Non-existence of normal tokamak equilibria with negative central current

Recent tokamak experiments employing off-axis, non-inductive current drive have found that a large central current hole can be produced. The current density is measured to be approximately zero in this region, though in principle there was sufficient current drive power for the central current density to have gone significantly negative. Recent papers have used a large aspect-ratio expansion to show that normal MHD equilibria (with axisymmetric nested flux surfaces, non-singular fields, and monotonic peaked pressure profiles) can not exist with negative central current. We extend that proof here to arbitrary aspect ratio, using a variant of the virial theorem to derive a relatively simple integral constraint on the equilibrium. However, this constraint does not, by itself, exclude equilibria with non-nested flux surfaces, or equilibria with singular fields and/or hollow pressure profiles that may be spontaneously generated.Comment: 5 pages, 3 figures. Submitted to Physics of Plasmas, Feb. 14, 2003. Revised Feb. 24, 2003. Vers. 2: revised May 29 to clarify points raised by referee, add references to recent work. July 18, accepted for publicatio

Magnetic flux pumping in 3D nonlinear magnetohydrodynamic simulations

A self-regulating magnetic flux pumping mechanism in tokamaks that maintains the core safety factor at $q\approx 1$, thus preventing sawteeth, is analyzed in nonlinear 3D magnetohydrodynamic simulations using the M3D-C$^1$ code. In these simulations, the most important mechanism responsible for the flux pumping is that a saturated $(m=1,n=1)$ quasi-interchange instability generates an effective negative loop voltage in the plasma center via a dynamo effect. It is shown that sawtoothing is prevented in the simulations if $\beta$ is sufficiently high to provide the necessary drive for the $(m=1,n=1)$ instability that generates the dynamo loop voltage. The necessary amount of dynamo loop voltage is determined by the tendency of the current density profile to centrally peak which, in our simulations, is controlled by the peakedness of the applied heat source profile.Comment: submitted to Physics of Plasmas (23 pages, 15 Figures

Construction and measurements of a vacuum-swing-adsorption radon-mitigation system

Long-lived alpha and beta emitters in the $^{222}$Rn decay chain on (and near) detector surfaces may be the limiting background in many experiments attempting to detect dark matter or neutrinoless double-beta decay, and in screening detectors. In order to reduce backgrounds from radon-daughter plate-out onto the wires of the BetaCage during its assembly, an ultra-low-radon cleanroom is being commissioned at Syracuse University using a vacuum-swing-adsorption radon-mitigation system. The radon filter shows ~20$\times$ reduction at its output, from 7.47$\pm$0.56 to 0.37$\pm$0.12 Bq/m$^3$, and the cleanroom radon activity meets project requirements, with a lowest achieved value consistent with that of the filter, and levels consistently < 2 Bq/m$^3$.Comment: 5 pages, 3 figures, Proceedings of Low Radioactivity Techniques (LRT) 2013, Gran Sasso, Italy, April 10-12, 201

A new explanation of the sawtooth phenomena in tokamaks

The ubiquitous sawtooth phenomena in tokamaks are so named because the central temperature rises slowly and falls rapidly, similar to the blades of a saw. First discovered in 1974, it has so far eluded a theoretical explanation that is widely accepted and consistent with experimental observations. We propose here a new theory for the sawtooth phenomena in auxiliary heated tokamaks, which is motivated by our recent understanding of "magnetic flux pumping." In this theory, the role of the (m, n) = (1, 1) mode is to generate a dynamo voltage, which keeps the central safety factor, q(0), just above 1.0 with low central magnetic shear. When central heating is present, the temperature on axis will increase until at some point, and the configuration abruptly becomes unstable to ideal MHD interchange modes with equal poloidal and toroidal mode numbers, m = n &gt; 1. It is these higher order modes and the localized magnetic stochasticity they produce that cause the sudden crash of the temperature profile, not magnetic reconnection. Long time 3D MHD simulations demonstrate these phenomena, which appear to be consistent with many experimental observations.</p

Supersonic regime of the Hall-magnetohydrodynamics resistive tearing instability

An earlier analysis of the Hall-magnetohydrodynamics (MHD) tearing instability [E. Ahedo and J. J. Ramos, Plasma Phys. Controlled Fusion 51, 055018 (2009)] is extended to cover the regime where the growth rate becomes comparable or exceeds the sound frequency. Like in the previous subsonic work, a resistive, two-fluid Hall-MHD model with massless electrons and zero-Larmor-radius ions is adopted and a linear stability analysis about a force-free equilibrium in slab geometry is carried out. A salient feature of this supersonic regime is that the mode eigenfunctions become intrinsically complex, but the growth rate remains purely real. Even more interestingly, the dispersion relation remains of the same form as in the subsonic regime for any value of the instability Mach number, provided only that the ion skin depth is sufficiently small for the mode ion inertial layer width to be smaller than the macroscopic lengths, a generous bound that scales like a positive power of the Lundquist numbe

Hall MHD Modeling of Two-dimensional Reconnection: Application to MRX Experiment

Two-dimensional resistive Hall magnetohydrodynamics (MHD) code is used to investigate the dynamical evolution of driven reconnection in the Magnetic Reconnection Experiment (MRX). The initial conditions and dimensionless parameters of the simulation are set to be similar to the experimental values. We successfully reproduce many features of the time evolution of magnetic configurations for both co- and counter-helicity reconnection in MRX. The Hall effect is shown to be important during the early dynamic X-phase of MRX reconnection, while effectively negligible during the late ''steady-state'' Y-phase, when plasma heating takes place. Based on simple symmetry considerations, an experiment to directly measure the Hall effect in MRX configuration is proposed and numerical evidence for the expected outcome is given

The acceleration and storage of radioactive ions for a neutrino factory

The term beta-beam has been coined for the production of a pure beam of electron neutrinos or their antiparticles through the decay of radioactive ions circulating in a storage ring. This concept requires radioactive ions to be accelerated to a Lorentz gamma of 150 for 6He and 60 for 18Ne. The neutrino source itself consists of a storage ring for this energy range, with long straight sections in line with the experiment(s). Such a decay ring does not exist at CERN today, nor does a high-intensity proton source for the production of the radioactive ions. Nevertheless, the existing CERN accelerator infrastructure could be used as this would still represent an important saving for a beta-beam facility. This paper outlines the first study, while some of the more speculative ideas will need further investigations.Comment: Accepted for publication in proceedings of Nufact02, London, 200

Modeling of lithium granule injection in NSTX using M3D-C1

In this paper, we present simulations of pedestal control by lithium granule injection (LGI) in NSTX. A model for small granule ablation has been implemented in the M3D-C1 code (Jardin et al 2012 Comput. Sci. Discovery 5 014002), allowing the simulation of realistic lithium granule injections. 2D and 3D simulations of Li injections in NSTX H-mode plasmas are performed and the effect of granule size, injection angle and velocity on the pedestal gradient increase is studied. The amplitude of the local pressure perturbation caused by the granules is found to be highly dependent on the solid granule size. Adjusting the granule injection velocity allows one to inject more particles at the pedestal top. 3D simulations show the destabilization of high order MHD modes whose amplitude is directly linked to the localized pressure perturbation, which is found to depend on the toroidal localization of the granule density source