating the high-power performance reveal that the matching probe of the Magic Tee and the phase shifting plate tip of the straight waveguide phase shifter could not satisfy the high-power requirement. Thus, the structure of the Magic Tee is improved in the proposed design, and a 3-dB bridge waveguide phase shifter was adopted. The simulation results show qualified performance for high-power applications, and other microwave performance characteristics also show great improvement. The study lays a good foundation for application in future high-vacuum and high-power environments. 

An $S$-Band Variable Waveguide Power Divider and Combiner for High-Vacuum and High-Power Applications

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 $$\hbox {TE}_{10}$$
 mode in rectangular waveguide to two polarization degenerated $$\hbox {TE}_{11}$$
 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 $$\hbox {TE}_{11}$$
 modes are −3.016, −3.005 dB and 90.02
 $$^\circ $$
 in HFSS, while it is −3.01, −3.01 dB and 89.79
 $$^\circ $$
 in CST. The simulated S parameters show agreement with the general scattering matrix. The $$\hbox {TE}_{114}$$
 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.

RF design of an S -band spherical cavity pulse compressor

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 TE
 $$_{10}$$
 mode in rectangular waveguide into two polarization degenerated TE
 $$_{11}$$
 modes in cylindrical waveguide. 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). The simulation results show that the magnitudes and phase difference of the two TE
 $$_{11}$$
 modes are − 3.0103/− 3.0104 dB and 89.79 degrees, respectively. The cold test results show good agreement with CST simulation. 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 TE
 $$_{11}$$
 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.

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

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RF design of an S -band spherical cavity pulse compressor

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