M. S. Yeh

Successful Operation of the 500 MHz SRF Module at TLS

A project to replace two existing room-temperature radio frequency (RF) cavities by one CESR-III 500 MHz superconducting radio frequency (SRF) module was initiated for the Taiwan Light Source (TLS) synchrotron ring in 1999. The goals are to double the photon flux of the synchrotron light by doubling the electron beam current and to increase the stability of the electron beam by taking the advantage of the ultra-weak high-order modes (HOM) of the SRF cavity. The SRF module has been routinely operated since February 2005. The NSRRC users have benefited from a very high photon flux stability (Δ I0/I0∼ 0.05%) that had never been achieved previously. Here, we report the initial operational experience of the SRF system.

Elastoplastic Buckling on the Bent Waveguide of CESR-Type SRF Cavity

A superconducting radio-frequency (SRF) cavity module of CESR-type has been adopted for some advanced accelerators. During operation, the pressure in the liquid-helium vessel acts on the cavity wall, but an ultrahigh vacuum must prevail inside the cavity; the cavity structure must be thus pressure-tested at ambient temperature as a standard procedure for safety. During the pressure test elastoplastic buckling might occur on this SRF cavity and its bent waveguide section, being a shell-like structure. A nonlinear finite-element model for computation is established to assess the mechanism of buckling and post-buckling of the bent waveguide. Some test results with copper bent waveguides are presented. The critical pressure and buckling pattern are strongly affected by the material property and the thickness of the structure and the U-channel.

Cryogenic‐related Performance of an SRF Cavity Module in NSRRC

A superconducting 500‐MHz cavity module has been installed into the electron storage ring of NSRRC. This SRF module is tested on both the RF and cryogenic performances, before and after installation into the electron ring. Calibrations and measurements on its cryogenic load at different operating helium bath pressures are described and concluded. The test results of unloaded quality factor are reported. Meanwhile the excellent regulation on helium bath pressure is so advantageous to all these measurements. During normal operation with RF power, fluctuations of the helium bath pressure and liquid helium level are +/− 1.38 mbar (0.02 psi) and +/−0.2%, respectively.

Commissioning and Operations Results of the Industry-Produced CESR-Type SRF Cryomodules

Upon signing a technology transfer agreement with Cornell University, ACCEL began producing turn-key 500 MHz superconducting cavity systems. Five such cryomodules have been delivered and commissioned to date. Four of them are installed in accelerators for operation (two in CESR and one each in Canadian Light Source and Taiwan Light Source) and one serves as an off-line spare at CLS. One more cryomodule is scheduled for testing in early 2005. It will be a spare unit for TLS. Three cryomodules for DIAMOND Light Source are being fabricated at ACCEL. The commissioning results and operational experience with the cryomodules in CESR, CLS and TLS are presented.

Effects of the Passive Harmonic Cavity on the Beam Bunch

In this paper, we present a computer tracking code, which can investigate the bunch length, energy spread and the threshold current of Robinson instability under the influence of the passive harmonic cavity. The effects of the radiation damping, quantum excitation and the beam loading of the harmonic cavity are included in the computation. The calculated result shows that the beam has a constant energy spread and blows up as the beam current increases from below to over the threshold current of the Robinson instability. It also indicates that the shunt impedance of the harmonic cavity is critical for whether the harmonic cavity can reach the designed goal, a stable and lengthening beam at the design beam current.

Digital Low Level Radio Frequency System for the Booster Ring of the Taiwan Photon Source

The purpose of a Low-Level Radio Frequency (LLRF) system is to control the accelerating cavity field amplitude and phase. For the Taiwan Photon Source (TPS) at NSRRC, the currently operating LLRF systems are based on analog technology. To have better RF field stability, precise control and high noise reduction, a digital LLRF control systems based on Field Programmable Gate Arrays (FPGA) was developed. We replaced the analog LLRF system with the digital version for the TPS booster ring at the beginning of 2018, and we will replace those in the storage rings in the future. Test results and operational performance of the TPS booster DLLRF sys-tem are reported here.

Quarter wavelength combiner for an 8.5kW solid state amplifier and conceptual study of hybrid combiners

Experimental results to combine ten 900W solid-state amplifier modules based on typical quarter wavelength 10-way combiners are described for a total of 8.5kW RF power output at 500 MHz. The power gain and phase distribution among the ten modules are measured and calculated to sense the combination efficiency. The combination efficiency of 100 modules differing in power gain and phase distribution is theoretically analysed. Groups of 5, 10, 25, 50 and 100 units are used in 4, 3, 2, and 1-stage power combination for total 100 units and the characteristics are calculated and investigated, including bandwidth, efficiency and even redundancy under various output VSWR levels. To simplify combining complexity and to eliminate the drawbacks of single stage combiners, a multi-way 2-stage coaxial to waveguide combiner is thus proposed as an expandable power combiner. INTRODUCTION A project for a home-made 60 kW solid-state power amplifier (SSPA) system is in progress at NSRRC. Numerous efforts have been made on the module design including planar balun with better heat dissipation, integrated water cooling plate [1], a temperature measurement circuit and a high directivity PCB directional coupler. After production of some modules, they were used for power combination. These handmade SSPAs were not very identical in gain and phase distribution, but the power combination efficiency still performed satisfactory. To find a relationship between gain and phase distribution on one side and their combination efficiency, circuit models with adjustable power gain and phase distribution on a RF circuit simulator were done based on the measurement data. The range of power and gain distribution among modules seems not to be very narrow at +/0.2dB and +/5deg, respectively [2]. A Gaussian distribution is applied to groups within +/-0.5 dB &+/-10deg and +/1dB & +/20deg to estimate the efficiency of a 100-unit power combination. In addition to the combination efficiency, combination methodology also affects the system performance as does insertion loss and bandwidth. In general, the SSPAs will be combined first in a basic group as a medium power unit, then basic groups are combined until the required power and quantity is reached. Varying the module numbers in a basic group for 1-stage to 4stage combinations of 100 units and complete combiner systems are analysed to obtain system bandwidth and redundancy capability under different output VSWR. To reduce the number of combination stages, a multi-port coaxial to waveguide combiner is proposed as an expandable twostage power combiner. COMBINATION EFFICIENCY ANALYSIS FOR AN 8.5 kW 10-WAY SSPA The maximum power combination efficiency can be obtained when all SSPA units are identical. However, the actual modules cannot be identical due to the dispersion of errors from different chips, passive components, to misaligned soldering etc. Each one contributes some error leading to big differences between modules which can be controlled by trim capacitors, but some errors still remain. How these errors affect the overall power combination efficiency is an interesting and practical point of study. We start from an experimental 8.5kW, 10-way combiner to a theoretical 100-way power combination for different error distribution ranges in this article. The measured power gain and phase as a function of output power for each handmade module is shown in Fig. 1 for a power gain and phase distribution within +/0.5dB and +/18 degrees of experimental values. According to measured power gain and phase distributions at the saturation point, the estimated and measured combination results are shown in Table 1. The measurement results are close to calculation while the combination efficiency is the ratio of combined output power to the total power of 10 modules. From the result, the combination efficiency seems not to be very sensitive to the range of error distribution. Figure 1: Power gain and phase distribution among the 10 handmade SSPA modules. Table 1: Calculated and Measured Combined Power Measurements of the 10 handmade SSPA Modules Calculated Measured Combined output power [kW] 8.87kW 8.68kW Combination efficiency [%] 99.37% 97.24% Combining Efficiency Under Various Dispersion The available overall SSPA efficiency derives from a combination of a few factors: the AC-DC conversion efficiency (EffAC-DC), the insertion loss/mismatch loss of passive transmission line components including combin16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5 20.0 0 200 400 600 80

Multipacting in a coaxial coupler with bias voltage for SRF operation with a large beam current

A superconducting radio-frequency (SRF) module is commonly used for a high-energy accelerator; its purpose is to provide energy to the particle beam. Because of the low power dissipation and smaller impedance of a higher-order mode for this module, it can provide more power to the particle beam with better stability through decreasing the couple bunch instability. A RF coupler is necessary to transfer the high power from a RF generator to the cavity. A coupler of coaxial type is a common choice. With high-power operation, it might suffer from multipacting, which is a resonance phenomenon due to re-emission of secondary electrons. Applying a bias voltage between inner and outer conductors of the coaxial coupler might increase or decrease the strength of the multipacting effect. We studied the effect of a bias voltage on multipacting using numerical simulation to track the motion of the electrons. The simulation results and an application for SRF operation with a large beam current are presented in this paper.

Operation of the Superconducting RF System at TLS

The Taiwan Light Source (TLS) is the first dedicated synchrotron light source facility that used a superconducting RF cavity to upgrade its existing RF system. More then one year has passed since the superconducting RF cavity was successfully integrated with the existing RF subsystem for routine operation. The operating status shows that the superconducting RF (SRF) system has effectively suppressed the instabilities of the beam that are caused by the interaction of the electron beam with the higher‐order‐modes of the cavity. The original goal of doubling the photon intensity has been reached by applying top‐up mode injection at the beam current of 300 mA. However, compared to the original RF system, the beam loading is much higher, and the frequency of the superconducting cavity is also more easily perturbed by the mechanical vibrations. Those make the operation of the SRF system facing the challenge on the machine reliability, especially for a dedicated synchrotron light source facility. Here we report ...

Upgrade issues of the TLS storage ring RF system

To achieve the goals in the TLS storage ring performance upgrade plan, the necessity of adding the third RF station to the storage ring for radiation loss compensation at 1.5 GeV/350 mA operation and lifetime improvement are analyzed. We estimated the RF power required to compensate the radiation losses by bending magnets and various insertion devices. Effects of the proposed passive Landau cavity on power consumption is also studied.