Plasma Processing Of Srf Cavities For The Next Generation Of Particle Accelerators

The cost-effective production of high frequency accelerating fields are the foundation for the next generation of particle accelerators. The Ar/Cl2 plasma etching technology holds the promise to yield a major reduction in cavity preparation costs. Plasma-based dry niobium surface treatment provides an excellent opportunity to remove bulk niobium, eliminate surface imperfections, increase cavity quality factor, and bring accelerating fields to higher levels. At the same time, the developed technology will be more environmentally friendly than the hydrogen fluoride-based wet etching technology. Plasma etching of inner surfaces of standard multi-cell SRF cavities is the main goal of this research in order to eliminate contaminants, including niobium oxides, in the penetration depth region. Successful plasma processing of multi-cell cavities will establish this method as a viable technique in the quest for more efficient components of next generation particle accelerators. In this project the single-cell pill box cavity plasma etching system is developed and etching conditions are determined. An actual single cell SRF cavity (1497 MHz) is plasma etched based on the pill box cavity results. The first RF test of this plasma etched cavity at cryogenic temperature is obtained. The system can also be used for other surface modifications, including tailoring niobium surface properties, surface passivation or nitriding for better performance of SRF cavities. The results of this plasma processing technology may be applied to most of the current SRF cavity fabrication projects. In the course of this project it has been demonstrated that a capacitively coupled radio-frequency discharge can be successfully used for etching curved niobium surfaces, in particular the inner walls of SRF cavities. The results could also be applicable to the inner or concave surfaces of any 3D structure other than an SRF cavity.

RF superconductivity (SRF) has become, over the last 2̃0 years, the technology of choice to produce RF cavities for particle accelerators. This occurred because of improvements in material and processing techniques as well as the understanding and remediation of practical limitations in SRF cavities. This development effort span 4̃0 years and Nb has been the material of choice for SRF cavity production. As the performances of SRF Nb cavities are approaching what are considered to be theoretical limits of the material, it is legitimate to ask what will be the future of SRF. In this article we will attempt to answer this question on the basis of near-future demands for SRF-based accelerators and the basic SRF properties of the available materials. Clearly, Nb will continue to play a major role in SRF cavities in the coming years but the use of superconductors with higher critical temperature than Nb is also likely to occur.

Superconducting radio-frequency (SRF) devices have extremely high quality factors which makes them ideal for applications in particle detection, in quantum computing, and in particle accelerators. From a fundamental standpoint, SRF cavities for particle accelerators are a particularly interesting use case because both high quality factor and high electromagnetic field amplitude are necessary. I use density-functional theory to build a better understanding of superconducting surface behavior under extreme conditions, to develop new recipes for higher quality factors and higher fields, and to study novel materials for SRF applications. I begin with my research on niobium, the gold-standard material for cavities and other SRF applications, focusing on the properties of niobium hydride defects and $\omega$-phase defects. I present new theories for the formation of both of these defects, addressing inconsistencies in the literature, explaining the effect of these defects on SRF performance, and developing new recipes. Next I discuss my work on the compound superconductor Nb$_3$Sn, which theoretically has the potential to far outperform niobium, but so far has been unable to reach high fields. Here I focus on developing new models for the nucleation and growth of the Nb$_3$Sn layer, especially concerning the behavior of antisite defects. I present my progress toward developing new recipes to improve layer uniformity and enhance cavity performance by minimizing the effect of compositional defects. Finally, I cover zirconium-doping recipes for SRF - a brand new area of research which I helped usher into existence. My calculations show the potential for zirconium to enhance superconducting properties significantly both in the bcc phase and in the rocksalt phase. I show that the previously-overlooked niobium-zirconium system is a promising pathway towards next-generation SRF technology.

A new in-situ plasma processing technique is being developed at the Spallation Neutron Source (SNS) to improve the performance of the cavities in operation. The technique utilizes a low-density reactive oxygen plasma at room temperature to remove top surface hydrocarbons. The plasma processing technique increases the work function of the cavity surface and reduces the overall amount of vacuum and electron activity during cavity operation; in particular it increases the field emission onset, which enables cavity operation at higher accelerating gradients. Experimental evidence also suggests that the SEY of the Nb surface decreases after plasma processing which helps mitigating multipacting issues. This article discusses the main developments and results from the plasma processing R & D are presented and experimental results for in-situ plasma processing of dressed cavities in the SNS horizontal test apparatus.

Superconducting Radio Frequency (SRF) cavities are responsible for the acceleration of charged particles to relativistic velocities in most modern linear accelerators, such as those employed at high-energy research facilities like Thomas Jefferson National Laboratory’s CEBAF and the LHC at CERN. Recognizing SRF as primarily a surface phenomenon enables the possibility of applying thin films to the interior surface of SRF cavities, opening a formidable tool chest of opportunities by combining and designing materials that offer greater benefit. Thus, while improvements in radio frequency cavity design and refinements in cavity processing techniques have improved accelerator performance and efficiency – 1.5 GHz bulk niobium SRF cavities have achieved accelerating gradients in excess of 35 MV/m – there exist fundamental material bounds in bulk superconductors limiting the maximally sustained accelerating field gradient (approximately 45 MV/m for Niobium) where inevitable thermodynamic breakdown occurs. With state of the art niobium based cavity design fast approaching these theoretical limits, novel material innovations must be sought in order to realize next generation SRF cavities. One proposed method to improve SRF performance is to utilize thin film superconducting-insulating-superconducting (SIS) multilayer structures to effectively magnetically screen a bulk superconducting layer such that it can operate at higher field gradients before suffering critically detrimental SRF losses. This dissertation focuses on the production and characterization of thin film superconductors for such SIS layers for radio-frequency applications.

The use of superconducting radio frequency (SRF) technology is a driving force in the development of particle accelerators. Scientists from around the globe are working together to develop the newest materials and techniques to improve the quality and efficiency of the SRF cavities that are essential for this technology.

A plasma wakefield accelerator is a device that converts the energy of a relativistic particle beam into a large-amplitude wave in a plasma. The plasma wave, or wakefield, supports an enormous electricfield that is used to accelerate a trailing particle beam. The plasma wakefield accelerator can therefore be used as a transformer, transferring energy from a high-charge, low-energy particle beam into a high-energy, low-charge particle beam. This technique may lead to a new generation of ultra-compact, high-energy particle accelerators. The past decade has seen enormous progress in the field of plasma wakefield acceleration with experimental demonstrations of the acceleration of electron beams by several gigaelectron-volts. The acceleration of positron beams in plasma is more challenging, but also necessary for the creation of a high-energy electron-positron collider. Part of the challenge is that the plasma responds asymmetrically to electrons and positrons, leading to increased disruption of the positron beam. One solution to this problem, first proposed over twenty years ago, is to use a hollow channel plasma which symmetrizes the response of the plasma to beams of positive and negative charge, making it possible to accelerate positrons in plasma without disruption. In this thesis, we describe the theory relevant to our experiment and derive new results when needed. We discuss the development and implementation of special optical devices used to create long plasma channels. We demonstrate for the first time the generation of meter-scale plasma channels and the acceleration of positron beams therein.

This third open access volume of the handbook series deals with accelerator physics, design, technology and operations, as well as with beam optics, dynamics and diagnostics. A joint CERN-Springer initiative, the "Particle Physics Reference Library" provides revised and updated contributions based on previously published material in the well-known Landolt-Boernstein series on particle physics, accelerators and detectors (volumes 21A,B1,B2,C), which took stock of the field approximately one decade ago. Central to this new initiative is publication under full open access.