Spoke Cavity Research Articles

Development of the double spoke cavity prototype for CSNS-II

The China Spallation Neutron Source (CSNS), located in Dongguan city, Guangdong Province, is the country’s first and only spallation neutron source. It is also the fourth spallation neutron source in the world. Meanwhile, it is one of the core large-scale scientific facilities of the Guangdong-Hong Kong-Macao Greater Bay Area Comprehensive National Science Center. The China Spallation Neutron Source phase II (CSNS-II) upgrade design will increase the total beam power from 100 kW to 500 kW and boost the beam energy from 80 MeV to 300 MeV in the linac by adding a superconducting linear accelerator to the existing accelerator complex. The double spoke resonator is used in the energy range of 80 MeV to 165 MeV. At present, the double spoke resonator (DSR) prototypes have been designed, fabricated and tested. The two prototypes underwent vertical testing five times, resulting in a maximum accelerating gradient of 15 MV/m. Additionally, the value of Q0 exceeded 4×1010 at an Eacc of 7.3 MV/m.

Suppression of multipacting growth in a superconducting spoke resonator by the use of higher order modes

Abstract Multipacting is a vacuum discharge phenomenon that constrains the performance of accelerator structures and various microwave devices significantly. The different ways to mitigate multipacting discharge have been a central research goal in Accelerator Physics & Technologies for a prolonged period. In this work, both Monte-Carlo and particle-in-cell (PIC) simulations show that the multipactor avalanche can be mitigated, by the use of Higher Order Modes (HOMs). The intuition behind the approach of using HOMs to suppress multipacting has been explained first. Then, the multipacting suppression has been demonstrated in a simple 1-D metallic gap, when it is exposed to a second carrier frequency in addition to fundamental rf mode, with the help of a Monte-Carlo simulation code. Finally, a detailed PIC simulation has been carried out using CST Microwave studio, to investigate the effect of various HOMs on the multipactor growth in a superconducting spoke cavity. It is observed that a slight excitation of some particular modes can drastically diminish the multipactor growth. Simulation has been done first without any HOMs, to calculate the growth rate and also to spot the discharge location. Next, among various HOMs (more specifically, quadrupole modes), only those have been selected, whose magnetic field locations coincide with the multipacting prone zone of the cavity. In a simulation study, the presence of these preselected quadrupole modes has been found to exhibit a substantial suppression effect on the multipacting growth of the cavity.

Fabrication progress of the prototype spoke cavity for the JAEA-ADS linac

The Japan Atomic Energy Agency (JAEA) has been proposing an accelerator-driven nuclear transmutation system (ADS) as a future nuclear system. In preparation for the actual design of the CW proton linac for the JAEA-ADS, we are now prototyping a low-β (≈ 0.2) single-spoke cavity. The cavity fabrication started in 2020. Most of the cavity parts were shaped in fiscal year 2020 by press-forming and machining. In 2021, we started welding the shaped cavity parts together. By preliminarily investigating the optimum welding conditions using mock-up test pieces, each cavity part was joined together with smooth welding beads. So far, we have fabricated the body section and the beam port sections. By measuring the resonant frequency of the temporarily assembled cavity, it was confirmed that there were no significant problems with the cavity fabrication.

Comparative thermodynamic analysis of China Spallation Neutron Source second phase (CSNSII) SRF system cooling Scheme

The China Spallation Neutron Source second phase (CSNS-II) accelerator upgrades consist of three major components: linac energy upgrade from 80 MeV to 300 MeV, injection system upgrade, and new magnetic alloy dual harmonic cavities. There are ten double spoke cavities cryomodules and eight elliptical cavities cryomodules in total. A comparative thermodynamic analysis of the superconducting radio frequency (SRF) systems can be of great importance to select a reliable, cost-effective and easy-maintenance cooling schemes. This is particularly critical for SRF systems that require long-term operation. A comparative thermodynamic analysis of both types of cooling schemes for SRF systems, referred to as Scheme 1 (a cryogenic distribution system with distributed heat exchangers) and Scheme 2 (a cryogenic distribution system with central heat exchangers), was presented in this paperThe length and size of process transfer lines, as well as the heat loads subjected to their components, of cryogenic distribution systems were introduced. In addition, the unsteady state cooldown process for the CSNS-II SRF cryomodules was investigated. The analysis results that included temperature and pressure profiles along the linac distance, as well as distributions of JT expansion effectiveness, mass flow rate, and cryomodule exegetic efficiency. Furthermore, the exergy analysis and refrigeration power are investigated using a neural network optimized by a novel sparrow search algorithm genetic algorithm (SSA-GA). Results show that, Scheme 1 is more advantageous than Scheme 2 because it can save maintenance time. From 300 K to 4.5 K, the total cooldown time of SRF cryomodules is approximately 28 h. The maximum ΔP is around 1.1 bara, which can be considered in the acceptable range. The prediction of ηex is stable at the 7th epoch after being optimized by SSA-GA, and the MSE is 1 × 10-5. And the error between the predicted and true values of the BP neural network after SSA optimization is very small (less than 0.1%), approaching zero, indicating that the BP optimized by SSA achieves good prediction. This paper's work can serve as a solid foundation for the construction and operation of the CSNS-II.

THE PROSPECTS FOR SMALL-GAP RESONATORS BASED ON COMBINED RF FOCUSING FOR USE IN PROTON LINACS

For high-current ion linacs operating over the medium energy range, the use of an accelerating structure that implements combined rf focusing (CRFF) is suggested. Considered is the focusing provided by short IH-, CH-, and two-gap spoke cavities. The absence of external focusing elements and the structural simplicity of the proposed resonators makes them promising for use in accelerators operating in continuous or low-duty mode, which are built on superconducting cavities.

Accelerator development at the FREIA Laboratory

The FREIA Laboratory at Uppsala University focuses on superconducting technology and accelerator development. It actively supports the development of the European Spallation Source, CERN, and MAX IV, among others. FREIA has developed test facilities for superconducting accelerator technology such as a double-cavity horizontal test cryostat, a vertical cryostat with a novel magnetic field compensation scheme, and a test stand for short cryomodules. Accelerating cavities have been tested in the horizontal cryostat, crab-cavities cavities in the vertical cryostat, and cryomodules for ESS on the cryomodule test stand. High power radio-frequency amplifier prototypes based on vacuum tube technology were developed for driving spoke cavities. Solid-state amplifiers and power combiners are under development for future projects. We present the status of the FREIA Laboratory complemented with results of recent projects and future prospects.

Development of a superconducting radio frequency double spoke cavity for CSNS

China Spallation Neutron Source (CSNS) has been in operation for about 2 years, and an upgrade is proposed to compensate the space charge effect. A superconducting radio frequency (SRF) linac with double spoke cavities and medium-beta elliptical cavities has been designed to increase the injection energy from 80MeV to 300MeV, and institute of high energy physics (IHEP) has developed a prototype double spoke cavity at β0 0.5. The cavity shape was optimized to minimize Ep/Eacc while keeping Bp/Eacc reasonably low. Meanwhile, mechanical design was applied to check stress, Lorentz force detuning and microphonic effects, and to minimize pressure sensitivity. A new input power coupling scheme was proposed to avoid electrons hitting directly on ceramic window. After fabrication and post processing of cavity, the cavity was vertical tested for several times, and mechanical polishing was applied to improve the performance. The cavity finally reached Bp of 140mT at Eacc = 16.1MV/m and Q0=1.42×1010 under vertical test at 2K.

Modeling of Superconducting Spoke Cavity with its Control Loops Systems for the MYRRHA Linac Project

Modeling of Superconducting Spoke Cavity with its Control Loops Systems for the MYRRHA Linac Project

RF design of a compact 648 MHz single spoke cavity at CSNS

In order to increase CSNS beam power from 100 to 500 kW, the linac injection energy needs to be increased from 80 to 300 MeV. The combined layout of superconducting spoke cavities and elliptical cavities will be adopted to accelerate H− beam to 300 MeV. In this paper, a compact single spoke cavity with Φ50-mm beam aperture was proposed, with the RF performance of the spoke cavity optimized to Ep/Eacc value. The simulation got optimum results of Epeak/Eacc = 4.97 and Bpeak/Eacc = 8.42 mT/(MV/m), and the max is 1.52 at Eacc = 7.74 MV/m. A 648 MHz βg = 0.4 single spoke cavity with beam aperture Φ50 mm was proposed in the paper. This compact cavity may be a very promising option for CSNS linac upgrading. In addition to the optimization of the RF parameters, the multipacting behavior of the cavity is also studied, and cavity shape is optimized to reduce value.

Design of a Balloon-Shaped Superconducting Single Spoke Resonator

We have investigated designs for a superconducting single spoke resonator with a resonant frequency of 325 MHz and an optimum beta of 0.51. In this paper, we suggest a balloon variant of a spoke cavity having an end-wall structure, which has a distinct advantage from the viewpoint of multipacting suppression. We determined the geometric parameters of the balloon-shaped single spoke resonator through electromagnetic, multipacting, and mechanical simulations. In addition, we discuss the resonant frequency shifts caused by the cavity processing, the power coupling, and the cavity tuning.

Thermal performance analysis and operation of the spoke cavity cryomodules for C-ADS Injector-I

The Chinese Accelerator Driven Sub-critical system (C-ADS) is an important strategic research that is based on a high energy proton beam for nuclear waste transmutation. To meet the requirements of stability and reliability of the lower energy part, two identical Injectors are operated in parallel. The Injector-I is designed and constructed by the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS) in Beijing, China. The Injector-I linac with two accelerating cryomodules (named CM1 and CM2) successfully accelerated the proton beam to 10.67 MeV with the beam current of 10.11 mA in pulse mode and 10.0 MeV with a beam current of 1.6 mA in CW (continuous wave) mode.The cryomodules have been designed, fabricated, assembled and commissioned at IHEP since the beginning of 2012. The thermal performances of the cryomodules were analyzed with numerical simulations and the empirical calculations take into account all the main contributors (support posts, multilayer insulation, current leads, power couplers and cavities) to the static and dynamic heat loads at various temperature levels. Two designs of current leads are developed and analyzed, and the case 2 current leads are tested and used in the CM2. The estimated static heat loads by analysis have been compared with the available experimental measurements. The operation experiences including the cool down, operating parameters and heat load measurements are reported, and the results were shown that the cryomodules can keep in a stable operation at 2 K and 30 mbar ±10 Pa.

Design and thermal performance of spoke cavity prototype cryomodule of the Chinese Accelerator Driven Sub-critical system Injector-I

The Chinese Accelerator Driven Sub-critical system (C-ADS) Injector-I is designed and constructed by the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS) in Beijing, China. The test cryomodule (TCM), housing two 325 MHz spoke cavities and two solenoids and operating at 2 K, has been designed, assembled and commissioned since the beginning of 2012. The C-ADS Injector-I linac with the TCM successfully accelerated the proton beam to 3.68 MeV with the peak current of 10.1 mA in pulse mode. It is the world’s first prototype of spoke cavity cryomodule with beam, and is an important step in the qualification of the cryomodule before series productions. This paper describes the detail of the design, operation and thermal performances of the prototype cryomodule. The analysis takes into account the heat loads of the main components (support posts, multilayer insulation and cavities) at various cryogenic temperature levels. The estimated heat loads of the cryomodule have been compared with the available experimental measurements, and the cryogenic deformations by simulation is verified by the experimental measurements by the wire position monitors (WPMs). The operation experiences including the cool down, alignment and heat load measurement are reported. • The 325 MHz spoke cavity prototype cryomodule has been designed, assembled and commissioned. • The thermal performance of the bottom-supported cryomodule has been proved firstly worldwide. • The cryomodule can be in a stable operation at 2 K and 31 mbar with beam. • The operation experiences including the cool down, alignment and measurement of heat loads are reported. • The linac with cryomodule had been accelerated successfully to 3.68 MeV@10.1 mA in pulse mode.

Characterization of a [formula omitted]=0.5 double spoke cavity with a fixed power coupler

ESS, the European Spallation Source, will adopt a single family of double spoke cavities for accelerating the beam from the normal conducting section to the first family of the elliptical superconducting cavities. It will be the first double spoke cavities in the world to be commissioned for a high power proton accelerator. The first double spoke cavity for the ESS project was tested with high power in the HNOSS cryostat at Uppsala University. A pulse-mode test stand based on a self-excited loop was used in this test. The qualification of the cavity package involves a double-spoke superconducting cavity, a fixed fundamental power coupler, tuner, a low-level radiofrequency (LLRF) system and a high-power radiofrequency (RF) station. The test represents an important verification milestone before the module assembly. This cavity had unfortunately a high dynamic loss of 12 W @ 9 MV/m, where potential causes for such a high value have been studied and corresponding suggestions are listed. This paper presents the test configuration, RF conditioning history, first high power performance and experience of this cavity package.

The superconducting magnet for ADS injection-I

The cryomodule I for ADS Injection-I had been designed, fabricated and online tested. The CM1 contains seven superconducting magnets, seven superconducting spoke cavities and seven beam position monitors. The superconducting magnet, which is a kind of multifunction magnet in a 170mm length axial space, contains a solenoid for beam focusing and two correctors for orbit correction. The design goals for the magnets are to meet the required integral field strength and to reduce the leakage field of less than 2 G at the nearby superconducting spoke cavities. The 2.1K, 31 mbar cryogenic system, where the magnets and spoke cavities shared with, force the magnet must select a kind of conduction cooled current leads. The first one of the batch magnets was tested in a vertical Dewar in HIT in July, 2014. The measurement results met the design requirements. Online operation of CM1 in September, 2015 showed that seven magnets can work at 230A under 4.2K and 2.1K respectively. This paper will present the magnet and current lead design, field measurement and the installation progress.

Design of the two-gap superconducting re-buncher

A new design of a spoke cavity for low relative velocities of heavy ions has been elaborated. Simulation results for a 2-gap spoke cavity with a resonance frequency of 216.816 MHz and a relative velocity of 0.07c are presented.

The Improvement of the Power Coupler for CADS SC Spoke Cavities

The Improvement of the Power Coupler for CADS SC Spoke Cavities

Design study of a radio-frequency quadrupole for high-intensity beams**Supported by Korea University Future Research Grant

The Rare isotope Accelerator Of Newness (RAON) heavy-ion accelerator has been designed for the Rare Isotope Science Project (RISP) in Korea. The RAON will produce heavy-ion beams from 660-MeV-proton to 200-MeV/u-uranium with continuous wave (CW) power of 400 kW to support research in various scientific fields. Its system consists of an ECR ion source, LEBTs with 10 keV/u, CW RFQ accelerator with 81.25 MHz and 500 keV/u, a MEBT system, and a SC linac. In detail, the driver linac system consists of a Quarter Wave Resonator (QWR) section with 81.25 MHz and a Half Wave Resonator (HWR) section with 162.5 MHz, Linac-1, and a Spoke Cavity section with 325 MHz, Linac-2. These linacs have been designed to optimize the beam parameters to meet the required design goals. At the same time, a light-heavy ion accelerator with high-intensity beam, such as proton, deuteron, and helium beams, is required for experiments. In this paper, we present the design study of the high intensity RFQ for a deuteron beam with energies from 30 keV/u to 1.5 MeV/u and currents in the mA range. This system is composed of an Penning Ionization Gauge ion source, short LEBT with a RF deflector, and shared SC Linac. In order to increase acceleration efficiency in a short length with low cost, the 2nd harmonic of 162.5 MHz is applied as the operation frequency in the D+ RFQ design. The D+ RFQ is designed with 4.97 m, 1.52 bravery factor. Since it operates with 2nd harmonic frequency, the beam should be 50% of the duty factor while the cavity should be operated in CW mode, to protect the downstream linac system. We focus on avoiding emittance growth by the space-charge effect and optimizing the RFQ to achieve a high transmission and low emittance growth. Both the RFQ beam dynamics study and RFQ cavity design study for two and three dimensions will be discussed.

MTCA.4-based LLRF control system for the C-ADS proton Linac injector I

The Chinese ADS proton Linac injector I composed of an electron cyclotron resonance ion source, a low-energy beam transport line, a radio frequency quadrupole (RFQ) accelerator, a medium-energy beam transport (MEBT) line, a superconducting section and beam dump with design goal to achieve CW proton beam with beam current 10 mA and energy gain around 10 MeV. In order to maintain the stability of the RF system, to compensate for the beam load effect and to provide the GUI required for the operation of the RF power system, we have established a LLRF control system. This LLRF system for ADS proton Linac injector I in IHEP consists of RF reference system and LLRF controllers for one RFQ cavity, two MEBT bunching cavities and 14 superconductive spoke cavities. We have adopted a new development mTCA.4-based hardware platform, and it has some characteristics such as faster bandwidth, standardized design, easy operation and maintenance and good expansibility. The LLRF system has been improved and optimized and has now been put into operation. It takes into account the pulse and CW beam flow mode, feedforward and feedback control functions. With the help of this system, we achieved stable operation with narrow pulses, long pulses, and CW modes under different beam loads. This article describes the implementation of this system and presents some initial the CW beam acceleration results with the help of this system at the end of 2016.

Measurement of the transfer function for a spoke cavity of C-ADS Injector I**Supported by Proton Linac accelerator I of China Accelerator Driven Sub-critical System (Y12C32W129)

The spoke cavities mounted in the China Accelerator Driven sub-critical System (C-ADS) have high quality factor (Q) and very small bandwidth, making them very sensitive to mechanical perturbations, whether external or self-induced. The transfer function is used to characterize the response of the cavity eigenfrequency to the perturbations. This paper describes a method to measure the transfer function of a spoke cavity. The measured Lorentz transfer function shows there are 206 Hz and 311 Hz mechanical eigenmodes excited by Lorentz force in the cavity of C-ADS, and the measured piezo fast tuner transfer function shows there are 12 mechanical eigenmodes from 0 to 500 Hz. According to these results, some effective measures have been taken to weaken the influence of helium pressure fluctuation, avoid mechanical resonances and improve the reliability of the RF system.

The ESS Spoke cryomodule and its test valve box

The European Spallation Source project aims at being the most powerful neutron source feeding multidisplinary researches. Based on superconducting radiofrequency technologies, the linear accelerator will operate for the first time a 56 meter long section of niobium double Spoke cavities. Paired in 13 cryomodules, each cavity will generate an accelerating pulsed field of 9MV/m. A prototype cryomodule housing two superconducting Spoke cavities and their RF power couplers is now being fabricated and assembled. It provides the cryogenic environment for their operation in a 2K saturated superfluid helium bath: a 50 K thermal shield, a cold magnetic shield, and integrates all the interfaces necessary to be operational within the linac machine. This prototype will be tested in 2016 at IPNO site and then at Uppsala university FREIA facilities. A valve box was designed to take into account the specific features of this prototype cryomodule and of both test sites. This valve box is also considered as a prototype of the cryogenic distribution of the linac Spoke section. This element will then be used for the tests of the series cryomodules.