arXiv subjects

J. Norem

Publications and source records attributed to J. Norem.

10 recordsLinked to original sources

arXiv2022

Understanding Vacuum Arcs and Gradient Limits

Although a general model of vacuum arcs and gradient limits would be widely useful, roughly 120 years after the first good experimental data on these arcs, this important field continues to be unsettled. This problem is a limitation in a number of technologies and has applications in many fields. Large tokamaks are sensitive to arcing on the plasma facing components, linac costs depend on their maximum operating fields, power transmission efficiency depends on the voltage that can be maintained, and the efficiency of Atom Probe Tomography depends on avoiding sample failures. A multidisciplinary study of this field could improve the precision and applicability of the theoretical models used. We outline the basic mechanisms involved in arcing and the issues that determine the physics of arcs. We divide the process into four stages; the trigger, plasma formation, plasma evolution and surface damage, in order to look at the physical principles involved. We try to identify the dominant mechanisms, critical issues and desirable aspects of an R&D program to produce a more precise and general model.

physics.plasm-ph↗

arXiv2020

An integrated Approach to Understanding Vacuum Arcs

Although used in the design and costing of large projects such as linear colliders and tokamaks, the theory of vacuum arcs and gradient limits is not well understood. Almost 120 years after the isolation of vacuum arcs, the exact mechanisms of the arcs and the damage they produce still being debated. We describe our simple and general model of the vacuum arc that can incorporate all active mechanisms and aims to explain all relevant data. Our four stage model, is based on experiments done at 805 MHz with a variety of cavity geometries, magnetic fields, and experimental techniques as well as data from Atom Probe Tomography and failure analysis of microelectronics. The model considers the trigger, plasma formation, plasma evolution and surface damage phases of the arc. Our data clearly shows surface damage produced by differential cooling capable of producing local high field enhancements $\beta \sim 200$, and arcing in subsequent pulses. We update the model and discuss new features while also pointing out where new data would be useful in extending the model to a wider range of frequencies.

physics.acc-ph↗

arXiv2014

Modeling of High-Gradient Vacuum Breakdown

Comsol finite element technique was applied to study heating and cooling of a microspot on the cathode surface. The reasons why there seems to be no common model for vacuum arcs, in spite of the importance of this field and the level of effort expended over more than one hundred years, were explored.

physics.comp-ph↗

arXiv2010

Modeling Vacuum Arcs

We are developing a model of vacuum arcs. This model assumes that arcs develop as a result of mechanical failure of the surface due to Coulomb explosions, followed by ionization of fragments by field emission and the development of a small, dense plasma that interacts with the surface primarily through self sputtering and terminates as a unipolar arc capable of producing breakdown sites with high enhancement factors. We have attempted to produce a self consistent picture of triggering, arc evolution and surface damage. We are modeling these mechanisms using Molecular Dynamics (mechanical failure, Coulomb explosions, self sputtering), Particle-In-Cell (PIC) codes (plasma evolution), mesoscale surface thermodynamics (surface evolution), and finite element electrostatic modeling (field enhancements). We can present a variety of numerical results. We identify where our model differs from other descriptions of this phenomenon.

physics.plasm-ph↗

arXiv2008

Improvement and protection of niobium surface superconductivity by Atomic Layer Deposition and heat treatment

A method to treat the surface of Nb is described which potentially can improve the performance of superconducting RF cavities. We present tunneling and x-ray photoemission spectroscopy (XPS) measurements at the surface of cavity-grade niobium samples coated with a 3 nm alumina overlayer deposited by Atomic Layer Deposition (ALD). The coated samples baked in ultra high vacuum (UHV) at low temperature reveal at first degraded superconducting surface. However, at temperatures above 450C, the tunneling conductance curves show significant improvements of the superconducting density of states (DOS) compared with untreated surfaces.

cond-mat.supr-con↗

arXiv2008

Tunneling study of cavity grade Nb: possible magnetic scattering at the surface

Tunneling spectroscopy was performed on Nb pieces prepared by the same processes used to etch and clean superconducting radio frequency (SRF) cavities. Air exposed, electropolished Nb exhibited a surface superconducting gap delta=1.55 meV, characteristic of clean, bulk Nb. However the tunneling density of states (DOS) was broadened significantly. The Nb pieces treated with the same mild baking used to improve the Q-slope in SRF cavities, reveal a sharper DOS. Good fits to the DOS were obtained using Shiba theory, suggesting that magnetic scattering of quasiparticles is the origin of the gapless surface superconductivity and a heretofore unrecognized contributor to the Q-slope problem of Nb SRF cavities.

cond-mat.supr-con↗

arXiv2005

Booster 6-GeV Study

Since a wider aperture has been obtained along the Booster beam line, this opens the opportunity for Booster running a higher intensity beam than ever before. Sooner or later, the available RF accelerating voltage will become a new limit for the beam intensity. Either by increasing the RFSUM or by reducing the accelerating rate can achieve the similar goal. The motivation for the 6-GeV study is to gain the relative accelerating voltage via a slower acceleration.

physics.acc-ph↗

arXiv2004

Experimental Estimate of Beam Loading and Minimum rf Voltage for Acceleration of High Intensity Beam in the Fermilab Booster

The difference between the rf voltage seen by the beam and the accelerating voltage required to match the rate of change of the Booster magnetic field is used to estimate the energy loss per beam turn. Because the rf voltage (RFSUM) and the synchronous phase can be experimentally measured, they can be used to calculate the effective accelerating voltage. Also an RFSUM reduction technique has been applied to measure experimentally the RFSUM limit at which the beam loss starts. With information on beam energy loss, the running conditions, especially for the high intensity beam, can be optimized in order to achieve a higher intensity beam from the Fermilab Booster.

physics.acc-ph↗

arXiv1996

Muon Collider Design

Muon Colliders have unique technical and physics advantages and disadvantages when compared with both hadron and electron machines. They should thus be regarded as complementary. Parameters are given of 4 TeV and 0.5 TeV high luminosity \mu^+ \mu^- colliders, and of a 0.5 TeV lower luminosity demonstration machine. We discuss the various systems in such muon colliders, starting from the proton accelerator needed to generate the muons and proceeding through muon cooling, acceleration and storage in a collider ring. Detector background, polarization, and nonstandard operating conditions are discussed.

acc-phys↗

arXiv1995

Beam Dynamics problems in a muon collider

We discuss the various beam dynamics problems in muon collider systems, starting from the proton accelerator needed to generate the muon beams and proceeding through the muon storage ring.

acc-phys↗