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DOE OSTI2022

High-Gradient Wien Spin Rotators at Jefferson Lab

Nuclear physics experiments performed in the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Laboratory (JLab) require spin manipulation of electron beams. Two Wien spin rotators in the injector keV region are essential at CEBAF to establish longitudinal polarization at the end station target, and to flip the polarization direction by ? rad to rule out false asymmetries. In a Wien filter, the homogeneous and independent electric and magnetic fields, along with the velocity vectors of the electrons that traverse it, form a mutually orthogonal system. The magnitude of the electrostatic field, established by biasing two highly-polished electrodes, defines the desired spin angle at the target yet deviates the beam trajectory due to the Lorentz force. The beam trajectory in the Wien is then re-established by adjusting the magnetic field, induced by an electromagnet encasing the device vacuum chamber. This contribution describes the evolution design and high voltage testing of Wien filters for spin manipulation at increased beam energies in the keV injector region, required by high precision parity violation experiments like MOLLER.

Palacios Serrano, G. G.↗

DOE OSTI2022

High-Gradient Wien Spin Rotators at Jefferson Lab

Palacios Serrano, G. G.↗

DOE OSTI2012

Superconducting DC and RF properties of ingot Niobium used in SRF cavity fabrication

Bulk niobium (Nb) has been the material of choice for the superconducting radiofrequency (SRF) cavity used in particle accelerator because of its highest transition temperature (Tc) and highest lower critical field (Hc1~ 190 mT) among the elemental type-II superconductors. The overall performance of these SRF cavities is measured by its quality factor, Q0 as a function of the accelerating gradient, Eacc. Higher Q0 for the reduction of cryogenic loss and higher Eacc for the use of high energy accelerators are desired. SRF cavities fabricated from ingot niobium have become a viable alternative to standard high-purity fine-grain Nb for the fabrication of high-performing SRF cavities with the possibility of significant cost reduction. In this contribution, we present the results on DC, low-frequency and RF measurements of Nb rods machined from ingots with different impurity concentrations, provided by CBMM, Brazil. The rods have been subjected to different treatments typically applied to SRF cavities. In addition, single-cell cavities are being fabricated from the same ingot material and initial results on one of them showed high Q0-values at 2 K up to peak surface magnetic fields of about 110 mT. Residual resistance values below 5 n have been consistently achieved on this cavity. A new induction furnace was commissioned for the heat treatment of these single-cells, which allows avoiding subsequent chemical etching of the cavity surface.

Myneni, Ganapati↗

DOE OSTI2024

Beryllium–tungsten graded density inner shells in double shell capsules for improved hydrodynamic stability

The outer surface of the high-Z inner shell in the double shell configuration of inertial confinement fusion experiments experiences Rayleigh–Taylor instability growth during the implosion process due to inverted density and pressure gradients between a highly compressed foam interstitial layer and the accelerating dense inner shell. Graded density layers have long been known to reduce instability growth rates. In this study, we employ high-fidelity radiation hydrodynamic simulations to demonstrate this improved stability when grading beryllium into tungsten. We first characterize the response to L-band preheat of these layers using a newly calibrated radiation drive. While graded layer capsules suffer reduced performance (here, measured as DD neutron yield from a CD foam fuel) in 1D simulations due to reduced kinetic energy coupling and reduced fuel compression, they suffer less of a performance drop when 2D instabilities are accounted for. With the improved stability of graded layers, we explore the performance of capsules with larger fuel radii and thinner shells as a preliminary study to find new designs in which graded layers produce the highest yields.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2023

Three-Dimensional Reconstruction of Nb 3 Sn Films by Focused Ion Beam Cross Sectional Microscopy

Niobium has been the material of choice for SRF cavities for several decades due to its formability and superconducting properties. The accelerating gradient of niobium cavities is, however, rapidly approaching a theoretical limit. To achieve higher accelerating gradients a new material is needed that can sustain high fields. Nb 3 Sn is a promising competitor with a higher superconducting transition temperature and a higher critical field than pure niobium. However, Nb 3 Sn is very brittle and cannot be formed readily into a cavity. The main method for creating Nb 3 Sn cavities is to form a Nb 3 Sn film into a niobium surface using a tin vapor-diffusion method. This technique creates a microcrystalline Nb 3 Sn thin film on the inner surface of the cavity. Tin depleted regions are known to form in the film during this process. Previous studies have analyzed these regions using transmission electron microscopy on cross-sectional lamellae prepared by focused ion beam/scanning electron microscope (FIB/SEM). This method does not provide any three-dimensional (3-D) information about the distribution of tin-deficient regions. In this study we employ a focused ion beam tomographic technique to analyze the 3-D structure of the film. Electron dispersive X-ray spectroscopy is used to image the tin concentration of the film in 3-D. Tin-deficient regions are discovered close to the surface of the Nb 3 Sn film.

Viklund, E.↗

DOE OSTI2011

Exploration of very high gradient cavities

Several of the 9-cell ILC cavities processed at Jlab within ongoing ILC R&D program have shown interesting behavior at high fields, such as mode mixing and sudden field emission turn-on during quench. Equipped with thermometry and oscillating superleak transducer (OST) system for quench detection, we couple our RF measurements with local dissipation measurements. In this contribution we report on our findings with high gradient SRF cavities.

43 PARTICLE ACCELERATORS↗

DOE OSTI2011

Dispersion And Energy Compensation in High-Gradient Linacs for Lepton Colliders

The shape of an rf pulse is distorted due to dispersion encountered in acceleration through traveling-wave linear accelerator structures. Simulations are made to ascertain the severity of this distortion in cavities designed to operate at various group velocities. The pulse suffers maximum degradation when propagated through accelerating cavities with a phase advance per cell in the vicinity of {pi}, where the group velocity reaches its minimum value. Several cavities are simulated to study the pulse distortion and compared with experiments performed on a high phase advance structure H60VG3, which has a phase advance of 5{pi}/6 per cell. A circuit model and a mode matching code are used to perform these simulations on accelerating structures consisting of multiple cells. Beam loading is taken into account and the implications on energy dispersion are also analyzed. Means of mitigating for this energy deviation are discussed.

43 PARTICLE ACCELERATORS↗

DOE OSTI2019

Schemes of Laser Muon Acceleration: Ultra-Short, Micron-Scale Beams

Experimentally accessible schemes of laser muon (μ±) acceleration are introduced and modeled using a novel technique of controlled laser-driven post-processing of cascade showers (or pair plasmas). The proposed schemes use propagating structures in plasma, driven as wakefields of femtosecond-scale high-intensity laser, to capture particles of divergent cascade shower of: (a) hadronic type from proton-nucleon or photo-production reactions or, (b) electromagnetic type. Apart from the direct trapping and acceleration of particles of a raw shower in laser-driven plasma, a conditioning stage is proposed to selectively focus only one of the charge states. Not only is the high gradient that is sustained by laser-driven plasma structures well suited for rapid acceleration to extend the lifetime of short-lived muons but their inherent spatiotemporal scales also make possible production of unprecedented ultrashort, micron-scale muon beams. Compact laser muon acceleration schemes hold the promise to open up new avenues for applications.

43 PARTICLE ACCELERATORS↗

DOE OSTI2012

Structural Properties of Niobium Thin Films Deposited on Metallic Substrates by ECR

Particle accelerator technologies rely on SRF cavities to create the accelerating gradient for beam lines. Solid niobium cavities are widely employed throughout the community despite high material, fabrication, and operation cost. New thin film technologies are being explored for the suitability of niobium coatings for accelerating cavities. Thin layers of high-quality niobium would be deposited on a base material that has lower material and fabrication cost. Copper is a strong candidate for the cavity base due to availability, cost, machinability, and potentially improved performance characteristics of the niobium SRF surface. Initial results of TEM, EBSD and XRD analyses of niobium thin films grown on copper substrates under controlled conditions are presented to demonstrate the feasibility of the technology and establish lower limits of performance characteristics. Correlation of RRR data with the structure of niobium thin films will demonstrate the importance of thin film structural quality. *Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.

Spradlin, Joshua↗

DOE OSTI2023

C-band high gradient research and facilities at Los Alamos National Laboratory [Slides]

Achieving high-gradient performance (low breakdown rates, low field emission, new materials for HOM absorption, cathodes at high gradient etc.) is a materials science problem. Los Alamos is, at core, a materials science laboratory with particular expertise in metallurgy. Los Alamos also considers itself the steward of accelerator science for the NNSA part of the DOE complex. Thus, Los Alamos has both an institutional interest in, and capability to address, this problem space. High gradient C-band work directly aligns with future NNSA and LANL missions.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Muon Acceleration-RLA and FFAG

Various acceleration schemes for muons are presented. The overall goal of the acceleration systems: large acceptance acceleration to 25 GeV and 'beam shaping' can be accomplished by various fixed field accelerators at different stages. They involve three superconducting linacs: a single pass linear Pre-accelerator followed by a pair of multi-pass Recirculating Linear Accelerators (RLA) and finally a non-scaling FFAG ring. The present baseline acceleration scenario has been optimized to take maximum advantage of appropriate acceleration scheme at a given stage. The solenoid based Pre-accelerator offers very large acceptance and facilitates correction of energy gain across the bunch and significant longitudinal compression trough induced synchrotron motion. However, far off-crest acceleration reduces the effective acceleration gradient and adds complexity through the requirement of individual RF phase control for each cavity. The RLAs offer very efficient usage of high gradient superconducting RF and ability to adjust path-length after each linac pass through individual return arcs with uniformly periodic FODO optics suitable for chromatic compensation of emittace dilution with sextupoles. However, they require spreaders/recombiners switchyards at both linac ends and significant total length of the arcs. The non-scaling Fixed Field Alternating Gradient (FFAG) ring combines compactness with very large chromatic acceptance (twice the injection energy) and it allows for large number of passes through the RF (at least eight, possibly as high as 15).

43 PARTICLE ACCELERATORS↗

DOE OSTI2019

Simulations and Experimental Plans for a High-Repetition-Rate Field-Enhanced Conduction-Cooled Superconducting RF Electron Source

We present a novel RF design for a field enhanced electron source driven by field emission cathodes. The proposed electron source relies on the enhanced high electric field gradients at the cathode to simultaneously extract and accelerate electrons. The system will be tested in a conduction-cooled superconducting radio-frequency cavity recently demonstrated at Fermilab. In this paper, we present electromagnetic and thermal simulations of the setup that support the feasibility of the design.

43 PARTICLE ACCELERATORS↗

DOE OSTI2023

Temperature, rf field, and frequency dependence performance evaluation of superconducting niobium half-wave coaxial cavity

Recent advancement in superconducting radio frequency cavity processing techniques, with diffusion of impurities within the RF penetration depth, resulted in high quality factor with increase in quality factor with increasing accelerating gradient. The increase in quality factor is the result of a decrease in the surface resistance as a result of nonmagnetic impurities doping and change in electronic density of states. The fundamental understanding of the dependence of surface resistance on frequency and surface preparation is still an active area of research. Here, we present the result of RF measurements of the TEM modes in a coaxial half-wave niobium cavity resonating at frequencies between 0.3 ? 1.3 GHz. The temperature dependence of the surface resistance was measured between 4.2 K and 1.6 K. The field dependence of the surface resistance was measured at 2.0 K. The baseline measurements were made after standard surface preparation by buffered chemical polishing.

Dhakal, P.↗

DOE OSTI2019

Simulations for Field-Enhanced Conduction-Cooled Superconducting RF Electron Source

We present a novel RF design for a field enhanced electron source driven by field emission cathodes. The proposed electron source relies on the enhanced high electric field gradients at the cathode to simultaneously extract and accelerate electrons. The system will be tested in a conduction-cooled superconducting radiofrequency cavity recently demonstrated at Fermilab. In this work, we present electromagnetic and thermal simulations of the setup that support the feasibility of the design.

43 PARTICLE ACCELERATORS↗

DOE OSTI2019

LANL Accelerator Activities for C-band

LANL has a range of accelerator activities that would benefit from improvements in RF resonator performance. XFEL (presently using a SRF baseline similar to LCLS-II) would benefit from much higher gradient and long-pulse operation of copper structures. Higher gradient RF structures for lower energy accelerators (~100 MeV) would also provide a path to transportable systems, such as ICS sources. Lastly, the utilization of dielectrics could be exploited for increased efficiency and compactness, e.g. for accelerator in space efforts. A LANL R&D effort aims at systematic improvement of copper structures employing material science simulations to explain and improve breakdown effects, combined with highly optimized designs, integration of cryo-cooling and the use of dielectric insertions. The work is integrated into a High-Gradient collaboration between SLAC, UCLA and LANL.

43 PARTICLE ACCELERATORS↗

DOE OSTI2020

TESLA's High Gradient March: Thirty years anniversary of TESLA Technology Collaboration (TTC)

In 2020, the TTC (TESLA Technology Collaboration) celebrates 30 years of collaborative efforts on SRF technologies. The first international TESLA workshop on a TeV Energy Superconducting Linear Accelerator was held at Cornell University on July 23-26, 1990. The purpose of the meeting was to work on a parameter list for a superconducting high energy physics collider in the Teravolt energy region and to explore ideas on how to increase the gradients and lower the costs for the accelerator structures. It was clear from the beginning that the needed technological progress would require a large international collaboration. This meeting was indeed the origin of a long preparation effort toward a future ILC (International Linear Collider), which eventually led to the realization of the TTF (TESLA Test Facility, since 2005 operated as the DESY user facility FLASH) and the European XFEL also at DESY, and created the conditions supporting the proposals for several large SRF-based research facilities like SNS, LCLS-II, ESS, PIP-II and SHINE, as well as a growing number of smaller facilities around the world. Highlights of this 30-year international collaboration are presented and celebrated.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2020

First demonstration of a cryocooler conduction cooled superconducting radiofrequency cavity operating at practical cw accelerating gradients

Abstract We demonstrate practical accelerating gradients on a superconducting radiofrequency (SRF) accelerator cavity with cryocooler conduction cooling, a cooling technique that does not involve the complexities of the conventional liquid helium bath. A design is first presented that enables conduction cooling an elliptical-cell SRF cavity. Implementing this design, a single cell 650 MHz Nb 3 Sn cavity coupled using high purity aluminum thermal links to a 4 K pulse tube cryocooler generated accelerating gradients up to 6.6 MV m −1 at 100% duty cycle. The experiments were carried out with the cavity-cryocooler assembly in a simple vacuum vessel, completely free of circulating liquid cryogens. We anticipate that this cryocooling technique will make the SRF technology accessible to interested accelerator researchers who lack access to full-stack helium cryogenic systems. Furthermore, the technique can lead to SRF based compact sources of high average power electron beams for environmental protection and industrial applications. A concept of such an SRF compact accelerator is presented.

43 PARTICLE ACCELERATORS↗

DOE OSTI2015

Proof-of-principle demonstration of Nb{sub 3}Sn superconducting radiofrequency cavities for high Q{sub 0} applications

Many future particle accelerators require hundreds of superconducting radiofrequency (SRF) cavities operating with high duty factor. The large dynamic heat load of the cavities causes the cryogenic plant to make up a significant part of the overall cost of the facility. This contribution can be reduced by replacing standard niobium cavities with ones coated with a low-dissipation superconductor such as Nb{sub 3}Sn. In this paper, we present results for single cell cavities coated with Nb{sub 3}Sn at Cornell. Five coatings were carried out, showing that at 4.2 K, high Q{sub 0} out to medium fields was reproducible, resulting in an average quench field of 14 MV/m and an average 4.2 K Q{sub 0} at quench of 8 × 10{sup 9}. In each case, the peak surface magnetic field at quench was well above H{sub c1}, showing that it is not a limiting field in these cavities. The coating with the best performance had a quench field of 17 MV/m, exceeding gradient requirements for state-of-the-art high duty factor SRF accelerators. It is also shown that—taking into account the thermodynamic efficiency of the cryogenic plant—the 4.2 K Q{sub 0} values obtained meet the AC power consumption requirements of state-of-the-art high duty factor accelerators, making this a proof-of-principle demonstration for Nb{sub 3}Sn cavities in future applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗