New materials beyond the standard bulk niobium have the potential to greatly improve the performance of Superconducting Radio Frequency (SRF) cavities. Specifically, thin coatings of normal conductors such as gold have the potential to improve the key RF performance metric of quality factor. We present progress on depositing thin gold layers onto 2.6 GHz SRF cavities and testing their RF performance.

The Cornell High Pulsed Power Sample Host Cavity (CHPPSHC) is a new system designed to measure the superheating field of candidate superconducting RF (SRF) materials, giving insight into their operational limits. This system is designed to reach peak magnetic fields of up to 0.5 T in only a few microseconds, allowing us to achieve a pure magnetic field quench on the sample. We present an overview of the CHPPSHC system and proof of principle data from a niobium sample.

Superconducting RadioFrequency (SRF) technology is a key component in many particle accelerators operating in a continuous wave, or high duty cycle, mode. The on-line performance of SRF cavities can be negatively impacted by the gradual reduction in the accelerating gradient that can be attained within a reasonable field emission level. Conventional cleaning procedures are both time- and resource-exhaustive as they are done *ex-situ*. As such, *in-situ* techniques are quite attractive. Plasma processing is an emerging *in-situ* method of cleaning which utilizes a mixture of oxygen and an inert gas to chemically remove hydrocarbon-based field emitters through plasma. At TRIUMF's Advanced Rare IsotopE Laboratory (ARIEL), an R&D program is in place to develop plasma processing procedures using fundamental power couplers on 1.3 GHz ARIEL 9-cell cavities. Single cell and multi-cell processing has been performed off-line. The studies involve varying the input parameters and testing the effectiveness of the treatment through RGA analysis. The progress on the developments will be reported.

It has been found in benchmark tests that some Single Spoke Resonator Type-2 (SSR2) cavities have early field emission onset as well as strong multipacting barriers. A longstanding hypothesis is that field-emitted electrons in the high electric field accelerating gap can migrate and ignite multipacting bands in the low electric field regions of the cavity periphery. In this study, we use simulation techniques to examine multipacting behavior in SSR2 cavities from electrons seeded in common field emitter locations. Additionally, we investigated seed locations for areas in SSR2 cavities which may have poor coverage during high pressure water rinsing and compared the multipacting behavior.

Recent studies indicate the magnitude of an anomalous decrease in the resonant frequency, so-called frequency dip, near critical temperature of superconducting niobium cavities, Tc, correlates to the cavity quality factor, Q0, and impurities introduced into the superconducting niobium surfaces, such as nitrogen or oxygen. We measured frequency dips in both 644 MHz fundamental mode (FM) and 1.45 GHz higher-order mode (HOM) of single-cell elliptical cavities for FRIB energy upgrade (FRIB400) R&D. These measurements were performed in cavities with the following surface treatments: 1) electropolished (EP) only, 2) nitrogen-doped (N-doping), 3) medium-temperature (mid-T) baked and then hydrofluoric (HF) acid rinsed. We will present measured frequency dips and compare them to cavity Q0 performance in the FM. Frequency-dependent behavior of frequency dips with various surface treatments will also be discussed as our experimental setup has a unique feature compared to previous studies, which allows for measurement of frequency dips in different modes within the same cavity, in other word, on the same surfaces.

Dust particulates are always present to some degree inside the vacuum space of particle accelerators, causing a variety of issues. At the LHC, beam loss events have been linked to the interaction of charged dust with the proton beams. In superconducting rf cavities, dust contamination leads to field emission, limiting the accelerating gradient and causing damage to external beamline components. Facilities such as the SLAC LCLS-II and TRIUMF electron linear accelerator see progressive onsets in field emission that cannot simply be explained by vacuum events. The environment of a particle accelerator provides an ideal opportunity for dust to gain charge, which is one of the main drivers of dust grain dynamics in vacuum. However, fundamental parameters such as the dust composition and charge to mass ratio of these grains are unique to each accelerator environment and remain largely unknown. We will present an analysis of dust samples taken from TRIUMF linear accelerators, detailing their size, composition and potential sources. Preliminary results from experimental studies on the charging, detachment and migration mechanisms acting on micron sized particulates will also be presented.

Nb$_3$Sn is the most promising alternative material for the future of superconducting radio-frequency (SRF) technology, steadily advancing towards practical applications. Having a critical temperature twice that of niobium, Nb$_3$Sn offers the potential for developing smaller, more powerful, and more efficient accelerators. We have designed a comprehensive study to synthesize and characterize substrate treatments at nucleation temperatures following the thermal vapor diffusion growth process to improve the uniformity of Nb$_3$Sn coatings, pushing its performance closer to fundamental limits.

In-situ plasma processing is a promising technique to reduce field emission in superconducting radio-frequency cavities and thus maintain maximum accelerator performance for long-term operation. Continuous-wave accelerators such as FRIB are more challenging than pulsed accelerators due to relatively weak coupling (Qext = 2E6 to 1E7 for FRIB) via the fundamental power coupler (FPC). This results in an unfavorable mismatch at room temperature and makes fundamental-mode plasma processing difficult. Hence we have investigated the use of higher-order-modes (HOMs) with less FPC mismatch. Several HOMs are promising for lower-mismatch plasma generation. However, HOMs often present a less favorable plasma distribution. To improve the plasma distribution, we are studying techniques to drive the plasma with two HOMs simultaneously. Plasma development results will be presented for FRIB beta = 0.085 quarter wave resonators including ignition threshold measurements and plasma distribution assessments.

The SRF community has shown that introducing certain impurities into high-purity niobium can improve quality factors and accelerating gradients. We question why some impurities improve RF performance while others hinder it. The purpose of this study is to characterize the impurities of niobium coupons with a low residual resistance ratio (RRR) and correlate these impurities with the RF performance of low RRR cavities so that the mechanism of impurity-based improvements can be better understood and improved upon. The combination of RF testing, temperature mapping, frequency vs temperature analysis, and materials studies reveals a microscopic picture of why low RRR cavities experience low BCS resistance behavior more prominently than their high RRR counterparts. We evaluate how differences in the mean free path, grain structure, and impurity profile affect RF performance. The results of this study have the potential to unlock a new understanding on SRF materials and enable the next generation of high Q/high gradient surface treatments.

The performance of superconducting radiofrequency (SRF) cavities is critical to enabling the next generation of efficient high-energy particle accelerators. Recent developments have focused on altering the surface impurity profile through in-situ baking, furnace baking, and doping to introduce and diffuse beneficial impurities such as nitrogen, oxygen, and carbon. However, the precise role and properties of each impurity are not well understood. In this work, we attempt to disentangle the role of nitrogen and oxygen impurities through time-of-flight secondary ion mass spectrometry of niobium samples baked at temperatures varying from 75-800 C with and without nitrogen injection. From these results, we developed treatments recipe that decouple the effects of oxygen and nitrogen in doping treatments. Understanding how these impurities and their underlying mechanisms drive further optimization in the tailoring of impurity profiles for high-performance SRF cavities.

New materials beyond the standard bulk niobium have the potential to greatly improve the performance of Superconducting Radio Frequency (SRF) cavities. Specifically, thin coatings of normal conductors such as gold have the potential to improve the key RF performance metric of quality factor. We present progress on depositing thin gold layers onto 2.6 GHz SRF cavities and testing their RF performance.

The world’s first 1.3 GHz cryomodule containing eight 9-cell superconducting radio-frequency (RF) cavities treated by medium-temperature furnace baking (mid-T bake) was developed, assembled and tested at the Institute of High Energy Physics (IHEP), Chinese Academy of Sciences for the Dalian Advanced Light Source (DALS). The 9-cell cavities in the cryomodule achieved an unprecedented high average intrinsic quality factor (Q0) of 3.8E10 at 16 MV/m and 3.6E10 at 21 MV/m in the horizontal test. The cryomodule can operate stably up to a total continuous wave (CW) RF voltage greater than 191 MV, with an average cavity usable accelerating gradient of more than 23 MV/m. The results significantly exceed the specifications of DALS and the other high repetition rate free electron laser facilities (LCLS-II, LCLS-II-HE, SHINE, S3FEL etc.). This paper reviews the cryomodule performance and discusses some important issues in cryomodule assembly and testing.

The Cornell High Pulsed Power Sample Host Cavity (CHPPSHC) is a new system designed to measure the superheating field of candidate superconducting RF (SRF) materials, giving insight into their operational limits. This system is designed to reach peak magnetic fields of up to 0.5 T in only a few microseconds, allowing us to achieve a pure magnetic field quench on the sample. We present an overview of the CHPPSHC system and proof of principle data from a niobium sample.

A new type tuner is designed for the double spoke superconducting cavity of the Spallation neutron Source Phase II project in China. The tuner is mounted on the side of the cavity, and each module contains two tuner systems. In this paper, the structure and working principle of the tuner are designed and analyzed, also the testing results of the tuner with the superconducting cavity system as a whole is introduced.

Nb$_3$Sn is the most promising alternative material for the future of superconducting radio-frequency (SRF) technology, steadily advancing towards practical applications. Having a critical temperature twice that of niobium, Nb$_3$Sn offers the potential for developing smaller, more powerful, and more efficient accelerators. We have designed a comprehensive study to synthesize and characterize substrate treatments at nucleation temperatures following the thermal vapor diffusion growth process to improve the uniformity of Nb$_3$Sn coatings, pushing its performance closer to fundamental limits.

The Cornell High Pulsed Power Sample Host Cavity (CHPPSHC) is a new system designed to measure the superheating field of candidate superconducting RF (SRF) materials, giving insight into their operational limits. This system is designed to reach peak magnetic fields of up to 0.5 T in only a few microseconds, allowing us to achieve a pure magnetic field quench on the sample. We present an overview of the CHPPSHC system and proof of principle data from a niobium sample.

New materials beyond the standard bulk niobium have the potential to greatly improve the performance of Superconducting Radio Frequency (SRF) cavities. Specifically, thin coatings of normal conductors such as gold have the potential to improve the key RF performance metric of quality factor. We present progress on depositing thin gold layers onto 2.6 GHz SRF cavities and testing their RF performance.

As part of the PIP-II project at Fermilab, a pre-production cryomodule featuring 325 MHz Single Spoke Resonator type 2 (SSR2) superconducting RF cavities is under construction. These SSR2 cavities are fabricated by industry partners and undergo initial cold testing at our collaborating institution, IJCLab in France, utilizing low-power coupler. Subsequently, the cavities are subjected to final qualification at Fermilab, complete with tuner and high-power coupler assemblies. This paper provides an overview of the ongoing efforts dedicated to high-power testing of jacketed SSR2 cavities in the Spoke Test Cryostat (STC) at Fermilab. Performance parameters obtained from these tests are presented, offering valuable insights into the cavities’ operational characteristics and readiness for integration into the PIP-II cryomodule.

To minimize the contamination of SRF cavities, remote installation techniques are needed during the installation of components. Recent work at Fermilab has been performed to begin the process of developing techniques for assembling cavities using robotics. Multiple alignment methods were prototyped including alignment and computer vision methods. Using a remotely controlled robotic arm, the alignment and installation of couplers have been successfully performed on prototype PIP-II SSR2 cavities in a cleanroom. The installation process will be shown to show to demonstrate the potential of future installations on other cavities and cavity ancillaries.

For assemblies of cavities in cleanrooms, single-use tooling systems are made for the alignment and installation of ancillary components such as couplers and bellows. To try and minimize the amount of tooling sets used, a design has been created to standardize alignment features to allow for assembly of different components with one set of tooling. A prototype set of tooling has been developed to with the required degrees of freedom for multiple assemblies while minimizing deformation during the assembly process. Prototype designs have been created for PIP-II SSR2 and 650 Cavities and for AUP Crab Cavities. Using 3D printing, this tooling can be quickly adjusted to allow for different ancillary components. The development process and status of the design will be discussed.

The PIP-II linac will include thirty-five 325 MHz Single Spoke Resonators Type 2 (SSR2) cavities. Each cavity will be equipped with a tuner for resonance control. The tuner consists of mechanical frame with a motor for coarse frequency tuning and a piezoelectric actuator for fine frequency tuning. The tuner was tested for the first time at Fermilab on an SSR2 cavity. This dressed cavity-tuner system was tested at the single spoke testing cryo-stat (STC) in Fermilab at 2 K. The tuner performance was evaluated and is presented. Lastly, cavity-tuner mechanical modes were measured via the piezos.

High-efficiency sub-GHz elliptical superconducting RF cavity are a critical enabling technology for multiple upcoming accelerator development projects such as for the Powerful Energy Recovery Linac for Experiments (PEARLE), the Future Circular Collider (FCC) FCC Booster, and for a certain realization of the FCC Collider ring. The ambitious quality factor and gradient requirements of these projects require strong R&D programs applying advanced surface processing techniques such as mid-T baking to 800 MHz cavities. We report the current achievements of our current high-Q development program including the first mid-T baking of an 800 MHz 5-cell elliptical niobium cavity compatible with PEARLE and FCC applications.

The implementation of High Pressure Rinse (HPR) not only ensures thorough cleaning of the inner high purity niobium surface of Superconducting Radio Frequency (SRF) cavities but also unlocks their full potential for achieving peak performance. By effectively removing contaminants and impurities, HPR sets the stage for enhanced superconducting properties, improved energy efficiency, and superior operational stability. A simulation tool has been developed, facilitating the accurate prediction of both the quality and effectiveness of the rinsing process before its execution in the cleanroom. This tool, the focus of this paper, stands as a pivotal advancement in optimizing Superconducting Radio Frequency (SRF) cavity preparation. Furthermore, our paper will also present correlations with cavity cold testing results, demonstrating the practical applicability and reliability of the simulation predictions in real-world scenarios.

The world’s first 1.3 GHz cryomodule containing eight 9-cell superconducting radio-frequency (RF) cavities treated by medium-temperature furnace baking (mid-T bake) was developed, assembled and tested at the Institute of High Energy Physics (IHEP), Chinese Academy of Sciences for the Dalian Advanced Light Source (DALS). The 9-cell cavities in the cryomodule achieved an unprecedented high average intrinsic quality factor (Q0) of 3.8E10 at 16 MV/m and 3.6E10 at 21 MV/m in the horizontal test. The cryomodule can operate stably up to a total continuous wave (CW) RF voltage greater than 191 MV, with an average cavity usable accelerating gradient of more than 23 MV/m. The results significantly exceed the specifications of DALS and the other high repetition rate free electron laser facilities (LCLS-II, LCLS-II-HE, SHINE, S3FEL etc.). This paper reviews the cryomodule performance and discusses some important issues in cryomodule assembly and testing.

At the Los Alamos Neutron Science Center (LANSCE), the accelerator operation is loss-dominated, and the losses are primarily minimized via operators’ intuition. The physics tune-up procedures for the linac, including the Drift Tube Linac (DTL) and the Side-Coupled Cavity Linac (CCL), does not take the bunch distribution into consideration. For the DTL, only statistical quantities like the full width half maximum are considered but not the whole phase scan distributions. For the CCL, a single particle model is used. In this work, we demonstrate an improved tuning tool to incorporate the simulated bunch distribution via the multi-particle High-Performance Simulator (HPSim) for the physicists to monitor the bunch distribution and losses during the tune-up process.

FRIB is developing a new N-doping method with a simplified recipe. This recipe is called wet nitrogen doping, by adding nitric acid to the conventional EP acid. Nitrogen doping introduces impurities to the SRF surface, and reduces the BCS resistance by shortening the mean free path, which leads to a higher Qo. Conventional nitrogen doping, developed at FNAL and Jlab, requires a high-temperature treatment (900 ºC), and an additional light EP to remove the over-contaminated layer. This recipe produces a decreasing Qo at extremely low fields but successfully achieves high Qo performance up to 25 MV/m. The wet doping method does not require additional high-temperature baking and light EP afterwards, therefore it is superior in terms of processing steps. This method produced a high Qo of 8x10^10 at a low field of 0.5MV/m without the decreasing trend on FRIB beta=0.53 HWR. In this presentation, we will show the related R&D results generated from the FRIB 0.53 HWRs.

It has been found in benchmark tests that some Single Spoke Resonator Type-2 (SSR2) cavities have early field emission onset as well as strong multipacting barriers. A longstanding hypothesis is that field-emitted electrons in the high electric field accelerating gap can migrate and ignite multipacting bands in the low electric field regions of the cavity periphery. In this study, we use simulation techniques to examine multipacting behavior in SSR2 cavities from electrons seeded in common field emitter locations. Additionally, we investigated seed locations for areas in SSR2 cavities which may have poor coverage during high pressure water rinsing and compared the multipacting behavior.

In-situ plasma processing is a promising technique to reduce field emission in superconducting radio-frequency cavities and thus maintain maximum accelerator performance for long-term operation. Continuous-wave accelerators such as FRIB are more challenging than pulsed accelerators due to relatively weak coupling (Qext = 2E6 to 1E7 for FRIB) via the fundamental power coupler (FPC). This results in an unfavorable mismatch at room temperature and makes fundamental-mode plasma processing difficult. Hence we have investigated the use of higher-order-modes (HOMs) with less FPC mismatch. Several HOMs are promising for lower-mismatch plasma generation. However, HOMs often present a less favorable plasma distribution. To improve the plasma distribution, we are studying techniques to drive the plasma with two HOMs simultaneously. Plasma development results will be presented for FRIB beta = 0.085 quarter wave resonators, including ignition threshold measurements and plasma distribution assessments.

Recent studies indicate the magnitude of an anomalous decrease in the resonant frequency, so-called frequency dip, near critical temperature of superconducting niobium cavities, Tc, correlates to the cavity quality factor, Q0, and impurities introduced into the superconducting niobium surfaces, such as nitrogen or oxygen. We measured frequency dips in both 644 MHz fundamental mode (FM) and 1.45 GHz higher-order mode (HOM) of single-cell elliptical cavities for FRIB energy upgrade (FRIB400) R&D. These measurements were performed in cavities with the following surface treatments: 1) electropolished (EP) only, 2) nitrogen-doped (N-doping), 3) medium-temperature (mid-T) baked and then hydrofluoric (HF) acid rinsed. We will present measured frequency dips and compare them to cavity Q0 performance in the FM. Frequency-dependent behavior of frequency dips with various surface treatments will also be discussed as our experimental setup has a unique feature compared to previous studies, which allows for measurement of frequency dips in different modes within the same cavity, in other word, on the same surfaces.

Field emission (FE) is a major contributor to degradation in the high-field performance of Superconducting Radio Frequency (SRF) cavities. The driver linac for the Facility for Rare Isotope Beams (FRIB) has been operating for user experiments since May 2022, using 104 quarter-wave resonators and 220 half-wave resonators in 46 cryomodules. We have used pulsed RF conditioning to mitigate the FE X-rays and maintain the cavities’ performance. During conditioning, we observe "electrical breakdown," a rapid (<1us) collapse of the field. We have found that the FE X-rays may be greatly reduced after a single to several electrical breakdown events, which are accompanied by a local discharge in the vacuum and burning out of the emitter on the cavity surface. On the other hand, when a slow (~ms) thermal breakdown (known as quench) is seen, it limits the field and hampers further FE conditioning. We have also investigated the field enhancement factor and the effective area of FE emitter, inferred by Fowler-Nordheim fitting of FE X-ray dose rate vs accelerating gradient. In this paper, we will present RF pulse conditioning results and analysis thereof for about 50 cavities in FRIB cryomodules.

Nb3Sn is one of the most promising materials for the next generation of superconducting RF (SRF) cavities. One reason is that Nb3Sn cavities can achieve high Q-values at 4 K, whereas conventional Nb cavities need to be cooled down to 2 K. This allows for the operation of SRF cavities with conduction cooling, eliminating the need for liquid helium, unlike conventional SRF cavities which require immersion cooling. KEK started Nb3Sn deposition tests on the single-cell cavity based on the Sn vapor diffusion method around 2019 and has steadily improved the cavity performance. In addition, a small deposition furnace for the sample study was constructed last year to investigate the relationship between Nb3Sn film quality and deposition parameters and to improve the throughput of the deposition study. We will report the results of deposition tests on samples and RF measurements of single-cell Nb3Sn cavities.

A heavy-ion accelerator facility was constructed for the Rare Isotope Science Project (RISP) at the Institute for Rare Isotope Science (IRIS) in Daejeon, Korea. A cryomodule with quarter-wave resonators (QWRs) and half-wave resonators (HWRs) was installed in the SCL (Superconducting Linac) 3 tunnel,and the initial beam commissioning using argon beams has been completed. Additionally, a cryomodule with single-spoke resonators (SSRs), power couplers,and tuners is currently under development for the SCL2 project. The geometry of the power couplers for the SSRs is a coaxial capacitive type based on a conventional 3-1/8 inch Electronic Industries Alliance (EIA) coaxial transmission line with a single ceramic window. A multi-physics analysis, incorporating electromagnetic, thermal, and mechanical aspects, was conducted to evaluate the design of the power coupler for the SSRs. This paper presents the results of the multi-physics analysis and the current design status of the power coupler for the SSRs.

A 325 MHz, optimal beta = 0.40 niobium half-wave resonator (HWR) called HWR040 for the superconducting driver linac of the China initiative Accelerator-Driven subcritical System (CiADS) has been designed and analysed at the Institute of Modern Physics, Chinese Academy of Sciences (IMP, CAS). The linac requires 60 HWR040s to accelerate protons from 45 MeV to 175 MeV. This paper mainly presents the multi-physics studies of the HWR040, include electromagnetic optimization, mechanical structure design and heat transfer simulation of the cavity, to predict the behaviour of the cavity under practical operating process.

Sustainability and cost reduction are key factors for the development of future large particle accelerators. This motivated INFN LASA to initiate an INFN-funded R&D program dedicated to improve the performance of SRF Nb cavities in terms of quality factor (High-Q) and accelerating gradient (High-G). The R&D program will start by exploiting state-of-the-art surface treatments on 1.3 GHz single-cell prototypes, in view of a possible industrialization process for large-scale productions. Integrating part of this program is the upgrade of our vertical test facility to enable qualification of such high-performance cavities. Ongoing activities include the construction of a new dedicated cryostat, which minimizes Liquid Helium consumption, reduces the impact of trapped magnetic flux and provides a wide range of diagnostics for quench, field emission, and magnetic flux expulsion studies.

The status of INFN LASA in-kind contribution to the PIP-II project at Fermilab is reported in this paper. The effort for the series production of the 38 INFN LASA designed, 5-cell cavities with beta 0.61 for the LB650 section of the linac commenced and the status of ongoing activities and major procurements is here conveyed. At the same time, preliminary tests on INFN LB650 cavity prototypes are progressing in order to optimize the complete preparation and qualification cycle. All cavities will be produced, and surface treated in industry to reach the unprecedented performances required, qualified through vertical cold test at state-of-the art infrastructures and delivered as installation ready at the string assembly site.

RadiaBeam is designing a 915 MHz, 25 kW CW Fundamental Power Coupler (FPC) to power a Nb3Sn coated superconducting radio-frequency (SRF) cavity. Unlike traditional FPCs for SRF cavities, the device relies only on conductive cooling by cryocoolers. The baseline design was adapted from the liquid helium cooled 805 MHz SNS FPC with the notable addition of an intermediate 50 K thermal intercept and associated RF shield. Engineering design details to address the thermomechanical, manufacturability, and structural challenges will be presented. Particular emphasis will be placed on static and dynamic heat load management along with finite element analysis to validate mechanical stability. Additionally, initial manufacturing studies of the coaxial window brazing will be discussed along with full device manufacturing and integration plans.

Plasma processing is a common technique where the free oxygen produced in a low-pressure RF plasma breaks down and removes hydrocarbons from surfaces. This increases the work function and reduces the secondary emission coefficient of the treated surfaces. Jefferson Lab has an ongoing R&D program in plasma processing. The experimental program investigated processing using argon/oxygen and helium/oxygen gas mixtures. The initial focus of the effort was processing C100 cavities by injecting RF power into the HOM coupler ports. We also developed the methods for establishing a plasma C75 cavities where the RF power is injected via the fundamental power-coupler. As part of the process development we processed, three C100 cryomodules in our off-line cryomodule test facility. In May 2023 we processed four C100 cryomodules in-situ in the CEBAF accelerator with the cryomodules returning to an operational status in Sept. 2023. The improvement in field emission free operation, as measured on a cavity by cavity basis, was 59 MeV or 24%. At the time that this abstract was written, the plans are to process an additional 5 to 7 cryomodules in the CEBAF accelerator in the summer of 2024. Methods systems and results from processing cryomodules and individual cavities in the vertical test will be presented. Current status and future plans will also be presented. Funding provided by SC Nuclear Physics Program through DOE SC Lab funding announcement DE-FOA-0002670.

Two 1.5 GHz CEBAF C75-shape 5-cell accelerator cavities were coated with Nb3Sn film using the vapor diffusion technique at Fermilab and Jefferson Lab coating facilities. Both cavities were measured at 4 K and 2 K in the vertical dewar test in each lab, then assembled into a CEBAF quarter cryomodule at Jefferson Lab. The cryomodule was tested in 4 K and 2 K in the CryoModule Test Facility at Jefferson Lab. RF test results for both cavities in the cryomodule are similar to those of the qualification test in VTS, with one cavity reaching Eacc = 7.5 MV/m and the other - 13 MV/m at 4 K. In this contribution we discuss the progress with assembling Nb3Sn cavities in a cryomodule and the first results from cryomodule testing.

Traveling-wave (TW) technology can push the accelerator field gradient of niobium SRF cavity to 70MV/m or higher beyond the limit of 50~60MV/m in Standing-wave (SW) technology. The early stages of TW SRF cavity developments had been funded by several SBIR grants to Euclid Techlabs and completed in collaboration with Fermilab through a 1-cell prototype and a proof-of-principle 3-cell TW cavity. The TW resonance excitation in the 3-cell TW cavity at 2K was demonstrated through the low power RF test in early 2024. A high-power test of the 3-cell in TW mode being prepared. To advance a design and technology to fabricate a novel high gradient TW SRF cavity, FNAL proposed a half-meter TW RF design and R&Ds to realize that are in progress. Here we will report the recent progress in the 3-cell TW cavity and the challenges towards a half-meter scale TW cavity.

Dust particulates are always present to some degree inside the vacuum space of particle accelerators, causing a variety of issues. At the LHC, beam loss events have been linked to the interaction of charged dust with the proton beams. In superconducting rf cavities, dust contamination leads to field emission, limiting the accelerating gradient and causing damage to external beamline components. Facilities such as the SLAC LCLS-II and TRIUMF electron linear accelerator see progressive onsets in field emission that cannot simply be explained by vacuum events. The environment of a particle accelerator provides an ideal opportunity for dust to gain charge, which is one of the main drivers of dust grain dynamics in vacuum. However, fundamental parameters such as the dust composition and charge to mass ratio of these grains are unique to each accelerator environment and remain largely unknown. We will present an analysis of dust samples taken from TRIUMF linear accelerators, detailing their size, composition and potential sources. Preliminary results from experimental studies on the charging, detachment and migration mechanisms acting on micron sized particulates will also be presented.

Nb3Sn is the most advanced potential successor for niobium in superconducting RF accelerator cavities. Nb3Sn has a significantly higher critical temperature (18.3 K) compared to that of niobium (9.2 K). This has a large effect on the BCS surface resistance, and therefore, on the dynamic RF losses at 4.5 K. The higher critical temperature allows two important changes for cavity and cryomodule design. First, the lower BCS losses allow the designer to use a higher frequency, translating to physically smaller cavities and cryomodules. Second, the low dynamic losses allow the use of stand-alone cryocoolers instead of complex helium refrigerators and distribution systems. Fabrication of a prototype 218 MHz cavity, test results, and continuing challenges are discussed.

Two 1.5 GHz CEBAF C75-shape 5-cell accelerator cavities were coated with Nb3Sn film using the vapor diffusion technique at Fermilab and Jefferson Lab coating facilities. Both cavities were measured at 4 K and 2 K in the vertical dewar test in each lab, then assembled into a CEBAF quarter cryomodule at Jefferson Lab. The cryomodule was tested in 4 K and 2 K in the CryoModule Test Facility at Jefferson Lab. RF test results for both cavities in the cryomodule are similar to those of the qualification test in VTS, with one cavity reaching Eacc = 7.5 MV/m and the other - 13 MV/m at 4 K. In this contribution we discuss the progress with assembling Nb3Sn cavities in a cryomodule and the first results from cryomodule testing.

Traveling-wave (TW) technology can push the accelerator field gradient of niobium SRF cavity to 70MV/m or higher beyond the limit of 50~60MV/m in Standing-wave (SW) technology. The early stages of TW SRF cavity developments had been funded by several SBIR grants to Euclid Techlabs and completed in collaboration with Fermilab through a 1-cell prototype and a proof-of-principle 3-cell TW cavity. The TW resonance excitation in the 3-cell TW cavity at 2K was demonstrated through the low power RF test in early 2024. A high-power test of the 3-cell in TW mode being prepared. To advance a design and technology to fabricate a novel high gradient TW SRF cavity, FNAL proposed a half-meter TW RF design and R&Ds to realize that are in progress. Here we will report the recent progress in the 3-cell TW cavity and the challenges towards a half-meter scale TW cavity.

Fermilab has optimized the surface processing conditions for PIP-II high beta 650 MHz cavities. This encompasses conditions for bulk electropolishing, heat treatment, nitrogen doping, post-doping final electropolishing, and post-processing surface rinsing. The technology has been effectively transitioned to industry. This paper highlights the efforts made to fine-tune the process and to smoothly share them with the partner labs and an associated vendor.

Mechanical grinding is commonly employed to eliminate surface defects such as scratches and pits from niobium cavity surfaces or sheets before cavity fabrication. Subsequently, chemically buffered polishing or electropolishing is often utilized to completely remove residues of the polishing media and any defects induced by mechanical grinding, ensuring a pristine surface. In this study, we conducted a systematic investigation to assess the influence of mechanical grinding using silicon car-bide and aluminum oxide polishing media on niobium surfaces. Additionally, the study examines the effects of post-mechanical grinding chemical treatments on surface quality.

The electropolishing process and cathodes have undergone modification and optimization for both low- and high-beta 650 MHz five-cell niobium cavities. Cavities treated with these novel electropolishing conditions exhibited superb surface quality and performance in baseline tests. Nonetheless, due to administrative constraints on project cavities, maximum gradient performance testing was not conducted. This paper presents a study conducted on a single-cell 650 MHz cavity utilizing the optimized electropolishing conditions, highlighting the maximum performance attained for this specific cavity.

Conduction-cooled SRF niobium cavities are being developed for use in compact, continuous-wave electron linear accelerators for a variety of industrial applications. A 915 MHz two-cell cavity has been designed to achieve an energy gain of 3.5 MeV. The design of the cell shape aims at minimizing the peak surface magnetic field. Field flatness is achieved by adjusting the length of the outer end half-cells. The higher-order mode analysis shows that absorbers are not required for a moderate beam current of 5 mA. One of the beam tubes has two side-ports for insertion of coaxial fundamental power couplers. The mechanical design and analysis were done to maintain a stress near or less than 15.5 MPa for all anticipated loading conditions. This is half the measured yield strength and is to provide relief from creep when the cavity is evacuated and stored with outside atmospheric pressure.

The Viton gate valves installed in the CEBAF beamline have significantly degraded after long-term operation in a radiation environment, generating numerous particles that cause heavy contamination and strong field emission. As a replacement, all-metal gate valves have been proposed for installation in the CEBAF beamline. In this paper, we present thorough comparison tests between the Viton gate valves and the all-metal gate valves, including evaluations of particle levels, aging tests of the gate valves, and analysis of the particle material.

The bipolar pulsed electropolishing (BPEP), due to its HF-free feature, can offer much safer, more environmentally friendly, and lower-cost operation compared to the conventional electropolishing, using concentrated HF and H2SO4 as electrolyte. Jefferson Lab has developed the BPEP system using diluted H2SO4 only for implementing final surface processing of niobium SRF cavities, including single cells, 7-cell CEBAF C100 cavity, and 9-cell TESLA-style cavities. The BPEP-treated cavity, followed by 120°C baking, has achieved an accelerating gradient (Eacc) of 37 MV/m with a quality factor (Q0) above 1e10 at 2K, which demonstrated the success of the system's development. The detailed BPEP parameter optimization and study of the surface engineering by BPEP will also be presented.

Dust particulates are always present to some degree inside the vacuum space of particle accelerators, causing a variety of issues. At the LHC, beam loss events have been linked to the interaction of charged dust with the proton beams. In superconducting rf cavities, dust contamination leads to field emission, limiting the accelerating gradient and causing damage to external beamline components. Facilities such as the SLAC LCLS-II and TRIUMF electron linear accelerator see progressive onsets in field emission that cannot simply be explained by vacuum events. The environment of a particle accelerator provides an ideal opportunity for dust to gain charge, which is one of the main drivers of dust grain dynamics in vacuum. However, fundamental parameters such as the dust composition and charge to mass ratio of these grains are unique to each accelerator environment and remain largely unknown. We will present an analysis of dust samples taken from TRIUMF linear accelerators, detailing their size, composition and potential sources. Preliminary results from experimental studies on the charging, detachment and migration mechanisms acting on micron sized particulates will also be presented.

Superconducting RadioFrequency (SRF) technology is a key component in many particle accelerators operating in a continuous wave, or high duty cycle, mode. The on-line performance of SRF cavities can be negatively impacted by the gradual reduction in the accelerating gradient that can be attained within a reasonable field emission level. Conventional cleaning procedures are both time- and resource-exhaustive as they are done *ex-situ*. As such, *in-situ* techniques are quite attractive. Plasma processing is an emerging *in-situ* method of cleaning which utilizes a mixture of oxygen and an inert gas to chemically remove hydrocarbon-based field emitters through plasma. At TRIUMF's Advanced Rare IsotopE Laboratory (ARIEL), an R&D program is in place to develop plasma processing procedures using fundamental power couplers on 1.3 GHz ARIEL 9-cell cavities. Single cell and multi-cell processing has been performed off-line. The studies involve varying the input parameters and testing the effectiveness of the treatment through RGA analysis. The progress on the developments will be reported.

The performance of superconducting radiofrequency (SRF) cavities is critical to enabling the next generation of efficient high-energy particle accelerators. Recent developments have focused on altering the surface impurity profile through in-situ baking, furnace baking, and doping to introduce and diffuse beneficial impurities such as nitrogen, oxygen, and carbon. However, the precise role and properties of each impurity are not well understood. In this work, we attempt to disentangle the role of nitrogen and oxygen impurities through time-of-flight secondary ion mass spectrometry of niobium samples baked at temperatures varying from 75-800 C with and without nitrogen injection. From these results, we developed treatments recipe that decouple the effects of oxygen and nitrogen in doping treatments. Understanding how these impurities and their underlying mechanisms drive further optimization in the tailoring of impurity profiles for high-performance SRF cavities.

The SRF community has shown that introducing certain impurities into high-purity niobium can improve quality factors and accelerating gradients. We question why some impurities improve RF performance while others hinder it. The purpose of this study is to characterize the impurities of niobium coupons with a low residual resistance ratio (RRR) and correlate these impurities with the RF performance of low RRR cavities so that the mechanism of impurity-based improvements can be better understood and improved upon. The combination of RF testing, temperature mapping, frequency vs temperature analysis, and materials studies reveals a microscopic picture of why low RRR cavities experience low BCS resistance behavior more prominently than their high RRR counterparts. We evaluate how differences in the mean free path, grain structure, and impurity profile affect RF performance. The results of this study have the potential to unlock a new understanding on SRF materials and enable the next generation of high Q/high gradient surface treatments.

RF superconducting cavities have been widely used in accelerators. The higher order modes caused by the wakefield radiation will lead to the beam instability, which is very harmful. So, it is necessary to depress the higher order modes. The photonic band gap (PBG) structure can effectively absorb higher order modes and suppress wakefield radiation. In addition, PBG cavities based on PBG structures have the advantage of adding waveguide ports directly to the cavity wall. Therefore, the PBG cavity can be used directly as a coupler, instead of the coupler attached to the end cell. So far, the PBG cavities have been tested and validated. On this basis, a PBG cavity working at 3.9 GHz was designed, and a couple of waveguide couplers are added to the cavity to ensure that all dangerous higher order modes in the cavity can be exported. After that, we used the CST microwave studio to calculate the electromagnetic parameters of the cavity. Accordingly, Q0=11488, Qe=1.149×1011, Eacc = 8.159×107, and Epeak/Eacc = 2.317.

In 2021, the Chinese ADS Front-end demo superconducting radio-frequency (SRF) linac, known as CAFe, successfully conducted a commissioning of a 10 mA, 200 kW continuous wave proton beam. During this commissioning, it was observed that the SRF cavity fault played a predominant role, contributing to approximately 70% of total beam trips. Upon the detection of fault signals, an acquisition process recorded 8 RF waveforms using digital low-level radio-frequency systems. A meticulous study of the cavity fault mechanisms was undertaken, leading to the identification and generalization of several fault patterns through the analysis of collected time-series data. The findings revealed that the dominant causes of SRF trips were field emission-triggered cavity faults and thermal quenches. We optimized the feature extraction methods for fault signals and developed a machine learning-based fault classification model. Comparative analysis with expert identification results demonstrated an accuracy rate of over 90% for the model. This research marks a significant stride towards enhancing the availability and reliability of operational beams for the future China Initiative Accelerator-Driven System project.

SRF technology using niobium accelerating cavities enables high performance and efficient acceleration for modern accelerator projects. While electron linacs accelerate particles with common structures designed for relativistic acceleration hadron linacs require acceleration over a broad velocity range. SRF technology is now being adopted at hadron energies in some cases starting from the RFQ exit but with top end energies such that a velocity range of a factor of ten has to be considered in the linac configuration and cavity design. Different structures in the TEM mode (coaxial) class (QWR, HWR, SSR, DSR) are employed with customized rf frequency, design beta and cavity structure. The coaxial cavities are now operating at very high performance rivaling the achievements in the 1.3GHz elliptical cavities. The talk should give an overview of the state of the art in the field.

HIAF is a heavy ion accelerator facility in China for nuclear physics research. The superconducting LINAC was used to accelerating beam energy up to 17MeV/u, then injecting to a Booster Ring. The linac are under construction since 2021, which includes 30 quarter-wave resonator (QWR) and 66 half-wave resonator (HWR). The first-batch production of cavity system have been completed. And the cavity's auxiliaries, such as coupler and tuner are ready too for first two cryomodules. This paper will present the current status of the HIAF SC cavity system.

The SRF world has made considerable advances in the last 15 years on the performance of bulk niobium cavities. Processing recipes like N-doping and 120C baking are now being accepted as standard and consistently delivered by industry. New treatments like mid-T baking are now being incorporated into some project processing recipes as well. Thin film research is advancing with the mission to pave the way to performance beyond bulk niobium. This talk should give a summary of present performance with a glimpse towards the fundamental processes at play in the treatments of today and some thoughts towards future directions.