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DOE OSTI2021

Superconducting Radio-frequency Cavity Fault Classification Using Machine Learning at Jefferson Laboratory

We report on the development of machine learning models for classifying C100 superconducting radiofrequency (SRF) cavity faults in the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab. Of the 418 SRF cavities in CEBAF, 96 are designed with a digital low-level RF system configured such that a cavity fault triggers recordings of RF signals for each of eight cavities in the cryomodule. Subject matter experts analyze the collected time-series data and identify which of the eight cavities faulted first and classify the type of fault. This information is used to find trends and strategically deploy mitigations to problematic cryomodules. However, manually labeling the data is laborious and time-consuming. By leveraging machine learning, near real-time - rather than postmortem - identification of the offending cavity and classification of the fault type has been implemented. We discuss the performance of the machine learning models during a recent physics run. We also discuss efforts for further insights into fault types through unsupervised learning techniques and present preliminary work on cavity and fault prediction using data collected prior to a failure event.

Tennant, C. D.↗

DOE OSTI2021

Measurements of Magnetic Field Penetration in Superconducting Materials for SRF Cavities

Superconducting radiofrequency (SRF) cavities used in particle accelerators operate in the Meissner state. To achieve high accelerating gradients, the cavity material should stay in the Meissner state under high RF magnetic field without penetration of vortices through the cavity wall. The field onset of flux penetration into a superconductor is an important parameter of merit of alternative superconducting materials other than Nb which can enhance the performance of SRF cavities. There is a need for a simple and efficient technique to measure the onset of field penetration into a superconductor directly. We have developed a Hall probe experimental setup for the measurement of the flux penetration field through a superconducting sample placed under a small superconducting solenoid magnet which can generate magnetic fields up to 500 mT. The system has been calibrated and used to measure different bulk and thin film superconducting materials. This system can also be used to study SIS multilayer coatings that have been proposed to enhance the vortex penetration field in Nb cavities.

Senevirathne, I. H.↗

DOE OSTI2021

Magnetron R&D for high efficiency CW RF sources for industrial accelerators

The scheme of using high-efficiency magnetrons to drive radiofrequency accelerators has been demonstrated at 2450 MHz in CW mode *. Magnetron test stands at JLab and GA have been set up to further test the noise figure and the locking speed of the injection phase-lock method. For higher power applications, power combining experiments using a TM010 cavity-type combiner and a magic tee for the binary combiner while using a single clean injection signal has been carried out at 2450 MHz. The frequency pulling effect between the magnetron and a low-Q cavity has been shown to enhance the frequency locking bandwidth compared to the injection phase-lock alone. The principle has been studied by the equivalent circuit simulation, analytical model, and finally confirmed experimentally on the magnetrons. Due to the pandemic delay in 2020, the equivalent high power tests using a 75kW, 915MHz industrial magnetron will be done in 2021 and will be reported in a future paper.

Wang, H.↗

DOE OSTI2022

Evaluating the Effects of Nitrogen Doping and Oxygen Doping on SRF Cavity Performance

Superconducting radiofrequency (SRF) cavities are resonators with extremely low surface resistance that enable accelerating cavities to have extremely high quality factors (Q₀). High Q₀ decreases the capital required to keep the accelerators cold by reducing power loss. The performance of SRF cavities is largely governed by the surface composition of the first §I{100}{nm} of the cavity surface. Impurities such as oxygen and nitrogen have been observed to yield high Q₀, but their precise roles are still being studied. Here, we compare the performance of cavities doped with nitrogen and oxygen in terms of surface composition and heating behavior with field. A simulation of the diffusion of oxygen into the bulk of the cavity was built using COMSOL Multiphysics software. Simulated results were compared to the actual surface composition of the cavities as determined from secondary ion mass spectrometry analysis. Understanding how these impurities affects performance allows us to have further insight into the underlying mechanisms that enable these surface treatments to yield high Q₀.

43 PARTICLE ACCELERATORS↗

DOE OSTI2023

A Compton transmission polarimeter for DC and SRF electron photo-injectors

The production of high-current and intense spin polarized electron beams is of great importance in electron-based facilities. Tests are planned to produce such beams in 2023 using GaAs-based photocathodes installed in the Brookhaven National Lab RHIC Coherent electron Cooling superconducting radiofrequency (SRF) photogun [1]. A fast and efficient electron polarimeter operating in the MeV energy range is required to measure the beam spin polarization. While Mott polarimeters provide larger measured asymmetries, a Compton Transmission polarimeter is well suited in the few MeV energy range. In this work, we report on a relatively compact and cost-effective Compton transmission polarimeter which has been built and calibrated at Jefferson Lab (JLab). First, we present the design of the polarimeter radiator, polarized target analyzing magnet, BGO detector assembly and data acquisition system. Next, results of a two-week commissioning study performed at the JLab Upgraded Injector Test Facility will be described. Here, a well-known polarized electron beam produced from a bulk GaAs photocathode in a dc high-voltage photogun was first measured in a 180 keV Mott scattering polarimeter, then used to characterize and calibrate the Compton transmission polarimeter as a function of the polarized target magnetization and beam properties. Finally, we report an effective analyzing power of the Compton polarimeter and compare experimental results with those produced via Geant4 simulations.

Blume, G.↗

DOE OSTI2023

Origins of quench in buffered chemical polished and low temperature baked SRF cavities

Electropolishing (EP) and buffered chemical polishing (BCP) are conventional surface preparation techniques for superconducting radiofrequency (SRF) cavities that remove damaged material from the cavity surface. One main issue with EP and BCP treated SRF cavities is high field Q-slope (HFQS), a drop in quality factor at high gradients that limits quench field. High gradient performance in EP cavities can be improved by applying a low temperature bake (LTB), but LTB does not consistently remove HFQS in BCP cavities. There is no consensus as to the why LTB is not effective on BCP prepared cavities, and the cause of HFQS in BCP cavities is not well understood. We examine the origins of quench in EP, BCP, EP+LTB, and BCP+LTB treated SRF cavities. We also show the effect of these treatments on the onset of HFQS, heating within the cavity up to quench, concentration of free hydrogen, and surface roughness.

43 PARTICLE ACCELERATORS↗

DOE OSTI2022

Examining the Effects of Oxygen Doping on SRF Cavity Performance

Superconducting radiofrequency (SRF) cavities are resonators with extremely low surface resistance that enable accelerating cavities to have extremely high quality factors (Q₀). High (Q₀) decreases the capital required to keep accelerators cold by reducing power loss. The performance of SRF cavities is largely governed by the surface composition of the first 100 nm of the cavity surface. Impurities such as oxygen and nitrogen have been observed to yield high Q₀, but their precise roles are still being studied. Here, we compare the performance of cavities doped with nitrogen and oxygen in terms of fundamental material properties to understand how these impurities affect performance. This enables us to have further insight into the underlying mechanisms that enable these surface treatments to yield high Q₀ performance.

43 PARTICLE ACCELERATORS↗

DOE OSTI2022

Accelerator Physics Lessons From CBETA, the First Multi-turn SRF ERL

The Cornell-BNL ERL Test Accelerator (CBETA) has been designed, constructed, and commissioned in a collaboration between Cornell and Brookhaven National Laboratory (BNL). It focuses on energy-saving measures in accelerators, including permanent magnets, energy recovery, and superconductors; it has thus been referred to as a green accelerator. CBETA has become the world's first Energy Recovery Linac (ERL) that accelerates through multiple turns and then recovers the energy in superconducting radiofrequency (SRF) cavities though multiple decelerating turns. The energy is then available to accelerate more beam. It has also become the first accelerator that operates 7 beams in the same large-energy aperture Fixed Field Alternating-gradient (FFA) lattice. The FFA is constructed of permanent combined function magnets and transports energies of 42, 78, 114, and 150 MeV simultaneously. Accelerator physics lessons from the commissioning period will be described and applications of such an accelerator from hadron cooling to EUV lithography and from nuclear physics to a compact Compton source will be discussed.

Deitrick, K. E.↗

DOE OSTI2022

Preliminary Results from Magnetic Field Scanning System for a Single-Cell Niobium Cavity

One of the building blocks of modern particle accelerators is superconducting radiofrequency (SRF) cavities. Niobium is the material of choice to build such cavities, which operate at liquid helium temperature (2 - 4 K) and have some of the highest quality factors found in Nature. There are several sources of residual losses, one of them is trapped magnetic flux, which limits the quality factor in SRF cavities. The flux trapping mechanism depends on different niobium surface preparations and cool-down conditions. Suitable diagnostic tools are not yet available to study the effects of such conditions on magnetic flux trapping. A magnetic field scanning system (MFSS) for SRF cavities using Hall probes and Fluxgate magnetometer has been designed, built, and is commissioned to measure the local magnetic field trapped in 1.3 GHz single-cell SRF cavities at 4 K. In this contribution, we will present the preliminary results from MFSS for a single cell niobium cavity.

Ciovati, G.↗

DOE OSTI2022

Cylindrical Magnetron Development for Nb3sn Deposition via Magnetron Sputtering

Due to its better superconducting properties (critical temperature Tc~ 18.3 K, superheating field Hsh~ 400 mT), Nb₃Sn is considered as a potential alternative to niobium (Tc~ 9.25 K, Hsh~ 200 mT) for superconducting radiofrequency (SRF) cavities for particle acceleration. Magnetron sputtering is an effective method to produce superconducting Nb₃Sn films. We deposited superconducting Nb₃Sn films on samples with magnetron sputtering using co-sputtering, sequential sputtering, and sputtering from a stoichiometric target. Nb₃Sn films produced by magnetron sputtering in our previous experiments have achieved DC superconducting critical temperature up to 17.93 K and RF superconducting transition at 17.2 K. A magnetron sputtering system with two identical cylindrical cathodes that can be used to sputter Nb₃Sn films on cavities has been designed and is under development now. We report on the design and the current progress on the development of the system.

43 PARTICLE ACCELERATORS↗

DOE OSTI2023

Comparing the Effectiveness of Low Temperature Bake in EP and BCP Cavities

Electropolishing (EP) and buffered chemical polishing (BCP) are conventional surface preparation techniques for superconducting radiofrequency (SRF) cavities. Both EP and BCP treated SRF cavities display high field Q-slope (HFQS) which degrades performance at high gradients. While high gradient performance in EP cavities can be improved by introducing oxygen via a low temperature bake (LTB) of 120^{°}C by 48 hours, LTB does not consistently remove HFQS in BCP cavities. There is no consensus as to why LTB is not effective on BCP prepared cavities. We examine quench in EP, BCP, EP+LTB, and BCP+LTB treated 1.3 GHz single-cell Nb cavities by studying the heating behavior with field using a temperature mapping system. Cavity performance is correlated to characterizations of surface impurity profile obtained via time of flight secondary ion mass spectrometry studies. We observe a difference in near surface hydrogen concentration following BCP compared to EP that may suggest that the causes of quench in EP and BCP cavities are different.

43 PARTICLE ACCELERATORS↗

DOE OSTI2020

R&D toward a proof-of-principle experiment on optical stochastic cooling at the Fermilab’s IOTA ring

The stochastic-cooling method was invented in the late 70s and played a key role in the accumulation of intense beams of antiprotons for a head-on collision with counter rotating proton beams at 500 GeV in the Super Proton Synchrotron (SPS) at CERN. Such an improvement led to the discovery of the W and Z bosons in 1983. In brief, stochastic cooling relies on recording information (position) associated with short longitudinal slices within the bunch. The radiation is encoded on a radiofrequency (RF) signal, manipulated and used to correct the bunch slices at a later time.The repetitive correction is applied as the beam circulates in the ring ultimately cool the beam.Stochastic cooling has been successfully implemented in several machines for particle cooling and accumulation. Cooling of bunched beams in proton-(anti)proton colliders was challenging due to the high phase-space density of the bunches but was recently demonstrated at the Relativistic Heavy Ion Collider (RHIC).

43 PARTICLE ACCELERATORS↗

DOE OSTI2020

2020 Review of Axis 2 rf

An intense relativistic electron beam can produce radiofrequency (rf) radiation in many ways. The DARHT II linear induction accelerator produces a 17 MeV, 1.65 kA beam. The dominant sources of rf at DARHT-II and the methods used to suppress this rf are well documented. This note will examine the observed rf over the past six years and its impact on the beam. The recorded waveforms for each hydro starting with 4011 from 10/29/14 are examined to identify the presence of rf. The hydros are chosen because they represent the optimal accelerator performance at any given time rather than tuning or beam studies.

43 PARTICLE ACCELERATORS↗

DOE OSTI2020

Development of Improved Noble Liquid Purity Measurements (Final Technical Report)

The main goal of the Detector R&D project supported by this grant, “Development of Improved Noble Liquid Purity Measurements”, was to perform a scoping study of the feasibility of trapping ions in a liquid argon medium using a radiofrequency (RF) quadrupole trap, and to subsequently use the population of trapped ions to make measurements of the properties of the liquid medium. Using RF quadrupole traps immersed in a noble liquid to trap ions may yield new capabilities to detect and quantify the presence of impurities, as well as enabling other measurements of the microphysics in a noble liquid environment. The major activities undertaken during this project include: the development of a simulation of ions in a liquid argon medium; the designing of a prototype RF quadrupole trap; and the setup of hardware to be used in testing the prototype in vacuum and gas conditions. The summary of these major activities is reported here.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2020

Thermal / structural analysis of the HB 650 thermal shield

Fermilab’s PIP-II project’s superconducting linear accelerator will drive the next generation of particle accelerators through a revolution in beam intensity. Key to beam intensification are the high efficiencies of niobium-tin superconducting radiofrequency (SRF) cavities operating at cryogenic temperatures near 5 Kelvin. A multifaceted approach is employed to achieve and maintain the extreme temperature. Vacuum provides the first barrier to thermal convection. Physical thermal intercept zones further isolate the exterior shell vacuum vessel at 300 Kelvin from the supercooled beamline. The first thermal intercept lies just inside the exterior vacuum vessel forming the 40 Kelvin zone. This 40K thermal shield is the focus of this investigation. In preparation for operation, the thermal shield is cooled from 300K to 40K in a slow process over 2-1/2 days. The factors limiting the cooling rate are a high thermal gradient which produces thermal strain and high mechanical stress. D ecreasing the time required to reach operational temperatures is the desired research goal. Physical testing data and finite element method (FEM) computer models from the Single Spoke Resonator – 1 (SSR1) and Linac Coherent Light Source (LCLS-II) cryomodules will be evaluated to determine needed design changes. The thermal shield design for the High Beta 650 MHz (HB650) will be modified and analyzed using FEM computer models to determine maximum cooling rate with a target of 50 thermal cycles of life. Maximum allowable stress will be determined through low cycle fatigue calculations.

43 PARTICLE ACCELERATORS↗

DOE OSTI2022

Design and demonstration of an economical SRF structure for Continuous Wave (CW), high-energy, Megawatt-class beams

Due to its very high RF-to-beam efficiency, superconducting radiofrequency (SRF) cavities have become the technology of choice for the construction of large linear accelerators (linacs) for basic science applications (examples, ESS and PIP-II). The technology is also attractive for high volume industrial applications such as electron irradiation processing of wastewater and flue gas. However, SRF’s current reliance on liquid helium for cryogenic cooling introduces infrastructural and safety challenges for use in industrial settings. Cryocooler conduction-cooling is a recently developed novel technique for operating SRF cavities without requiring liquid helium. This project aims for technical design and economic assessment of a medium energy, high average power e-beam linac based on the conduction-cooled SRF technique for high volume irradiation treatment of wastewater. The goals of this project are (1) to design a medium energy (10 MeV), high average power (1 MW) electron beam accelerator around the cryocooler conduction-cooled SRF cavity technology, (2) demonstrate the required accelerating voltage on a prototype cavity, and (3) build a cost-model to evaluate capital and operating expense of the accelerator.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Wall plug efficiency analysis of a compact SRF industrial accelerator [Poster]

Superconducting radiofrequency cavities offer extremely high (>90%) RF-to-beam energy conversion efficiency. Recent breakthroughs in the Nb 3 Sn SRF cavity technology, successful demonstration of the conduction cooling technique, and the availability of high cooling capacity cryocoolers has made possible the use of SRF for industrial particle accelerators. However, the overall wall plug-to-beam efficiency of such an SRF accelerator can be limited by the wall plug-to-RF power efficiency of the RF source powering this accelerator. The present study quantifies the wall plug-to-beam efficiency of a conduction-cooled SRF accelerator considering several different choices of the RF power source. The study concludes that the wall plug efficiency of the accelerator is indeed limited by that of the power source.

43 PARTICLE ACCELERATORS↗

DOE OSTI2022

Wall plug efficiency analysis of a compact SRF industrial accelerator [Poster]

43 PARTICLE ACCELERATORS↗