Ming-Hsun Tsai

Effects of Material Properties on the Elastoplastic Buckling of an SRF Cavity Under External Pressure

An SRF cavity is generally manufactured with a shell structure to decrease the temperature of the inner surface and consequently to decrease the rf surface resistance. During operation, the SRF cavity is immersed in a bath of liquid helium while its interior is maintained under ultra-high vacuum. To be loaded under a condition of external pressure at a cryogenic temperature, a pressure test at room temperature is requested for safety examination. Explicit calculation and estimation of buckling to prove its structural strength is thus essential. With a great stress on the cavity, the nonlinear behavior of the stress-strain curve of niobium generates the elastoplastic buckling behavior different from elastic buckling. The stress-strain curve of niobium depends on the formation, fabrication, and treatment, thus modifying the buckling behavior. We investigated the buckling behavior of a 500-MHz SRF cavity under external pressure, for which various stress-strain curves were applied. Finite-element software (ANSYS) was used to calculate the limit pressure and post-buckling behavior, with an incremental arc-length control scheme to include effects of nonlinearities of both the geometry and the material property. Not only the limit pressure but also the buckling mode vary with the assigned material property and boundary conditions.

Calculation of the Cryogenic Loads on a Superconducting RF Module

The thermal radiation of a superconducting RF (SRF) module is simulated using software (ANSYS FLUENT) for three-dimensional computational fluid dynamics (CFD). The heat radiating from beam tubes and an input-power coupler (of waveguide type) of the SRF module into the 500-MHz niobium cavity is calculated simultaneously with partially diffuse and partially specular reflection. The total heat loads on liquid helium (LHe) and liquid nitrogen (LN2) are estimated on introducing both radiation and conduction. This estimated total heat load on liquid helium is verified with a direct experimental measurement of the heat load; the calculated results match the measurement data. The radiative surface properties are investigated on varying the magnitude of the diffuse fraction of surfaces to assess the effect of the surface condition on the thermal radiation; a tabulated list of total heat load is given to understand how most effectively to decrease the cryogenic load of an SRF module. Our results provide guidance for selecting in advance a surface treatment and the direction of design to minimize the cryogenic load from radiated heat.

Design and Upgrade the Safety System for the SRF Electronic System at the Taiwan Photon Source

This paper presents some new designs and upgrades of an SRF interlock and electronic system. Based on the experience from Taiwan Light Source (TLS) that uses one Cornell-type superconducting cavity made by ACCEL in the storage-ring RF system [1], in the new TPS SRF system [2] home-made LLRF and SRF electronics [3] are constructed for two KEKB-type superconducting cavities [4] that are installed in the storage ring of circumference 518 m. For reliable operation of the TPS SRF system, enhanced safety functions of the system were added to improve the original SRF system in TLS. The improved functions can provide both the operators and the RF systems with a safer environment and clearer messages for trouble-shooting and malfunction status indications.

Development of a 500 MHz Solid-state RF Amplifier as a Combination of Ten Modules

The recent development of semiconductor technology has proved that a solid-state RF amplifier is an attractive alternative high-power RF source for numerous accelerator applications. Because of the great redundancy and reliability of solid-state amplifiers present in many facilities worldwide, the development of a kW-level RF power per module using compact planar baluns has also been undertaken in NSRRC. Ten amplifier modules are combined to achieve stable output power 8 kW as an initial conceptual realization of a basic power unit within a combined network. This article describes each portion of the amplifier with the experimental results. INTRODUCTION Solid-state RF amplifiers have proved to be reliable and attractive CW RF power sources for accelerator applications [1-3]. Some R&D work has been done to improve the design and structure of the amplifier module itself [4] in NSRRC. The main feature of the developed solid-state amplifier circuit in NSRRC is a planar balun and stable operation of CW at output power 1 kW. With such great power per module, the required module numbers for systematic power combination become decreased by a third relative to the present CW 500-MHz power module, about 650 W [5]. Fewer modules would be built for the same total RF output power. Under such a condition, the importance of each module becomes great because of its enhanced contribution to the overall output power. The failure of one module during operation would bring a larger impact on the other modules and the total output power. The health status of each module thus becomes more important than in those systems using modules of less power. An early diagnosis of the health status of each module would be helpful to prevent multiple module failure at the same machine shift. To decrease the impact of one module failure within a combined group, the unit number within a basic combination group must also be increased. The reliability thus becomes important, and is improved on increasing the capacity of cooling with water and air. SYSTEM DESCRIPTION A high-power solid-state amplifier consists of several basic combined groups; the basic combining group typically contains eight modules in parallel [1-2]. In this case, if one module of the eight fails, to maintain the original total output power, the other modules would need to increase output power about 0.58 dB (14.3 %). There is also a combined group containing nine modules [3]; when one of the nine modules fails, a power increment of each other module by 0.52 dB (12.5 %) would be required to maintain a constant total output power. For a combined group comprising ten modules, one failure would cause a power increment about 0.46 dB (11.1 %) of the other nine modules. For a PA module with saturation power 1 kW, maintaining a constant output power with one failure can support maximum output power 9 kW in a nominal operation with ten units in combination. To include cable and combination losses, the nominal maximum output power must be 8.5 kW with 850 W from ten modules or 950 W from nine modules. The basic combined group, shown in Fig. 1, would be a basic unit for more power combination. Figure 1: Basic combined group of ten modules for total output power 8.5 kW. Block Diagram for a Power Amplifier Module The power amplifier module generally contains an amplifier circuit, a circulator and a stripline load. With the output circulator and load, the amplifier would be unconditionally stable because of the isolation of the output and the amplifier circuit. As the amplifier module itself can contribute RF power up to 950 W, the status of each amplifier module becomes significant, such as its temperature, output power and air-cooling fan rate, and can serve as indicators of its health. With this status information, personnel can find the worst one in advance and replace or repair it during machine maintenance (see Fig. 2). Figure 2: Block diagram of a complete amplifier module. 1kW x10 500W x1 10-way power divider 10-way power combiner

Effect of Bandwidth of Low Level Radio Frequency System on the Instability of an Electron Beam

The analog Low Level Radio Frequency (LLRF) system is used at Taiwan Photon Source (TPS) RF system. It is composed of three feedback loops to control the amplitude and phase of accelerating field and the frequency of RF cavity. Instability of electron beam and accelerating field due to the bandwidth of LLRF system were observed during the TPS commissioning. This effect was studied and the results would be presented in this paper. INTRODUCTION Taiwan Photon Source (TPS) is a modern light source with 3 GeV electron energy located in NSRRC, Taiwan [1]. There were two phases for the TPS commissioning. The phase I commissioning, which was done in the end of March 2015 with maximum stored beam current of 100 mA, was operated with 5-cells Petra cavities and without insertion devices. The phase II commissioning started in the third quarter of 2015 with two superconducting RF (SRF) cavities and 10 sets of insertion devices. Figure 1: The block diagram for the LLRF system. The analog Low Level Radio Frequency (LLRF) system is used at TPS RF system. It is composed of three feedback loops to control the amplitude and phase of accelerating field and the frequency of RF cavity. Figure 1 shows the block diagram for the LLRF system of TPS. The initial designed bandwidth of the gap voltage controller of the amplitude loop is about 7.2 kHz. With this bandwidth, the instability of electron beam at about 20 mA was observed during phase I commissioning with two Petra cavities operated at 1200 kV. The maximum stored beam current could not exceed about 25 mA. After reducing the bandwidth of amplitude loop to 720 Hz, the longitudinal instability was observed at about 80 mA and the stored beam current could reach designed goal of 100 mA [2]. For the phase II commissioning, the bandwidth of the gap voltage controller was further reduced to 596 Hz, and the stored beam current reached 520 mA at the end of December 2015 with two SRF cavity operated at 1400 kV. However, the beam processing was needed for the RF coupler every week due to the heavy gas loading [3]. During the beam processing, the beam current was fixed at certain level and then the gap voltage was decreased. The instability of LLRF system was observed during beam processing. A small signal model for the beam-cavity interaction with feedback loop [4] was used to analyse the effect of bandwidth of LLRF system on the instability of RF system. The model and the results are presented in the following sections. PEDERSEN MODEL The Pedersen model with feedback loops was used to simulate the instability of RF system for a given steady state condition, as shown in Fig 2. It describes the transmissions from small modulations of beam current and generator current to the cavity voltage, for both amplitude and phase. Due to the slow response of tuner loop, only amplitude and phase loop were considered in the simulation. Transfer Functions in the Pedersen Model The transfer functions from the total current to the cavity voltage are given by [4]: } ) ( ) ( ) ( ) ( { 2 1 ) ( ) (

Structural Reinforcement on a Superconducting Radio-Frequency Cavity

A pressure test on a liquid-helium vessel with the niobium cavity in a cryostat to the maximum allowable operational pressure is required as the main item for a safety test near 300 K, to prepare for application for a license to operate a superconducting radio-frequency (SRF) module. Because the niobium cavity has a shell structure and is part of the liquid-helium vessel, to prevent its buckling during test at high pressure becomes a critical issue in designing a 500-MHz SRF module. To meet the safety requirement, currently available 500-MHz SRF modules hence have a limit on the maximum allowable operational pressure that is marginally above the routine operational pressure. Safety devices such as relief valves are, correspondingly, chosen to meet the pressure limit at only 10 or 20 kPa above the operational pressure, thus causing operational inconvenience and a risk of damage of the shell-like cavity structure. A local reinforcement of the cavity is proposed herein to increase its buckling strength and thus its maximum allowable operational pressure. Illustrated with a 500-MHz SRF cavity of KEK type, detailed investigations of its elastic buckling modes and the corresponding strengths are computed with linear, 3-D, and finite-element models to determine an optimized geometry and the location of the reinforcement rings. Variations of the corresponding buckling modes and stress distributions are examined. Although the required tuning force is slightly increased but within an acceptable range from an engineering point of view, maximum allowable operational pressure of the 500-MHz SRF module can be effectively increased up to 200 kPa or even more when properly implementing one pair of reinforcement rings on the niobium cavity cell.