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Different from common coaxial couplers used for superconducting radio frequency (SRF) accelerators, power couplers with capacitive coupling in which the inner conductor is separated into two unconnected parts on both sides of the cold ceramic window transmit RF power by capacitive coupling. Capacitive coupling type power couplers have the advantages of easy fabrication of ceramic window, reduced thermal conduction from room temperature side to the cryogenic system, and convenient assembling to cryomodule. To further investigate the performance of this type of RF power couplers, an upgraded power coupler with capacitive coupling has been designed and fabricated at Peking University. The high-power test demonstrated that RF power up to 15 kW in continuous wave (CW) mode can be transmitted steadily. In this paper, the structure, simulation, and high-power test results of this power coupler are presented in detail.

We have developed a method to obtain mechanical centers of nine cell superconducting radio frequency (SRF) cavities from localized dipole modes, that is one of the higher order modes (HOM) induced by low-energy beams. It is to be noted that low-energy beams, which are used as alignment probes, are easy to bend in fringe fields of accelerator cavities. The estimation of the beam passing orbit is important because only information about the beam positions measured by beam position monitors outside the cavities is available. In this case, the alignment information about the cavities can be obtained by optimizing the parameters of the acceleration components over the beam orbit simulation to consistently represent the position of the beam position monitors measured at every beam sweep. We discuss details of the orbit estimation method, and estimate the mechanical center of the localized modes through experiments performed at the STF accelerator. The mechanical center is determined as x , y = ( 0 . 44 ± 0 . 56 mm , − 1 . 95 ± 0 . 40 mm ) . We also discuss the error and the applicable range of this method.

Conduction cooled current leads, which bring the current from the room temperature terminal down to the cryogenic environment, were used in common recently in large scale superconducting accelerators for its low cost, sample design and low heat load. In practice, the current lead is designed contained in a stainless steel tube. The heat load can be incepted in steps by thermal anchors, where one end is attached to the stainless steel tube, while the other end is connected with the cold shield of the cryomodule. Since the limitation of the welding technique, a thicker stainless steel tube needs to be used, and hence the thermal anchors cannot provide enough pressure to deform the tube enough to be in direct contact with the current lead, which may lead to temperature instability and bring extra heat load to the cryogenic system. An excellent option of epoxy filled current lead can realize the fully contact and reduce the heat load effectively. This paper will present the process of the current lead design, optimization, numerical simulation and cryogenic test, the test results show that the current lead can keep in a stable operation and low heat load.

A total of ten jacketed single-spoke resonators type 1 (SSR1) have been fabricated for Fermilab’ injection experiment (PIP2IT). PIP2IT is a test bed for Fermilab’s future accelerator named proton improvement plan II that is currently under development. SSR1 cavities operate at 325 MHz to accelerate a proton beam at a relative (to speed of light) velocity ( $\beta = 0.22$ ). In this paper, we present Fermilab’s experience in developing those spoke resonators starting from the design and analysis phase, to fabrication and extensive testing to qualify cavities for cryomodule assembly.

Capacitive input couplers based on a 50 Ω coaxial transmission line are frequently used to transmit RF power to superconducting (SC) resonators operating in CW mode. It is well known that coaxial transmission lines are prone to multipacting phenomenon in a wide range of RF power level and operating frequency. The Facility for Rare Isotope Beams (FRIB) being constructed at Michigan State University includes two types of quarter wave SC resonators (QWR) operating at 80.5 MHz and two types of half wave SC resonators (HWR) operating at 322 MHz. As was reported in ref. [1] a capacitive input coupler used with HWRs was experiencing strong multipacting that resulted in a long conditioning time prior the cavity testing at design levels of accelerating fields. We have developed an insert into 50 Ω coaxial transmission line that provides opportunity to bias the RF coupler antenna and protect the amplifier from the bias potential in the case of breakdown in DC isolation. Two of such devices have been built and are currently used for the off-line testing of 8 HWRs installed in the cryomodule.

The heat load of the original cryomodules for the continuous electron beam accelerator facility is ~50% higher than the target value of 100 W at 2.07 K for refurbished cavities operating at an accelerating gradient of 12.5 MV/m. This issue is due to the quality factor of the cavities being ~50% lower in the cryomodule than when tested in a vertical cryostat, even at low RF field. Previous studies were not conclusive about the origin of the additional losses. We present the results of a systematic study of the additional losses in a five-cell cavity from a decommissioned cryomodule after attaching components, which are part of the cryomodule, such as the cold tuner, the He tank, and the cold magnetic shield, prior to cryogenic testing in a vertical cryostat. Flux-gate magnetometers and temperature sensors are used as diagnostic elements. Different cool-down procedures and tests in different residual magnetic fields were investigated during the study. Three flux-gate magnetometers attached to one of the cavities installed in the refurbished cryomodule C50-12 confirmed the hypothesis of high residual magnetic field as a major cause for the increased RF losses.

The new LCLS-II Linear Superconducting Accelerator at SLAC needs superconducting magnet packages installed inside SCRF (superconducting radio frequency technology) cryomodules to focus and steer an electron beam. Two magnet prototypes were built and successfully tested at Fermilab. Magnets have an iron-dominated configuration, quadrupole and dipole NbTi superconducting coils, and splittable in the vertical plane configuration. Magnets inside the cryomodule are conductively cooled through pure Al heat sinks. Both magnets' performance was verified by magnetic measurements at room temperature, and during cold tests in liquid helium. Test results including magnetic measurements are discussed. Special attention was given to the magnet performance at low currents where the iron yoke and the superconductor hysteresis effects have large influence. Both magnet prototypes were accepted for the installation in FNAL and JLAB prototype cryomodules.

Five solenoid-based focusing lenses designed for use inside the SSR1 cryomodule of the PXIE test stand at Fermilab have been fabricated and tested. In addition to a focusing solenoid, each lens is equipped with a set of windings that generate magnetic field in the transverse plane and can be used in the steering dipole mode or as a skew quadrupole corrector. The lenses will be installed between superconducting cavities in the cryomodule, so getting sufficiently low fringe magnetic field was one of the main design requirements. Beam dynamics simulations indicated a need for high accuracy positioning of the lenses in the cryomodule, which triggered a study towards understanding uncertainties of the magnetic axis position relative to the geometric features of the lens. This report summarizes the efforts towards certification of the lenses, including results of performance tests, fringe field data, and uncertainty of the magnetic axis position.

A significant advance in magnetic field management in a fully assembled superconducting radiofrequency cryomodule has been achieved and is reported here. Demagnetization of the entire cryomodule after assembly is a crucial step toward the goal of average magnetic flux density less than 0.5 μT at the location of the superconducting radio frequency cavities. An explanation of the physics of demagnetization and experimental results are presented.

The use of non-evaporable getter (NEG) pumps in particle accelerators has increased significantly over the past few years because of their large pumping speed, particularly for hydrogen, compared to the size of the pump. A concern about using such pumps in superconducting radio-frequency (SRF) accelerators is the possibility of shedding particulates which could then migrate into the SRF cavities and produce field emission, therefore degrading the cavity performance. One option to mitigate such issue is to use sintered getter materials which intrinsically offer superior mechanical and particle retention properties. In this article we present the results from cryogenic RF tests of a high-gradient SRF cavity after being evacuated several times with an NEG pump equipped with sintered getter disks and placed in close proximity to the cavity. The results showed that the cavity performance was not affected by the pump up to the quench gradient of 34 MV/m. As a result of this study, two such NEG pumps have been installed next to a cryomodule in the CEBAF accelerator to maintain ultra-high vacuum in the SRF cryomodule and two adjacent warm girder sections.

A helium cryogenic system is designed by the Institute of Modern Physics, Chinese Academy of Sciences, to supply different cooling powers to the cryomodules of ion-Linac (iLinac) accelerator, which serves as the injector of the High Intensity Heavy-Ion Accelerator Facility project. The iLinac is a superconducting heavy-ion accelerator approximately 100 m long and contains 13 cryomodules cooled by superfluid helium. This article describes the cryogenic system design of the iLinac accelerator. The requirements of the cryogenic system, such as cooling mode, refrigeration temperature, operating pressure and pressure stability, are introduced and described in detail. In addition, heat loads from different sources are analyzed and calculated quantitatively. An equivalent cooling capacity of 10 kW at 4.5 K was determined for the cryogenic system according to the total heat load. Furthermore, a system process design was conducted and analyzed in detail. Further, the system layout and the main equipment are presented.

The thesis works describe in this manuscript deal with the modelling and the control ofLINAC cryogenic process. The components of the LINAC cryogenic system (cryomodule,valves, pipes,...) are modeled as interconnectable objects. Those same objects are usedto model the SPIRAL2 cryogenic system. The model trueness is evaluated through comparisonbetween experimental and simulated data. Once the model prediction capabilityhas been validated, the models are used to synthetized control laws. More specifically, alinear quadratic command and an extended Kalman filter has been realized for the SPIRAL2cryomodule. Finally, a hierarchical control for a cryogenic refrigerator has beenstudied. A method is proposed for the control of multiple interconnected subsystemsregulated through PID, LQ and MPC controllers.; Les travaux de thèse décrits dans ce manuscrit s’intéressent à la modélisation et lecontrôle de procédés cryogéniques associés à un LINAC supraconducteur. Les élémentsconstituants le système cryogénique du LINAC (cryomodules, vannes, tuyaux,...) sontmodélisés sous forme d’objets inter-connectables. Ces mêmes objets sont utilisés pourmodéliser le système cryogénique de l’accélérateur SPIRAL2. La justesse de ces modèlesest évaluée à travers des comparaisons entre simulations non-linéaires et mesures expérimentales.Une fois la capacité de prédiction des modèles validée, ces derniers sont utiliséspour synthétiser des lois de commande. Tout particulièrement, une commande linéairequadratique ainsi qu’un filtre de Kalman étendu ont été réalisés pour les cryomodulesde SPIRAL2. Enfin, la réalisation d’une commande hiérarchisée appliquée au cas d’unréfrigérateur cryogénique est étudiée. Une méthode est proposée pour le contrôle de plusieurssous-systèmes inter-connectés et régulés par des contrôleurs PID, LQ et MPC.

International audience; PLC provides a large part of the SPIRAL 2 project’s commands. The SPIRAL2 project is based on a multi-beam driver in order to allow both ISOL and low-energy in-flight techniques to produce Radioactive Ion Beams (RIB). A superconducting light/heavy-ion linac with an acceleration potential of about 40 MV capable of accelerating 5 mA deuterons up to 40 MeV and 1 mA heavy ions up to 14.5 MeV/u is used to bombard both thick and thin targets. The PLCs provide vacuum control, access control, part of the machine protection system, control of the cryogenic distribution system, cooling controls, control of RF amplifiers, they are associated with the safety control system. The standards used are presented as well as the general synoptic of the PLC control system. The details of the major systems are presented, the Cryo distribution, the machine protection system, a safety system.

International audience; CEA IRFU Saclay* is taking part of ESS (European Spallation Source)** construction through several packages and, especially in the last three years on the Elliptical Cavity and Cryomodule Test stand Demonstrator (ECCTD)***. The project consists of RF test, conditioning, cryogenic cool-down and regulations of eight cryomodules with theirs four cavities each. For now, two medium beta cavities cryomodules have been successfully tested. This paper describes the context and the realization of the control system for cryogenic and RF processes, added to cavities tuning motorization relying on COTS solutions: Siemens PLC, EtherCAT Beckhoff modules, IOxOS fast acquisition cards and MRF timing cards.

In order to meet the requirements of the Circular Electron Positron Collider (CEPC), a 650-MHz 2-cell superconducting cavity was proposed and designed. In this study, we show the progress in the cavity design for the CEPC. The radio frequency and mechanical designs are ready for the first design version, called CEPC-V1. A cryomodule, which consists of two 650-MHz 2-cell cavities, was scheduled the last year. Therefore, prototypes of the 2-cell cavity based on this design were fabricated and tested the following year. The vertical test results show that multipacting (MP) exists from 10 to 19 MV/m. This will increase the operational risk of the CEPC, while the continuous-wave operation for the 650-MHz cavity is 19.7 MV/m. Two trapped modes were also found, which exist because of the identical half-cell design. Therefore, a few improvements regarding MP and more efficient extraction of higher-order mode power are discussed. A new type of cavity design, called CEPC-V2, is also discussed in this paper.

The IFMIF accelerator aims to provide an accelerator-based D-Li neutron source to produce high intensity high energy neutron flux to test samples as possible candidate materials to a full lifetime of fusion energy reactors. A prototype of the low energy part of the accelerator (LIPAc) is under construction at Rokkasho Fusion Institute in Japan. It includes one cryomodule containing 8 half-wave resonators (HWR) operating at 175 MHz and eight focusing solenoids. The talk will cover the progress of developments in the IFMIF/EVEDA cryomodule: the qualification of 8 cavities, the RF conditioning results of 8 high-power couplers, the manufacturing and test of the 8 superconducting solenoids and the equivalent operational equivalent tests performed at Saclay. The assembling status of the cryomodule at Rokkasho site will also be presented., Proceedings of the 19th International Conference on RF Superconductivity, SRF2019, Dresden, Germany

International audience; The prototype IFMIF cryomodule encloses eight superconducting 175 MHz beta 0.09 Half-Wave Resonators (HWR). They are designed together with the power coupler to accelerate a high intensity deuteron beam (125 mA) from to 5 to 9 MeV. One prototype HWR and the 8 cavities to be hosted in the cryomodule have been manufactured, prepared and tested. The paper describes the phases of the cavities development, including fabrication, processing, and RF resonant frequency management. We focus on the results of the RF tests which have been performed for all bare and jacketed HWRs in a vertical cryostat.

Optimization of Fermilab string assembly procedure and infrastructure has yielded a significant improvement of cryomodule particulate counts. Late production of LCLS-II cryomodules were tested at CMTF at Fermilab and showed little to no x-ray up to administrative limit. The paper describes the field emission measurement instrumentation, field emission results of LCLS-II cyomodules, clean room infrastructure upgrade and procedure optimization., Proceedings of the 19th International Conference on RF Superconductivity, SRF2019, Dresden, Germany

The prototype IFMIF-EVEDA cryomodule encloses eight superconducting 175 MHz β=0.09 Half-Wave Resonators (HWR). They are designed together with the power coupler to accelerate a high intensity deuteron beam (125 mA) from to 5 to 9 MeV. Two cavity packages, complete with tuning system and power couplers, have been tested in a dedicated horizontal test cryostat - SaTHoRI (Satellite de Tests HOrizontal des Résonateurs IFMIF). The successful operational equivalent tests and tuning of the SRF accelerating units is reported., Proceedings of the 19th International Conference on RF Superconductivity, SRF2019, Dresden, Germany

CEA Saclay is in charge of the cavity prototypes that is designing, manufacturing, testing and integrating them into demonstrator cryomodules. We have manufactured 6 medium beta and 5 high beta cavities. As part of these activities we are interested in field emission as one of the limiting factors for cavity performances. We are currently collecting data from cavities operated in vertical cryostat and inside cryomodules. Analysis are carried out by means of particle tracking simulation and comparison with radiation dose monitor and scintillators., Proceedings of the 19th International Conference on RF Superconductivity, SRF2019, Dresden, Germany

The large inkind scope of the elliptical superconducting RF linac of the ESS facility implies the handling of handover conditions between the cavities fabrication and testing phases performed at INFN and STFC, to the assembly of cryomodules at CEA and later to ESS in Lund. The performance qualification at the module test stand, and later the commissioning and operation phases require the availability of the cavity performance and frequency data under all environmental conditions during preparation (e.g. temperature, vacuum in beam line/He vessel/vacuum vessel, tuner state). Availability of the data needs to be guaranteed for the long term maintainability of the accelerator. For these reasons a cavity database has been set up at ESS, integrating the data contained in the handover documentation from the inkind partners and extending it during the activities at ESS after receiving the modules. The database has been used to analyze the preparation steps of the prototype demonstrator cryomodule for the tests at ESS, by benchmarking with the data collected during the tests at CEA, and is currently used during the series cavities handover phases., Proceedings of the 19th International Conference on RF Superconductivity, SRF2019, Dresden, Germany

Shanghai high-repetition-rate XFEL and extreme light facility (SHINE), the first hard XFEL based on a superconducting accelerated structure in China, is now under development at the Shanghai Institute of Applied Physics, Chinese Academy of Sciences. In this paper, power losses caused by trapped longitudinal high-order modes (HOM), steady-state loss, and transient loss generated by untrapped HOMs in the 1.3-GHz SHINE cryomodule are investigated and calculated. The heat load generated by resistive wall wakefields is considered as well. Results are presented for power losses of every element in the 1.3-GHz cryomodule, caused by HOM excitation in the acceleration RF system of the continuous-wave linac of SHINE.

SNRC and CEA collaborate to the upgrade of the SARAF accelerator to 5 mA CW 40 MeV deuteron and proton beams (Phase 2). CEA is in charge of the design, construction and commissioning of the MEBT line and the superconducting linac (SARAF-LINAC Project). The prototypes of the 176 MHz NC rebuncher, SC cavities, RF coupler and SC solenoid-Package have been tested recently. Meanwhile, the cryomodules technical specifications have been written and called for tender. This paper presents the status of the SARAF-LINAC Project at April 2019., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

CEA is committed to delivering a Medium Energy Beam Transfer line and a Super Conducting Linac (SCL) for SARAF accelerator in order to accelerate 5mA beam of either protons from 1.3MeV to 35MeV or deuterons from 2.6 MeV to 40.1MeV. The SCL consists in 4 cryomodules separated by warm section housing beam diagnostics. The two first identical cryomodules hosts respectively 6 and 7 half-wave resonator (HWR) low beta (0.091) cavities 176MHz. In order to test the cavity with its tuner and coupler and validate some design consideration, the Equipped Cavity Test Stand (ECTS) has been designed and will be presented., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

PERLE is a proposed high power Energy Recovery Linac, designed on multi-turn configuration, based on SRF technology, to be hosted at Orsay-France in a col-laborative effort between local laboratories: LAL and IPNO, together with an international collaboration involv-ing today: CERN, JLAB, STFC ASTeC Daresbury, Liverpool University and BINP Novosibirsk. PERLE will be a unique leading edge facility designed to push advances in accelerator technology, to provide intense and highly flexible test beams for component development. In its final configuration, PERLE provides a 500 MeV elec-tron beam using high current (20 mA) acceleration during three passes through 801.6 MHz cavities. This presenta-tion outlines the technological choices, the lattice design and the main component descriptions., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

Two full cool-down of the SPIRAL2 superconducting LINAC have been performed in 2017 and 2018 respectively, followed by a total of around 5 months of tests at 4 K. Several cool-down strategies were tested, in order to minimize 100 K effect on the SC cavities. Helium bath regulations (level and pressure) have been tested and optimized. Effects of pressure instabilities and coupling with the cryogenic plant have also been observed. Cryogenic performances of each cryomodule have been measured. Low-level RF measurements were also performed on all cavities and showed unidentified modulations at frequencies around 5Hz. These turned out to be thermoacoustic oscillations (TAO) on the cryogenic lines, which generate important pressure instabilities. Several solutions to remove TAO and cure these instabilities have been tested and one has been successfully deployed., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

In the framework of the IFMIF/EVEDA project, the cryomodule of the Linear IFMIF Prototype Accelerator (LIPAc) will be assembled then tested at Rokkasho in 2019. Eight Series Power Couplers (PC) operating at 175 MHz were manufactured under a CEA contract, in order to equip this Cryomodule. They were all successfully RF conditioned up to 100 kW CW in TW and SW configurations. All the high RF power tests were performed under CIEMAT responsibility in BTESA Company premises, according to the CEA requirements. In order to fix difficulties encountered during the fab process, manufacturing and quality control have been analyzed in depth. Thanks to the corrective actions implemented, every PC reached the performances targeted for qualification. This paper will give details about this manufacturing phase and provide an overview of the obtained RF test results., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

International audience; The goal of the IFMIF-DONES (International Fusion Materials Irradiation Facility-DEMO Oriented Neutron Source) project is to build an irradiation facility that will provide a sufficient neutron flux to study and characterize structure materials foreseen for future fusion power plant. In order to accelerate the required 125mA/40 MeV continuous deuteron beam from 5 MeV to 40 MeV, a superconducting radio-frequency (SRF) linac, housed in five cryomodules, is proposed. The design is based on two beta families (β=0.11 and β=0.17) of half-wave resonators (HWR) at 175MHz. The transverse focusing is achieved using one solenoid coil per focusing period. This paper presents the extensive multiparticle beam dynamics simulations that have been performed to adapt the beam along the SRF-linac in such a high space charge regime. As one of the constraints of the IFMIF linac is a low level of beam losses, specific optimizations have been done to minimize the beam occupancy in the line (halo). A Monte Carlo error analysis has also been carried out to study the effects of misalignments or field imperfections (static errors) and also vibrations or power supplies ripple (dynamic errors). The results of these errors studies are presented and discussed.

The Double Quarter Wave (DQW) Crab Cavity was designed to rotate the colliding bunches and in consequence to increase the luminosity of the LHC machine. Prior to launching series production detailed RF validation tests, both without and with beam, were performed at CERN. For the cavity preparation and RF performance validation before installation in the cryomodule, a comprehensive programme of RF surface preparation and cavity performance evaluation in liquid helium temperatures were carried out. Due to the unusual geometry of the DQW cavity there were a number of challenges both in preparation and RF testing that had to be addressed. The results and conclusions of the preparation process and cavity performance in the vertical test cryostat are presented in this paper in the form of a short technical report.

The European Spallation Source (ESS) is currently under installation phase. It requires involvement of highly specialized engineering teams, working together in order to install all necessary equipment and to commission the facility. The engineering team from IFJ PAN is contributing to this common effort at Lund and team of twenty engineers and technicians are working at ESS site. The team is involved in three main tasks: installation of Radio Frequency Distribution System, Cryomodule tests and installation of Power Converters.

Field emission is one of the most serious issues in the degradation of superconducting cavity performance. However, surveying field emission sources during the cavity performance test (called the ``vertical test'') and cryomodule operation is difficult. Therefore, to precisely investigate electron emissions from the field emission source, we developed a diagnostic system for the energy recovery linac 1.3 GHz nine-cell superconducting cavity for both vertical tests and the cryomodule operation. The developed system is comprised of two types of sensors: a carbon sensor and a Si $p$-$i$-$n$ diode, that measure the temperature rise and the radiation produced by electron emissions, respectively. Rotating the sensor array around the cavity axis in the vertical test provides detailed information on the entire cavity surface. The precise x-ray mapping profile measured by the system in the vertical test enables identification of the local emission source. This paper describes how the field emission source is identified in the vertical test from the standpoint of not only experimental results obtained with the newly developed rotating mapping system but also the detailed approach based on precise simulations. In addition, field emission studies with the optimized instrumentation have been extended to cryomodule tests, both standalone and in the beam line. These developments allow us to monitor changes in the cavity field emission signatures introduced by cryomodule assembly and beam operation.

Superconducting Radio Frequency (SRF) cavities performance preservation is crucial, from vertical test to accelerator operation. Field Emission (FE) is still one of the performance limiting factors to overcome and plasma cleaning has been proven successful by the Spallation Neutron Source (SNS), in cleaning field emitters and increasing the work function of Nb. A collaboration has been established between Fermi National Accelerator Laboratory (FNAL), SLAC National Accelerator Laboratory and Oak Ridge National Laboratory (ORNL) with the purpose of applying plasma processing to the Linac Coherent Light Source-II (LCLS-II) cavities, in order to minimize and overcome field emission without affecting the high Q of nitrogen-doped cavities. The cleaning for LCLS-II will follow the same plasma composition adopted at SNS, which allows in-situ processing of cavities installed in cryomodules from hydrocarbon contaminants. A novel method for plasma ignition has been developed at FNAL: a plasma glow discharge is ignited using high order modes to overcome limitations imposed by the fundamental power coupler, allowing in-situ cleaning for cavities in cryomodule. The plasma can be easily ignited and tuned in each of the cavity cells using low RF power. A method for plasma detection has been developed as well, which allows the detection of the plasma location in the cavity without the need of cameras at both cavity ends. The presented method can be applied to other multi-cell cavity designs, even for accelerators where the coupling for the fundamental modes at room temperature is very weak.

The twofold goals of this work are (i) to reduce the lack of experimental data in literature concerning the effects of temperature and mechanical strain on magnetic properties of the Ni-Fe alloy Cryophy, and (ii) to validate its use as magnetic shield material for last-generation cryomodule prototypes of crab cavities used in particle accelerators for transverse deflection. The relative magnetic permeability was measured at room and cryogenic temperature, and its lowest value at 4 K fits the minimum design criteria of 100,000 for the crab cavities experiment at CERN. Permeability after uniaxial plastic deformation between 0% and 3% was also measured by means of an Epstein frame. Results show that deformation induces a significant decrease in the magnetic performances, underlining that particular care must be taken during all stages of handling and operation. Finally, the attenuation inside the magnetic shields was tested for the prototype Super Proton Synchrotron at CERN. Results highlight that at 150 mm from the opening, the magnetic field is shielded as required.

The staged installation and commissioning of LIPAc is ongoing at Rokkasho Fusion Institute of QST, Japan for validating the low energy section of the IFMIF deuteron accelerator up to 9 MeV. The LIPAc Superconducting Radio Frequency accelerator (SRF) cryomodule is assembled under the responsibility of the EU Home Team, and the assembly work recently started at Rokkasho in March 2019. To fulfil the cleanliness requirements for the assembly process, QST took the responsibility to prepare the infrastructure of a cleanroom and associated devices. In this present paper, the details of the preparation work for the cryomodule assembly made by QST will be presented., Proceedings of the 19th International Conference on RF Superconductivity, SRF2019, Dresden, Germany

For future experiments with heavy ions near the coulomb barrier within the super-heavy element (SHE) research project a multi-stage R&D program of GSI/HIM and IAP is currently in progress. It aims for developing a supercon-ducting (sc) continuous wave (CW) LINAC with multiple CH cavities as key components downstream the High Charge State Injector (HLI) at GSI. The LINAC design is challenging due to the requirement of intense beams in CW mode up to a mass-to-charge ratio of 6, while covering a broad output energy range from 3.5 to 7.3 MeV/u with unchanged minimum energy spread. Testing of the first CH-cavity in 2016 demonstrated a promising maximum accelerating gradient of E_{a} = 9.6 MV/m; the worldwide first beam test with this sc multi-gap CH-cavity in 2017 was a milestone in the R&D work of GSI/HIM and IAP. In the light of experience gained in this research so far, the beam dynamics layout for the entire LINAC has recently been updated and optimized.

Journal of physics / Conference series Conference series [...] 1350, 012089 (2019). doi:10.1088/1742-6596/1350/1/012089, Published by IOP Publ.39969, Bristol

The goal of the MYRRHA project is to demonstrate the technical feasibility of transmutation in a 100 MW Accelerator Driven System (ADS) by building a new flexible irradiation complex at Mol (Belgium). The MYRRHA facility requires a 600 MeV accelerator delivering a maximum proton current of 4 mA in continuous wave operation, with an additional requirement for exceptional reliability. Supported by SCK•CEN and the Belgium government the project has entered in its phase I: which consists in the development and the construction of the linac first part, up to 100 MeV. We review the design updates of the superconducting linac, with its enhanced fault-tolerance capabilities. The linac capabilities at 100 MeV (Phase I) and 600 MeV (ADS operation) are exposed and discussed., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

The Mainz Energy-recovering Superconducting Accelerator (MESA) will be an electron accelerator allowing operation in energy-recovery linac (ERL) mode. It provides the opportunity to operate scattering experiments at energies of ~100 MeV with thin gas-targets. The MESA Internal Gas Target Experiment (MAGIX) aims to operate windowless jet targets and different gases up to Xenon to search for possible dark photon interactions, to precisely measure the magnetic proton radius and astrophysical S-factors. Investigations on the impact of the target on beam dynamics and beam losses are required for machine safety and to examine limits to ERL operation. The goal of this work is to understand target induced halo in the different experimental setups, track halo particles through downstream sections to examine beam losses and include a suitable collimation system and shielding into the accelerator layout to protect the machine from direct and indirect damage through beam losses and radiation. The present status of the investigations is presented., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

MESA is a two pass energy recovery linac (ERL) currently under construction at the Johannes Gutenberg-University in Mainz. MESA uses four 1.3 GHz TESLA type cavities with 12.5 MVm-1 of accelerating gradient in two modified ELBE type cryomodule with improved thermal connection of the HOM antennas and cw operation. In the first stage of MESA operation 1mA of beam current is foreseen, which will later be upgraded to 10 mA. One potential limit to maximum beam current in ERLs is the transverse beam breakup (BBU) instability induced by dipole Higher Order Modes (HOMs). These modes can be excited by bunches passing through the cavities off axis. Following bunches are then deflected by the HOMs, which results in even larger offsets for recirculated bunches. This feedback can even lead to beam loss. Simulation results for HOM spectra of a single TESLA cavity are available for example in [1]. It was possible to measure the HOM spectra in the cold, not tuned cavities at DESY and in the cold string tuned to the 1.3 GHz fundamental mode at Mainz. Results for the maximum beam current for MESA, limited by BBU, for the various HOM spectra are presented.

CEA is in charge of the 30 elliptical medium and high-beta cryomodules to be installed in the ESS tunnel in Lund, Sweden. Before launching the assembly of the series cryomodules, CEA developed a medium-beta cryomodule technology demonstrator in a collaboration with IPNO, LASA and ESS. This paper briefly presents the cryomodule assembly and summarizes the main results of the high RF power tests performed in 2018 in a dedicated test stand in CEA Saclay. The main ESS requirements were reached: E_{acc} = 16.7 MV/m in cavities, Pforward = 1.1 MW in power couplers, RF pulses length = 3.6 ms at 14 Hz. The piezo tuners efficiently compensated the Lorentz forces detuning and could stabilize the accelerating field better than 1% over the full length of the expected ESS 2.86 ms beam pulse without any LLRF regulation system. Following this successful validation CEA started the assembly of the first ESS medium-beta series cryomodule, Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

166.6-MHz superconducting cavities have been chosen for the High Energy Photon Source (HEPS) as the main accelerating structures to provide 900 kW of beam power and 5.4 MV of accelerating voltage. A proof-of-principle cavity adopting the quarter-wave beta = 1 geometry was previously developed. Excellent performance was achieved in vertical tests at cryogenic temperatures. The cavity was later welded with a helium jacket, dressed with a power coupler and other ancillaries, and high-power tested in a test cryomodule. Performance degradation was observed and analyzed. Evidence from temperature sensor readout and heat loss measurement results suggested an overheating in the cavity–coupler interface region causing a “thermal runaway” and eventually quenching the cavity at its design voltage. Electromagnetic-fluid-thermal coupled simulation has thus been conducted, and the hypothesis was nicely validated. Finally, solutions were proposed including an elongated niobium extension tube at the coupler port and an optimized helium gas cooling of the power coupler’s outer conductor. These modifications have been subsequently applied on the 166.6-MHz higher-order-mode damped superconducting cavities for the HEPS. Heat loss at 4.2 K contributed by the power coupler can be largely reduced with a modest gas cooling scheme. Similar design approaches can also be applied to other non-elliptical superconducting structures with on-cavity high-power coupler mountings.

650 MHz 2-cell superconducting cavities are proposed for the main ring of the Circular Electron–Positron Collider (CEPC). The cavity is designed with two detachable double-notch higher order mode (HOM) couplers to suppress the fundamental mode and extracted the HOM power. The present paper describes the design, fabrication, surface treatment and vertical tests of the cavities and HOM couplers. The performances of the cavity at 2 K is not affected after the installation of the HOM coupler. The maximum intrinsic quality factor Q 0 of the cavity dressed with the HOM coupler reached 3 . 1 × 1 0 10 at operation accelerating gradient. The vertical tests results showed that the fundamental mode external quality factor Q e of all HOM couplers is an order of magnitude larger than Q 0 of the cavity. The HOM damping results for the 650 MHz 2-cell cavity were also measured at cryogenic temperature and compared with the simulated and measured results at room temperature. Two 650 MHz 2-cell cavities each with two HOM couplers will be assembled in the CEPC 650 MHz test cryomodule after helium vessel dressing.

Following a review of the design options for a superconducting ion linac, we present an alternative design for the pre-stripper section of the superconducting driver linac for the rare isotope science project (RISP) of the Institute for Basic Science in Korea. The proposed alternative design takes advantage of the recent accelerator developments at Argonne, namely for the recent ATLAS intensity and efficiency upgrade and the Fermilab proton improvement plan project (PIP-II). In particular, the state-of-the-art performance of quarter-wave (QWRs) and half-wave resonators (HWRs), the integrated steering correctors and cold beam position monitors (BPMs) for a compact cryomodule design. In order to simplify the design and avoid frequency transitions, we used two types of QWRs at 81.25 MHz while the baseline design has QWRs at 81.25 MHz and HWRs at 162.5 MHz. The new QWR types were optimized for β ∼ 0.05 and ∼ 0.11, respectively, and corrected for beam steering effects. Nine cryomodules are required to reach the stripping energy of 18.5 MeV/u for uranium beam. An alternative radio-frequency quadrupole (RFQ) design was also developed. It is optimized with the multi-harmonic buncher (MHB) to produce a much smaller longitudinal emittance, offering much needed flexibility for beam tuning and more tolerance to errors. Following the lattice design optimization, end-to-end beam dynamics simulations including most important sources of machine error were performed. The results showed that the design is robust and tolerant to errors with no beam loss observed for typical machine errors.

A horizontal test facility is set up at the Raja Ramanna Centre for Advanced Technology to test the superconducting radio frequency dressed cavities. Along with the cryomodule, control instrumentation, and the power coupler, this facility incorporates a high-power solid-state amplifier for establishing the desired cavity voltage gradient during the testing. This article describes the design, construction, rigorous testing, and measured results of this high-power solid-state radio frequency amplifier and its constituent components. Its maximum output power is 36 kW (average) at the operating frequency of 650 MHz. Its main features are its modular and scalable design with in-house developed constituent components. These components include 500 W, 20 dB gain modules, novel two-tier radial dividers, combiners, power sensors, and aperture-coupled directional couplers. Their excellent reprise performance for the multiple quantities confirms the design methodology presented here. The measured wall plug efficiency of this 36 kW amplifier is 43.6%, and its power gain is 86 dB. The designed radial combiner is highly efficient (power-combining efficiency of 98.4%), and the directional coupler exhibits a very low loss (insertion loss of 0.05 dB).