Zhang Bao-Cheng

Design of a low level radio frequency control system for the direct current-superconductive photocathode injector at Peking University

A digital low level radio frequency (LLRF) control system based on a high precision field-programmable gate array (FPGA) is being developed for the stable operation of the upgraded direct current-superconductive (DC-SC) photocathode injector at Peking University The design of this LLRF control system is described, including both the hardware and the internal algorithm. Analysis of disturbances shows that the system can achieve the requirement of ±0.1% for amplitude stability and ±0.1° for phase stability. Through experiments, preliminary results are presented in the paper.

The First Nine-Cell TESLA Cavity Made in China

A totally home-made 9-cell TESLA type superconducting cavity is made at Peking University. The cavity fabrication is according to DESY specification. The cavity is made of high purity niobium from OTIC, Ningxia. The electron beam welding is carried out at Harbin Institute of Technology, Harbin. By the cooperation, the cavity is tested at Thomas Jefferson National Accelerator Facility, USA. The preliminary result shows the acceleration gradient Eacc is 23MV/m without quench and has potential for improvement.

Design of a capacitive main power coupler for RF superconducting accelerators

An updated main coupler has been designed for the superconducting accelerator of Free Electron Laser (FEL) project under construction in Peking University. A capacitive structure is chosen for the main coupler. Numerical investigation using CST Microwave Studio demonstrates the cold window part. The other nonstandard structures such as holding rods and antenna are also optimized. The coupler uses a 95% purity Al2O3 ceramic cold window. The VSWR (Vottage Standing Wave Ratio) is 1.02 at 1.3 GHz and the frequency bandwidth is 45 MHz with VSWR<1.1. The electric field intensity is 8.5×10−2 kV/mm around the window with 20 kW Continuous Wave (CW) transmitted power. The Qext is designed variable from 5×106 to 1×107.

Cold RF test and associated mechanical features correlation of a TESLA-style 9-cell superconducting niobium cavity built in China

The RF performance of a 1.3 GHz 9-cell superconducting niobium cavity was evaluated at cryogenic temperatures following surface processing by using the standard ILC-style recipe. The cavity is a TESLA-style 9-cell superconducting niobium cavity, with complete end group components including a higher order mode coupler, built in China for practical applications. An accelerating gradient of 28.6 MV/m was achieved at an unloaded quality factor of 4 × 109. The morphological property of mechanical features on the RF surface of this cavity was characterized through optical inspection. Correlation between the observed mechanical features and the RF performance of the cavity is attempted.

Amplitude and phase stability studies in a DC superconducting injector

The DC superconducting injector will be used in the PKU-THz facility which consists of a DC-gun and a 3+1/2-cell superconducting cavity. The cavity must accelerate the electron beam to 5.82 MeV which is susceptible to perturbations because of its narrow bandwidth. In this paper, the sources and influences of the perturbations in the 3+1/2-cell cavity are discussed. It is shown that the control system is essential for the cavity. The design of a feedback based digital RF low level control system for the 3+1/2-cell cavity is accomplished.

Analysis of the impacts of mechanical errors on the RF performance of a single spoke cavity

Mechanical errors can not be avoided in fabrication. They will cause geometry errors and have impacts on the cavity performance. This paper systematically analyzes the impacts of mechanical errors on the RF performance of Peking University single spoke cavity. The various kinds of shape and size errors are considered, the influences on the resonation frequency and field flatness are studied by numerical simulation and the theoretical models are analyzed. The results show that the single spoke cavity is robust with respect to the mechanical tolerance. It also indicates the most essential factors for fabrication.

Measuring the performance of the coaxial HOM coupler on a 2-cell TESLA-shape copper cavity

Coaxial High Order Mode (HOM) couplers have been fabricated at Peking University and their RF performance has been measured on a test device consisting of a coaxial transmission line and a 2-cell TESLA-shape copper cavity. The test results on the 2-cell TESLA-shape copper cavity with HOM couplers indicate that the coupler can cut off the fundamental mode TM010 and absorb HOMs effectively after a careful adjustment. The optimal angle of the HOM coupler with the beam tube is found. The initial test results of HOM couplers are presented in this paper.

Stop band characteristics of a TESLA cavity coaxial-type HOM coupler

A TTF-type coaxial higher order modes (HOM) coupler has been used in a TESLA 9 cell cavity. It is impossible to measure the stop band characteristics of the HOM coupler with the cavity. A measurement device for the coaxial transmission line type for the HOM coupler has been designed at Peking University. Experimentally it was shown that the average voltage standing wave ratio of the coaxial transmission line is smaller than 1.08. The experimental results of the stop band characteristics of the TTF-type HOM coupler have been fitted for the simulation. This paper describes the design of the measurement device and discusses the experimental and simulation results of stop band characteristics of the HOM coupler.

Impact on the magnetic compressor due to CSR

When an electron bunch is compressed in a chicane compressor, the CSR (coherent synchrotron radiation) will induce energy redistribution along the bunch. Such energy redistribution will affect the longitudinal emittance as a direct consequence. It will also excite betatron oscillation due to the chromatic transfer functions, and hence a transverse emittance change. So, it is indispensable for us to find a way to alleviate the CSR-caused emittance dilution and the bad result of chicane compressor in PKU-FEL.

Development of high purity niobium used in SRF accelerating cavity

Niobium is widely used in SRF (Superconducting Radio Frequency) cavities due to its excellent superconductivity and workability. With the continuous development of technology, higher demands of material are raised. One of the key issues is that RRR (Residual Resistance Ratio) of the Nb material should be more than 300, which requires that the Nb ingot have even higher RRR. This article introduces the development and the experimental results of high purity niobium in OTIC in Ningxia (Ningxia Orient Tantalum Industry Co. Ltd.), and the test results of the single cell TESLA (Tera Electron volt energy Superconducting Linear Accelerator) shaped cavity manufactured by Peking University using Nb material from OTIC.