Superconducting RF Test Facility Research Articles

Development of EPICS-Based Data Acquisition system for Beam Loss Monitor and sX-map

Beam commissioning is underway at KEK’s Superconducting RF Test Facility (STF). Since beam diagnostics are important to realize stable operation, we have developed a time-resolved beam loss distribution monitor as one of the beam diagnostics. This monitor uses up to 16 PIN photodiodes as X-ray sensors to observe X-rays generated due to beam loss of a pulsed beam with a width of approximately 800 μs, which is divided into 100 μs intervals. Thus, 128 data can be acquired per bunch. The data is read out by an EPICS-based data acquisition system. On the other hand, we are also developing an EPICS-based data acquisition system for an inspection method for the superconducting cavities(sX-map) for the vertical test of bare cavities. sX-map strips are inserted inside the stiffener ring at the iris of superconducting cavities. In this presentation, an overview of these EPICS-based data acquisition systems and their integration with the existing control system will be presented.

Study on Conduction Cooling of Superconducting Magnets for the ILC Main Linac

In the main linac of the International Linear Collider (ILC), superconducting magnets for beam focusing and steering will be located periodically in superconducting RF (SRF) cavity string for beam acceleration in common cryomodules. A concept of conduction cooling of the combined-functioned, splittable superconducting magnets has been proposed and investigated to adapt much different features and to meet different requirements for the superconducting magnet and SRF cavity in fabrication, assembly, and operation. It is required to integrate the superconducting magnet after the SRF cavity string assembly which completed under an ultra-clean environment. The magnet must be conductively cooled down through thermal links to a liquid helium supply pipe. According to this concept, a model magnet development was carried out in cooperation with Fermilab and KEK, and has been demonstrated in KEK superconducting RF test facility (STF). In addition, an important issue has been recently identified. High gradient SRF cavities naturally emit field emission electron flux from the inner surface, so-called dark current. It may pass through the subsequent SRF cavity string and penetrate into the superconducting magnets placed downstream. It may heat up the superconducting coils, and may cause a quench. Therefore, further study on reliable conduction cooling and to secure the superconducting magnet operation with a keeping sufficient safety margin is quite essential. In this paper, we report the installation, the improvement achieved in STF, and the R&D progress in the study on the conduction cooling of the superconducting magnet for the ILC main linac.

Development of a compact Local Power Distribution System for the ILC

The Local Power Distribution System (LPDS) of the International Linear Collider (ILC) transmits RF power from a 10 MW klystron to 39 cavities. A variable hybrid and variable phase shifter are adopted in the LPDS to have been adopted in the LPDS to achieve the efficient beam acceleration in the ILC. As it is difficult to construct the LPDS in an accelerator tunnel, the LPDS can be assembled on a cryomodule on the ground. Then, the cryomodule integrated with the LPDS can be installed in the tunnel. Furthermore, production cost can be decreased by utilizing identical variable hybrids and variable phase shifters. In this study, a compact LPDS was developed to satisfy these two requirements. A compact variable hybrid and variable phase shifter were designed for a compact LPDS. The power transmitted to a cavity was adjusted by the variable hybrid to achieve the maximal accelerating field, which exhibited a variation of ±20% depending on cavity performance. The variable phase shifter compensated the phase shift of the variable hybrid. Fixed phase shifters were designed to adjust the phase difference between adjacent cavities for beam acceleration. These RF components were manufactured, and the S parameters and power handling capability of the components were tested. An L-band resonant ring was constructed at the Super-conducting RF Test Facility (STF) at KEK. The power handling capability of these RF components were tested using a power of 5.2 MW with a pulse width of 200 μs and a power of 2.0 MW with a pulse width of 2 ms. The S parameters and power handling capability satisfied the requirements of the compact LPDS. The compact LPDS will be constructed using these RF components and demonstrated at the STF.

Design, Procurement, Installation and Commissioning of the Cryogenic Infrastructure for a New Superconducting RF Test Facility with Beam at CERN

The High-Luminosity LHC project (HL–LHC), aiming at peak luminosity above 5.0 × 1034 cm −2 s −1, considers replacing the matching sections on both sides of the ATLAS and CMS experiments. To complement new focusing quadrupoles, this upgrade considers using the so-called superconducting crab cavities, never operated before with protons and therefore requiring qualification with beam. To this aim, a new cryogenic infrastructure for a superconducting RF test facility was initiated and recently installed at CERN SPS accelerator in 2018. From the early studies of heat load and design principles to the successful tests passed during late 2018, this paper describes the main cryogenic requirements for such a test facility, its design challenges, procurement, installation, and commissioning up to stable operation of the crab cavities module in superfluid helium at 2 K.

Digital low level rf control system for the International Linear Collider

The International Linear Collider (ILC) aims to employ field programmable gate array (FPGA)-based digital low level radio frequency (LLRF) control system to achieve its rf stability requirements. In this paper, we present a demonstration and evaluation of the minimal configuration of the digital LLRF control system for the ILC. The achieved stabilities are shown to fulfill the ILC requirements. We also developed signal monitoring capabilities using direct sampling technique and we successfully demonstrate the filling on resonance (FoR) method to reduce the required klystron power. Furthermore, we present the possible configurations of digital LLRF control system for the ILC in this paper. We performed the experiments at Superconducting rf Test Facility (STF) in High Energy Research Organization (KEK), Japan during the cavity conditioning in the autumn of 2016.

Fabrication and Evaluation of Superconducting Single-Cell Cavities Manufactured Using Various Materials and Methods

Six cavities of the same type (TESLA-like shape) made from six different niobium sources or types were produced at KEK using various materials and fabrication methods. Throughout these sequential in-house fabrications, comparisons of the properties of the various materials for use in superconducting radio-frequency cavities were conducted. The advantages and disadvantages of these properties, together with different fabrication methods, were examined. The cavities were fabricated primarily using existing facilities within KEK to control the fabrication quality. Vertical tests to evaluate single-cell cavity performances were conducted at the Superconducting RF Test Facility in KEK. The technical details of each fabrication method and the obtained high-gradient test results are described and discussed.

Digital low level rf control system with four different intermediate frequencies for the International Linear Collider

A field programmable gate array--based digital low level rf (LLRF) control system will be used in the International Linear Collider (ILC) in order to satisfy the rf stability requirements. The digital LLRF control system with four different intermediate frequencies has been developed to decrease the required number of analog-to-digital converters in this system. The proof of concept of this technique was demonstrated at the Superconducting RF Test Facility in the High Energy Accelerator Research Organization, Japan. The amplitude and phase stability has fulfilled the ILC requirements.

Control systems for the 2 K cryogenic systems at KEK-STF and KEK-cERL

The Superconducting RF Test Facility and the compact Energy Recovery Linac are accelerator facilities at KEK that employ 1.3 GHz superconducting cavities fabricated using niobium. They should be cooled with liquid He at 2 K, for which cryogenic systems were developed at KEK. Home-made control systems were also introduced for stable and effective operation of the 2 K cryogenic systems. The performance of these cryogenic control systems has been improved in accordance with each beam operation.

Residual Magnetic Field Inside the Superconducting RF Cavity Shield

Magnetic shielding is used to reduce ambient magnetic field surrounding superconducting RF cavities in order to decrease surface resistance and achieve high quality value. The tolerance of the residual magnetic field can be as low as 0.1 μT. Therefore, it is necessary to choose a proper shielding material to reduce the magnetic field to within permissible range. In this paper, field measurements are carried out using a flux gate sensor inside the magnetic shield for the superconducting RF test facility nine-cell cavity system. The measurement results at room and cryogenic temperatures are reported. A modeling method for the actual structure of the shield is developed, and the simulation results are compared with the measured results.

X-ray generation by inverse Compton scattering at the superconducting RF test facility

Quasi-monochromatic X-rays with high brightness have a broad range of applications in fields such as life sciences, bio-, medical applications, and microlithography. One method for generating such X-rays is via inverse Compton scattering (ICS). X-ray generation experiments using ICS were carried out at the superconducting RF test facility (STF) accelerator at KEK. A new beam line, newly developed four-mirror optical cavity system, and new X-ray detector system were prepared for experiments downstream section of the STF electron accelerator. Amplified pulsed photons were accumulated into a four-mirror optical cavity and collided with an incoming 40MeV electron beam. The generated X-rays were detected using a microchannel plate (MCP) detector for X-ray yield measurements and a new silicon-on-insulator (SOI) detector system for energy measurements. The detected X-ray yield by the MCP detector was 1756.8±272.2 photons/(244 electron bunches). To extrapolate this result to 1ms train length under 5Hz operations, 4.60×105 photons/1%-bandwidth were obtained. The peak X-ray energy, which was confirmed by the SOI detector, was 29keV, and this is consistent with ICS X-rays.

High-gradient near-quench-limit operation of superconducting Tesla-type cavities in scope of the International Linear Collider

We report the successful demonstration of an ILC-like high-gradient near-quench-limit operation at the Superconducting RF Test Facility at the High Energy Accelerator Research Organization (KEK) in Japan. Preparation procedures necessary for the accelerator operation were conducted, such as rf phase calibration, beam-based gradient calibration, and automated beam compensation. Test runs were performed successfully for nominal operation, high-loaded Q (${\mathrm{Q}}_{\mathrm{L}}$) operation, and automated ${\mathrm{P}}_{\mathrm{k}}{\mathrm{Q}}_{\mathrm{L}}$ operation. The results are described in terms of the achieved precision and stabilities of gradients and phases.

Development of a 4-mirror optical cavity for an inverse Compton scattering experiment in the STF

To obtain high-brightness quasi-monochromatic X-rays via inverse Compton scattering (ICS), an optical cavity for intensifying laser beams was designed and implemented in a new beam line at the KEK Superconducting RF Test Facility (STF) accelerator. The optical cavity adopts a planar configuration consisting of 4 mirrors. This confocal type resonator provides stable laser storage even with a long mirror distance, enabling head-on collision with the electron beams. To overcome the well-known astigmatism problems of the planar-type optical cavity, two forcibly bendable cylindrical mirrors were introduced instead of flat mirrors. With this new function for laser profile adjustment, an almost round laser profile at the waist point in the accelerator environment was successfully achieved. Estimated waist sizes were 43.7μm for the horizontal and 50.8μm for the vertical dimensions. The feedback control of this 4-mirror optical cavity worked with a stiff plate supporting all 4 mirrors. 1.7×103 finesse and 2.8-kW stored power for a 1-ms duration with 5Hz were achieved.

Achieving high gradient performance of 9-cell cavities at KEK for the international linear collider

Since 2008, vertical tests of 1.3GHz 9-cell cavities at the superconducting RF test facility (STF) in KEK have been carried out for the International Linear Collider (ILC). The tested cavities are mainly KEK-05 through KEK-22. Test results showed that KEK-12, KEK-13, KEK-17, and KEK-21 attained the unloaded Q value (Q0) of 0.8×1010 at the accelerating gradient (Eacc) of 35MV/m (ILC specification) only by the electro-polishing (EP) process. These four cavities had no problematic defect on any electron beam welding seams at their equators; however, field emission was still observed in the vertical tests. This paper reports on the recent progress of the cavity performance at KEK-STF, along with a detailed analysis of problematic defects and the effect of the local mechanical grinding.

L-band Photocathode RF gun at KEK-STF

The superconducting RF test facility (STF) in KEK is a facility to promote R&D of the International Linear Collider (ILC) cavities and cryomodule. L-band photocathode RF gun has been developed at KEK-STF as an electron beam source for cryomodule test scheduled in autumn of 2011. The RF cavity of the gun will be operated with a 1.3 GHz RF frequency, 1 msec RF pulse width, 5 Hz repetition rate at normal conductivity. The cavity was prepared by collaborative work with DESY and FNAL, and fabricated by FNAL. The RF conditioning of the cavity has been started since April 2010. A cesium telluride thin film as a photocathode material has been adopted, and the preparation equipment for cesium telluride has been newly designed and constructed. By using this new system, a fabrication and a performance estimation of the cesium telluride thin film as a photocathode are the next step of the research.

Magnetic Shielding Effect at Cryogenic Temperature Evaluated for the STF Cryomodule

The Superconducting RF Test Facility (STF) is a project at KEK to build and operate a test linac with high-gradient superconducting cavities. Two types of high gradient accelerating cavities, with design accelerating gradients of 35 MV/m and 45 MV/m, will be tested along with other essential technologies needed for the International Linear Collider main linac systems at the STF. In order to achieve the design accelerating gradients, the ambient magnetic fields inside the cavities must be well shielded, to a level of mG . The permeability of the shielding material shows a strong temperature dependence, which often results in a degradation of the shielding effect in a cryomodule when cooled down to the operating temperature. The magnetic field inside the magnetic shield for the STF cavities is measured along the STF beam axis at cryogenic temperature and compared with measurements at room temperature. The measurements at both temperatures are compared with simulation where the permeability curves of the shielding material obtained at room temperature, liquid nitrogen temperature and liquid helium temperature are used. The shielding effect dependence on temperature of the magnetic shield inside the STF cryomodule is presented along with a comparison between measurements and simulation.

Cryomodule tests of four Tesla-like cavities in the Superconducting RF Test Facility at KEK

A 6-m cryomodule including four Tesla-like cavities was developed, and was tested in the Superconducting RF Test Facility phase-I at KEK. The performance as a total superconducting cavity system was checked in the cryomodule tests at 2 K with high rf power. One of the four cavities achieved a stable pulsed operation at $32\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, which is higher than the operating accelerating gradient in the ILC. The maximum accelerating gradient (${E}_{\mathrm{acc},\mathrm{max}}$) obtained in the vertical cw tests was maintained or slightly improved in the cryomodule tests operating in a pulse mode. Compensation of the Lorentz force detuning at $31\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ was successfully demonstrated by a piezo tuner and predetuning.

Thermal simulation and analysis of the STF cryomodule

STF is a superconducting RF test facility constructed at the high energy accelerator research organization of Japan (KEK), as a main part of a R&D project for the proposed International Linear Collider (ILC) in Asia. Thermal study of the STF 1.3 GHz 9-cell cavity cryomodule was carried out within a collaboration between China and Japan. Static and dynamic thermal behaviors of the STF cryomodule were simulated and analyzed with the FEM method, and some simulation results were compared with the available experimental data. This paper presents the details.

New HOM coupler design for ILC superconducting cavity

Four superconducting cavities have been developed at the superconducting RF test facility (STF) at KEK as candidates for the International Linear Collider (ILC) accelerating cavity. These cavities, which are termed TESLA-type STF baseline cavities, are equipped with newly designed higher-order-mode (HOM) couplers that have a short outer cylinder and a single welding point on the antenna that acts as the inner conductor. Each STF baseline cavity is equipped with two types of these new HOM couplers: STF I-type and STF L-type. The HOM couplers were designed and developed, and their RF performance was evaluated by using a Cooper model. Then, niobium models were fabricated and cool down tests were conducted with these cavities. In all, 14 cool down tests were performed for the four cavities using a vertical setup that involved the newly designed HOM couplers. Electrochemical polishing of the inner surface of the cavity and frequency tuning of the accelerating mode were performed before each cool down test in order to maintain the RF performance and obtain a higher gradient. These new HOM couplers were operated up to a gradient of 32 MV/m in the accelerating field of the end cell without any serious problem. Their damping performance is comparable to that of the TESLA cavity except for the TM 011 mode, whose performance can be easily corrected by changing the installation angle of the next fabricated cavity. In this paper, the development of the new HOM couplers for STF baseline cavities is described.

Study of Magnetic Shield for the STF Cryomodules

The Superconducting RF Test Facility (STF) is being constructed at KEK. Two types of high gradient accelerating cavities, with accelerating gradients of 35 MV/m and 45 MV/m, will be tested along with other essential technologies needed for the International Linear Collider main linac systems at the STF. In order to achieve the design accelerating gradients, the ambient magnetic field inside the cavities must be well shielded, to the level of several mG. The earth's magnetic field in the STF tunnel, and the ambient magnetic fields inside the cryomodule and inside the magnetic shield were measured. The results are presented along with the permeability measurement data of several materials at 300 K, 77 K and 4 K, which will be used to estimate the shielding effect when the cryomodules are cooled down to the operating temperature in the future.