Development of a 500 MHz high power RF test stand

A flexible high power RF test stand has been designed and constructed at IHEP to test a variety of 500 MHz superconducting RF components for the upgrade project of the Beijing Electron Positron Collider (BEPCⅡ), such as the input coupler, the higher order modes (HOMs) absorber and so on. A high power input coupler has been conditioned and tested with the RF power up to 250 kW in continuous wave (CW), traveling wave (TW) mode and 150 kW CW in standing wave (SW) mode. A prototype of the HOMs absorber has been tested to absorb power of 4.4 kW. An introduction of the test stand design, construction and high power tests is presented in this paper.     

325 MHz低β半波长谐振腔设计

对质子加速器中半波长谐振腔（HWR）型的设计进行了研究,完成一种新型HWR超导腔的初步设计。通过对超导腔设计方法及设计原则的研究,结合相关半波长谐振腔的研究现状,充分利用电磁设计软件的功能,对325 MHz低大孔径的半波长谐振腔进行了设计研究。在建模中,针对腔体的不同部位进行比较分析,优化模型形状;在计算中,采用新型有限元网格,使计算快速而结果稳定;在后处理中,使用参数扫描,实现了腔形参数优化;在腔形分析中,计算了二次电子倍增效应,验证了腔体形状的可行性。通过此设计过程,使所设计的新型为0.12的HWR腔具有较好的电磁参数,满足质子加速器的要求,并可应用到实际工程中。     

A coaxial HOM coupler for a superconducting RF cavity and its low-power measurement results

A resonant buildup of beam-induced fields in a superconducting radio frequency(RF)cavity may make a beam unstable or a superconducting RF cavity quench. Higher-order mode(HOM)couplers are used for damping higher-order modes to avoid such a resonant buildup. A coaxial HOM coupler based on the TTF (TESLA Test Facility)HOM coupler has been designed for the superconducting RF cavities at the Proton Engineering Frontier Project(PEFP)in order to overcome notch frequency shift and feed-through tip melting issues. In order to confirm the HOM coupler design and finalize its structural dimensions, two prototype HOM couplers have been fabricated and tested. Low-power testing and measurement of the HOM couplers has shown that the HOM coupler has good filter properties and can fully meet the damping requirements of the PEFP low-beta superconducting RF linac.     

加速100 mA质子束的低β超导半波长谐振腔内的高阶模（英）

基于超导器的连续波运行的高流强质子/氘束被越来越多的大科学装置所采用。在流强低于10 mA情况下，用于加速低段粒子的超导腔的高阶模影响基本可以忽略，但是对于加速百mA强流的低超导腔，其高阶模损耗情况有待研究。近期北京大学设计并加工了一支=0.09、运行频率162.5 MHz的超导半波长谐振腔，用于研究非相对论低超导半波长谐振腔加速100 mA强流质子束时可能涉及到的关键物理问题。重点研究了这支半波长谐振腔内部高阶模损耗的问题，用时域和频域两种方法分别计算了腔的高阶模损失因子，同时计算了考虑加工误差下腔内发生高阶模共振激发的概率。     

Development of fundamental power coupler for C-ADS superconducting elliptical cavities

5-cell elliptical cavities have been selected for the main linac of the China Accelerator Driven subcritical System(C-ADS) in the medium energy section. According to the design, each cavity should be driven with radio frequency(RF) energy up to 150 kW by a fundamental power coupler(FPC). As the cavities work with high quality factor and high accelerating gradient, the coupler should keep the cavity from contamination in the assembly procedure. To fulfil the requirements, a single-window coaxial type coupler was designed with the capabilities of handling high RF power, class 10 clean room assembly, and heat load control. This paper presents the coupler design and gives details of RF design, heat load optimization and thermal analysis as well as multipacting simulations. In addition, a primary high power test has been performed and is described in this paper.     

Study and design of RF coupler for Chinese ADS HWR superconducting cavity

An RF power coupler is a key component of the superconducting accelerating system in Chinese ADS proton linac injector I, which is used to transmit 15 kW RF power from the power source to the superconducting HWR cavity. According to the requirement of working frequency, power level, transmission capability and cooling condition, the physics design of coupler has been finished, which includes RF structure optimization, thermal simulation, thermal stress analysis and so on. Based on this design, the prototype of HWR coupler has been fabricated, and it has successfully passed the high power test.     

Fabrication of the high power input coupler for BEPCⅡ superconducting cavities

The BEPCII storage ring adopts two 500 MHz superconducting cavities (SCC). Each one is equipped with a 500 MHz input power coupler. The coupler is to feed 150 kW power in continuous wave (CW) mode with both standing and traveling wave modes. Due to high power feeding and high frequency of the coupler, its fabrication is a big challenge. The fabrication started with two key components，the window and the antenna. Up to now, two sets including windows and antennas have been made by IHEP. And a 270 kW RF power in CW has passed through the coupler during the high power test. The fabrication details are presented in this paper.     

A coaxial HOM coupler for a superconducting RF cavity and its low-power measurement results

A resonant buildup of beam-induced fields in a superconducting radio frequency (RF) cavity may make a beam unstable or a superconducting RF cavity quench. Higher-order mode (HOM) couplers are used for damping higher-order modes to avoid such a resonant buildup. A coaxial HOM coupler based on the TTF (TESLA Test Facility) HOM coupler has been designed for the superconducting RF cavities at the Proton Engineering Frontier Project (PEFP) in order to overcome notch frequency shift and feed-through tip melting issues. In order to confirm the HOM coupler design and finalize its structural dimensions, two prototype HOM couplers have been fabricated and tested. Low-power testing and measurement of the HOM couplers has shown that the HOM coupler has good filter properties and can fully meet the damping requirements of the PEFP low-beta superconducting RF linac.     

RF thermal and new cold part design studies on a TTF-Ⅲ input coupler for Project-X

An RF power coupler is one of the key components in a superconducting (SC) linac. It provides RF power to the SC cavity and interconnects different temperature layers (1.8 K, 4.2 K, 70 K and 300 K). The TTF-Ⅲ coupler is one of the most promising candidates for the High Energy (HE) linac of Project X, but it cannot meet the average power requirements because of the relatively high temperature rise on the warm inner conductor, so some design modifications will be required. In this paper, we describe our simulation studies on the copper coating thickness on the warm inner conductor with RRR values of 10 and 100. Our purpose is to rebalance the dynamic and static loads, and finally lower the temperature rise along the warm inner conductor. In addition, to get stronger coupling, better power handling and less multipacting probability, one new cold part design was proposed using a 60 mm coaxial line; the corresponding multipacting simulation studies have also been investigated.     

Status of the IHEP 1.3 GHz superconducting RF program

The 1.3 GHz superconducting radio-frequency (SRF) technology is one of the key technologies for the ILC and future XFEL and ERL projects in China. With the aim to develop 1.3 GHz SRF technology, IHEP has started a program to build an SRF Accelerating Unit. This unit contains a 9-cell 1.3 GHz superconducting cavity, a short cryomodule, a high power input coupler, a tuner and a low level RF system. This program also includes the SRF laboratory upgrade, which will permit the unit to be built and tested at IHEP. The unit will be used for the 1.3 GHz SRF system integration study, high power horizontal test and possible beam test in the future. In this paper, we report the recent R&D status of this program. The first large grain low-loss shape 9-cell superconducting RF cavity made by IHEP reached 20 MV/m in the first vertical test in July, 2010. The prototype tuner and low level RF (LLRF) system are under test. The high power input coupler and cryomodule are under fabrication. Several key SRF facilities for 9-cell cavity surface treatment and pre-tuning were successfully commissioned and are in operation.     

Fabrication of the high power input coupler for BEPCⅡ superconducting cavities

The BEPCII storage ring adopts two 500 MHz superconducting cavities (SCC). Each one is equipped with a 500 MHz input power coupler. The coupler is to feed 150 kW power in continuous wave (CW) mode with both standing and traveling wave modes. Due to high power feeding and high frequency of the coupler, its fabrication is a big challenge. The fabrication started with two key components, the window and the antenna. Up to now, two sets including windows and antennas have beam made by IHEP. And a 270 kW RF power in CW has passed through the coupler during the high power test. The fabrication details are presented in this paper.     

Physics design for the C-ADS main linac based on two dif f erent injector design schemes

The China ADS（C-ADS） project proposes to build a 1000 MW Accelerator Driven sub-critical System around 2032. The accelerator will work in CW mode with 10 mA in beam current and 1.5 GeV in final beam energy. The linac is composed of two major sections： the injector section and the main linac section. There are two diferent schemes for the injector section. The Injector-scheme is based on a 325 MHz RFQ and superconducting spoke cavities of the same RF frequency and the Injector-scheme is based on a 162.5 MHz RFQ and superconducting HWR cavities of the same frequency. The main linac design will be diferent for diferent injector choices. The two diferent designs for the main linac have been studied according to the beam characteristics from the diferent injector schemes.     

A vertical test system for China-ADS project injector Ⅱ superconducting cavities

To test superconducting cavities, a vertical test system has been designed and set up at the Institute of Modern Physics (IMP). The system design is based on VCO-PLL hardware and the NI Labview software. The test of the HWR010#2 superconducting cavity shows that the function of this test system is satisfactory for testing the low frequency cavity.     

HOMs simulation and measurement results of IHEP02 cavity

In accelerator RF cavities, there exists not only the fundamental mode which is used to accelerate the beam, but also higher order modes(HOMs). The higher order modes excited by the beam can seriously affect beam quality, especially for the higher R/Q modes. 1.3 GHz low-loss 9-cell superconducting cavity as a candidate for ILC high gradient cavity, the properties of higher order mode has not been studied carefully. IHEP based on existing low loss cavity, designed and developed a large grain size 1.3 GHz low-loss 9-cell superconducting cavity(IHEP02cavity). The higher order mode coupler of IHEP02 used TESLA coupler's design. As a result of the limitation of the mechanical design, the distance between higher order mode coupler and end cell is larger than TESLA cavity.This paper reports on measured results of higher order modes in the IHEP02 1.3 GHz low-loss 9-cell superconducting cavity. Using different methods, Q e of the dangerous modes passbands have been obtained. The results are compared with TESLA cavity results. R/Q of the first three passbands have also been obtained by simulation and compared with the results of the TESLA cavity.     

一种可调耦合度型高功率输入耦合器的设计

旨在设计一支新型、耦合度可调、连续波型高功率输入耦合器，外部品质因数的调节范围为1.5×105~2×106，以适应先进光源技术研发与测试平台超导电子直线加速器束流试验装置不同束流模式下的需要。这是国内首次开展耦合度可调式连续波型高功率输入耦合器的研制。采用HFSS三维电磁仿真软件完成了耦合器的高频结构设计，对耦合器的微波传输、场分布进行了计算；同时采用CST微波工作室对不同内导体天线插入深度下耦合器的外部品质因数进行了仿真计算，结合650 MHz 2单元超导腔进行了耦合度调节实验，仿真计算与实验测试结果基本一致。     

Study on the HOMs coupler design of spoke cavity

The superconducting spoke cavity has been proposed to accelerate the proton in the low energy section of high power proton linac for an Accelerator Driven Sub-critical System (ADS). Considering that the High Order Modes (HOMs) in the superconducting cavity have far reaching influence on power dissipation and beam stability, the analysis of HOMs of the spoke cavity is needed. In this paper, we put emphasis on the analysis of HOMs of the spoke cavity and the HOMs coupler design.     

Study on the HOMs coupler design of spoke cavity

The superconducting spoke cavity has been proposed to accelerate the proton in the low energy section of high power proton linac for an Accelerator Driven Sub-critical System (ADS).Considering that the High Order Modes (HOMs)in the superconducting cavity have far reaching influence on power dissipation and beam stability,the analysis of HOMs of the spoke cavity is needed.In this paper,we put emphasis on the analysis of HOMs of the spoke cavity and the HOMs coupler design.     

RF power coupling for the CSNS DTL

The China Spallation Neutron Source (CSNS) drift tube linac (DTL) consists of four tanks and each tank is fed by a 2.5 MW klystron. Accurate predication of RF coupling between the RF cavity and ports is very important for DTL RF coupler design. An iris-type coupler is chosen to couple the RF power to the DTL accelerating cavity. The physical design of the DTL coupler and the calculations of RF coupling between the cavity and coupler are carried out. The results from the numerical simulations are in excellent agreement with the analytical results.     

RF power coupling for the CSNS DTL

The China Spallation Neutron Source(CSNS)drift tube linac(DTL)consists of four tanks and each tank is fed by a 2.5 MW klystron.Accurate predication of RF coupling between the RF cavity and ports is very important for DTL RF coupler design.An iris-type coupler is chosen to couple the RF power to the DTL accelerating cavity.The physical design of the DTL coupler and the calculations of RF coupling between the cavity and coupler are carried out.The results from the numerical simulations are in excellent agreement with the analytical results.     

High power conditioning of the input coupler for BEPCⅡ superconducting cavity

High power conditioning of the input coupler for BEPCⅡ supercOnducting cavity has been performed.After room temperature conditioning,the RF power of 150 kW with continuous wave at standing wave mode passed through the coupler without any problem.Meanwhile,a series of methods have also been studied to improve the performance of the coupler during the beam operation.Up to now,the input coupler can feed a RF power up to 100 kW stably with high current of 250 mA at 2.5 GeV.