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Nitrogen-doping and furnace-baking are advanced high-Q0 recipes developed for 1.3 GHz TESLA-type cavities. These treatments will significantly benefit the high-Q0 linear accelerator community if they can be successfully adapted to different cavity styles and frequencies. Strong frequency- and geometry- dependence of these recipes makes the technology transfer amongst different cavity styles and frequencies far from straightforward, and requires rigorous study. Upcoming high-Q0 continuous-wave linear accelerator projects, such as the proposed Michigan State University Facility for Rare Isotope Beam Energy Upgrade, and the underway Fermilab's Proton Improvement Plan-II, could benefit enormously from adapting these techniques to their beta_opt = 0.6 ~650 MHz 5-cell elliptical superconducting rf cavities, operating at an accelerating gradient of around ~17 MV/m. This is the first investigation of the adaptation of nitrogen doping and medium temperature furnace baking to prototype 644 MHz beta_opt = 0.65 cavities, with the aim of demonstrating the high-Q0 potential of these recipes in these novel cavities for future optimization as part of the FRIB400 project R&D. We find that nitrogen-doping delivers superior Q0, despite the sub-GHz operating frequency of these cavities, but is sensitive to the post-doping electropolishing removal step and experiences elevated residual resistance. Medium temperature furnace baking delivers reasonable performance with decreased residual resistance compared to the nitrogen doped cavity, but may require further recipe refinement. The gradient requirement for the FRIB400 upgrade project is comfortably achieved by both recipes., Comment: 16 pages, 5 figures

Upcoming projects requiring high-Q ~650 MHz medium-to-high-${\beta}$ elliptical cavities drive a need to understand magnetic RF loss mechanisms and mitigations in greater detail. High-temperature annealing and fast-cooldowns have proven effective techniques for promoting magnetic flux expulsion in cavities, however the extent of their effectiveness has been observed to vary between niobium material lot and vendor. We explore the fast-cooldown method, and high-temperature annealing (900{\deg}C) in 644-650 MHz cavities fabricated from two different niobium vendors: Tokyo-Denkai, and Ningxia. which promote flux-expulsion efficiency. Using EBSD and PPMS methods, we aim to trace cavity flux expulsion efficiency to specific, measurable properties of the bulk niobium material, which, if identified, can lead to methods by which the flux expulsion properties of Nb material can be predicted prior to cavity fabrication, and can enable fine-tuning of cavity temperature treatments., Comment: IPAC 2023

This paper discusses recently proposed Higgs Energy LEptoN (HELEN) $e+e-$ linear collider based on advances in superconducting radio frequency technology. The collider offers cost and AC power savings, smaller footprint (relative to the ILC), and could be built at Fermilab with an interaction region within the site boundaries. After the initial physics run at 250 GeV, the collider could be upgraded either to higher luminosity or to higher (up to 500 GeV) energies.

The Linac Coherent Light Source (LCLS) is an X-ray science facility at SLAC National Accelerator Laboratory. The LCLS-II project (an upgrade to LCLS) is in the commissioning phase; the LCLS-II-HE (High Energy) project is another upgrade to the facility, enabling higher energy operation. An electron beam is accelerated using superconducting radio frequency (SRF) cavities built into cryomodules. It is planned to build 24 1.3 GHz standard cryomodules and one 1.3 GHz single-cavity Buncher Capture Cavity (BCC) cryomodule for the LCLS-II-HE project. Fourteen of these standard cryomodules and the BCC are planned to be assembled and tested at Fermilab. Procurements for standard cryomodule components are nearing completion. The first LCLS-II-HE cryomodule, referred to as the verification cryomodule (vCM) was assembled and tested at Fermilab. Fermilab has completed the assembly of the second cryomodule. This paper presents LCLS-II-HE cryomodule production status at Fermilab, emphasizing the changes done based on the successes, challenges, mitigations, and lessons learned from LCLS-II; validation of the changes with the excellent vCM results.

The PIP II linear accelerator includes different types of niobium SRF cavities including 650 MHz elliptical low (0.61) and high (0.92) beta cavities. The elliptical cavity surface is processed with the electropolishing method. The elliptical cavities especially the low-$\beta$ 650 MHz cavities showed a rough equator surface after the EP was per-formed with the standard EP conditions. This work was focused to study the effect of different EP parameters, including cathode surface area, temperature and voltage, and optimize them to improve the cavity surface.

Cold electropolishing (EP) of a nitrogen-doped (N-doped) niobium (Nb) superconducting RF (SRF) cavity was found to improve its quality factor. In order to understand the effect of EP temperature on N-doped and undoped surfaces, a systematic EP study was conducted with 2/0 N-doped and heat-treated Nb samples in a beaker. The Nb samples were electropolished at different surface temperatures ranging from 0 to 42 C. The results showed that the doped surface was susceptible to the sample temperature during EP. EP resulted in the surface pitting on the doped samples where the number density of pits increased at a higher temperature. The surface results were compared with the surface of cutouts from a 9-cell cavity which was 2/0 N-doped and electropolished. This paper shows de-tailed surface features of the N-doped and undoped Nb surfaces electropolished at different temperatures.

Radio frequency superconductivity is a cornerstone technology for many future HEP particle accelerators and experiments from colliders to proton drivers for neutrino facilities to searches for dark matter. While the performance of superconducting RF (SRF) cavities has improved significantly over the last decades, and the SRF technology has enabled new applications, the proposed HEP facilities and experiments pose new challenges. To address these challenges, the field continues to generate new ideas and there seems to be a vast room for improvements. In this paper we discuss the key research directions that are aligned with and address the future HEP needs., Comment: contribution to Snowmass 2021

Field emission (FE) is one of the main limiting factors of superconducting radio-frequency (SRF) cavities operating in accelerators and it occurs whenever contaminants, like dust, metal flakes or even absorbates, are present on the surface of the cavity high electric field region. Field emission reduces the maximum achievable accelerating field and generates free electrons that may interact with the beam, damage or activate the beamline. One practical method that can be used to mitigate this problem is in-situ plasma cleaning, or plasma processing. The development of a processing that can be applied in-situ is extremely advantageous, since it enables the recovery of the cryomodule performance without the need of disassembling the whole cryomodule, which is an extremely expensive and time-consuming process. On the other hand, plasma processing only requires the cryomodule warm-up to room-temperature and the subsequent processing of the contaminated cavities. The entire process is reasonably quick and involves a limited number of personnel. For these reasons we would like to advocate for continuing to invest in the R\&D of plasma processing to optimize its applicability in cryomodules and for extending the technique to other frequency ranges and cavities geometries., Comment: contribution to Snowmass 2021

This Snowmass 2021 contributed paper discusses a Higgs-Energy LEptoN (HELEN) $e^+e^-$ linear collider based on advances superconducting radio frequency technology. The proposed collider offers cost and AC power savings, smaller footprint (relative to the ILC), and could be built at Fermilab with an Interaction Region within the site boundaries. After the initial physics run at 250 GeV, the collider could be upgraded either to higher luminosity or to higher (up to 500 GeV) energies. If the ILC could not be realized in Japan in a timely fashion, the HELEN collider would be a viable option to build a Higgs factory in the U.S., Comment: contribution to Snowmass 2021

High Q-factors are of utmost importance to minimize losses of superconducting radio-frequency cavities deployed in continuous wave particle accelerators. This study elucidates the surface treatment that can maximize the Q-factors in high-beta 650 MHz elliptical niobium cavities. State-of-the-art surface treatments are applied in many single-cell cavities, and surface resistance studies are performed to understand the microwave dissipation at this unexplored frequency. The nitrogen doping treatment is confirmed to be necessary to maximize the Q-factors at medium RF fields. We applied this treatment in five-cell high-beta 650 MHz cavities and demonstrated that extremely high Q-factors were obtained at medium RF fields with this treatment. We also demonstrated that adding a cold electropolishing step after N-doping is crucial to push the quench field of multicell cavities to higher gradients.

An 8-cavity, 1.3 GHz, LCLS-II-HE cryomodule was assembled and tested at Fermilab to verify performance before the start of production. Its cavities were processed with a novel nitrogen doping treatment to improve gradient performance. The cryomodule was tested with a modified protocol to process sporadic quenches, which were observed in LCLS-II production cryomodules and are attributed to multipacting. Dedicated vertical test experiments support the attribution to multipacting. The verification cryomodule achieved an acceleration voltage of 200 MV in continuous wave mode, corresponding to an average accelerating gradient of 24.1 MV/m, significantly exceeding the specification of 173 MV. The average Q0 (3.0x10^10) also exceeded its specification (2.7x10^10). After processing, no field emission was observed up to the maximum gradient of each cavity. This paper reviews the cryomodule performance and discusses operational issues and mitigations implemented during the several month program., Comment: 15 pages, 24 figures

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.

In this paper we present the discovery of a new surface treatment applied to superconducting radio frequency (SRF) niobium cavities, leading to unprecedented accelerating fields of 49 MV/m in TESLA-shaped cavities, in continuous wave (CW); the corresponding peak magnetic fields are the highest ever measured in CW, about 210 mT. For TESLA-shape cavities the maximum quench field ever achieved was ~45 MV/m - reached very rarely- with most typical values being below 40 MV/m. These values are reached for niobium surfaces treated with electropolishing followed by the so called mild bake, a 120C vacuum bake (for 48 hours for fine grain and 24 hours for large grain surfaces). We discover that the addition during the mild bake of a step at 75C for few hours, before the 120C, increases systematically the quench fields up to unprecedented values of 49 MV/m. The significance of the result lays not only in the relative improvement, but in the proof that niobium surfaces can sustain and exceed CW radio frequency magnetic fields much larger than Hc1, pointing to an extrinsic nature of the current field limitations, and therefore to the potential to reach accelerating fields well beyond the current state of the art.

The radio-frequency surface resistance of niobium resonators is incredibly reduced when nitrogen impurities are dissolved as interstitial in the material, conferring ultra-high Q-factors at medium values of accelerating field. This effect has been observed in both high and low temperature nitrogen treatments. As a matter of fact, the peculiar anti Q-slope observed in nitrogen doped cavities, i.e. the decreasing of the Q-factor with the increasing of the radio-frequency field, come from the decreasing of the BCS surface resistance component as a function of the field. Such peculiar behavior has been considered consequence of the interstitial nitrogen present in the niobium lattice after the doping treatment. The study here presented show the field dependence of the BCS surface resistance of cavities with different resonant frequencies, such as: 650 MHz, 1.3 GHz, 2.6 GHz and 3.9 GHz, and processed with different state-of-the-art surface treatments. These findings show for the first time that the anti Q-slope might be seen at high frequency even for clean Niobium cavities, revealing useful suggestion on the physics underneath the anti Q-slope effect., Comment: To be published in Proceedings of SRF 2017

We report the finding of new surface treatments that permit to manipulate the niobium resonator nitrogen content in the first few nanometers in a controlled way, and the resonator fundamental Mattis-Bardeen surface resistance and residual resistance accordingly. In particular, we find surface infusion conditions that systematically a) increase the quality factor of these 1.3 GHz superconducting radio frequency (SRF) bulk niobium resonators, up to very high gradients; b) increase the achievable accelerating gradient of the cavity compared to its own baseline with state-of-the-art surface processing. Cavities subject to the new surface process have larger than two times the state of the art Q at 2K for accelerating fields > 35 MV/m. Moreover, very high accelerating gradients ~ 45 MV/m are repeatedly reached, which correspond to peak magnetic surface fields of 190 mT, among the highest measured for bulk niobium cavities. These findings open the opportunity to tailor the surface impurity content distribution to maximize performance in Q and gradients, and have therefore very important implications on future performance and cost of SRF based accelerators. They also help deepen the understanding of the physics of the RF niobium cavity surface.

Previous work has demonstrated that the radio frequency surface resistance of niobium resonators is dramatically reduced when nitrogen impurities are dissolved as interstitial in the material. The origin of this effect is attributed to the lowering of the Mattis and Bardeen surface resistance contribution with increasing accelerating field. Meanwhile, an enhancement of the sensitivity to trapped magnetic field is typically observed for such cavities. In this paper we conduct the first systematic study on these different components contributing to the total surface resistance as a function of different levels of dissolved nitrogen, in comparison with standard surface treatments for niobium resonators. Adding these results together we are able to show for the first time which is the optimum surface treatment that maximizes the Q-factor of superconducting niobium resonators as a function of expected trapped magnetic field in the cavity walls. These results also provide new insights on the physics behind the change in the field dependence of the Mattis and Bardeen surface resistance, and of the trapped magnetic vortex induced losses in superconducting niobium resonators.

Quench of superconducting radio-frequency cavities frequently leads to the lowered quality factor Q0, which had been attributed to the additional trapped magnetic flux. Here we demonstrate that the origin of this magnetic flux is purely extrinsic to the cavity by showing no extra dissipation (unchanged Q0) after quenching in zero magnetic field, which allows us to rule out intrinsic mechanisms of flux trapping such as generation of thermal currents or trapping of the rf field. We also show the clear relation of dissipation introduced by quenching to the orientation of the applied magnetic field and the possibility to fully recover the quality factor by requenching in the compensated field. We discover that for larger values of the ambient field, the Q-factor degradation may become irreversible by this technique, likely due to the outward flux migration beyond the normal zone opening during quench. Our findings are of special practical importance for accelerators based on low- and medium-beta accelerating structures residing close to focusing magnets, as well as for all high-Q cavity-based accelerators.

Even when cooled through its transition temperature in the presence of an external magnetic field, a superconductor can expel nearly all external magnetic flux. This Letter presents an experimental study to identify the parameters that most strongly influence flux trapping in high purity niobium during cooldown. This is critical to the operation of superconducting radiofrequency cavities, in which trapped flux degrades the quality factor and therefore cryogenic efficiency. Flux expulsion was measured on a large survey of 1.3 GHz cavities prepared in various ways. It is shown that both spatial thermal gradient and high temperature treatment are critical to expelling external magnetic fields, while surface treatment has minimal effect. For the first time, it is shown that a cavity can be converted from poor expulsion behavior to strong expulsion behavior after furnace treatment, resulting in a substantial improvement in quality factor. Future plans are described to build on this result in order to optimize treatment for future cavities., Comment: 5 pages

Cool-down dynamics of superconducting accelerating cavities became particularly important for obtaining very high quality factors in SRF cavities. Previous studies proved that when cavity is cooled fast, the quality factor is higher than when cavity is cooled slowly. This has been discovered to derive from the fact that a fast cool-down allows better magnetic field expulsion during the superconducting transition. In this paper we describe the first experiment where the temperature all around the cavity was mapped during the cavity cool-down through transition temperature, proving the existence of two different transition dynamics: a sharp superconducting-normal conducting transition during fast cool-down which favors flux expulsion and nucleation phase transition during slow cool-down, which leads to full flux trapping.

Previous studies on magnetic flux expulsion as a function of cooling details have been performed for superconducting niobium cavities with the cavity beam axis placed parallel respect to the helium cooling flow, and findings showed that for sufficient cooling thermogradients all magnetic flux could be expelled and very low residual resistance could be achieved. In this paper we investigate the flux trapping and its impact on radio frequency surface resistance when the resonators are positioned perpendicularly to the helium cooling flow, which is representative of how superconducting radio-frequency (SRF) cavities are cooled in an accelerator. We also extend the studies to different directions of applied magnetic field surrounding the resonator. Results show that in the cavity horizontal configuration there is a different impact of the various field components on the final surface resistance, and that several parameters have to be considered to understand flux dynamics. A newly discovered phenomenon of concentration of flux lines at the cavity top leading to cavity equator temperature rise is presented.