Energetic-Electron-Driven Instability in the Helically Symmetric Experiment
C. B. Deng, D. L. Brower, B. N. Breizman, D. A. Spong, A. F. Almagri, D. T. Anderson, F. S. B. Anderson, W. X. Ding, W. Guttenfelder, K. M. Likin, and J. N. Talmadge
Abstract
Energetic electrons generated by electron cyclotron resonance heating are observed to drive instabilities in the quasihelically symmetric stellarator device. The coherent, global fluctuations peak in the plasma core and are measured in the frequency range of 20-120 kHz. Mode propagation is in the diamagnetic drift direction of the driving species. When quasihelical symmetry is broken, the mode is no longer observed. Experimental observations indicate that the unstable mode is acoustic rather than Alfvénic. © 2009 The American Physical Society
DOI:10.1103/PhysRevLett.103.025003
Super-X divertors for solving heat and neutron flux problems of fusion devices
P. M. Valanju, M. Kotschenreuther, and S. M. Mahajan
Abstract
We present a new magnetic geometry, called the Super X divertor (SXD), that could potentially solve the
enormous heat exhaust problem of next-generation high power-density experiments and fusion reactors.
With only small changes in net coil currents, the axisymmetric SXD modification of the standard divertor
(SD) coils greatly increases the divertor radius, the line length, and the plasma-wetted area. The lower B
at large R decreases parallel heat flux and hence lowers the plasma temperature at SXD plates to below
10 eV, allowing higher divertor radiation fractions. The SXD could safely exhaust five times more heat
than an SD, is unique in allowing adequate shielding of divertor target from neutron damage, and can
enable much improved, reactor-relevant core plasma performance. © 2009 Elsevier B.V. All rights reserved.
DOI:10.1016/j.fusengdes.2009.06.001
Super-X divertors and high power density fusion devices
P. M. Valanju, M. Kotschenreuther, S. M. Mahajan, and J. Canik
Abstract
The Super-X Divertor (SXD), a robust axisymmetric redesign of the divertor magnetic geometry that can allow a fivefold increase in the core power density of toroidal fusion devices, is presented. With small changes in poloidal coils and currents for standard divertors, the SXD allows the largest divertor plate radius inside toroidal field coils. This increases the plasma-wetted area by 2–3 times over all flux-expansion-only methods (e.g., plate near main X point, plate tilting, X divertor, and snowflake), decreases parallel heat flux and hence plasma temperature at plate, and increases connection length by 2–5 times. Examples of high-power-density fusion devices enabled by SXD are discussed; the most promising near-term device is a 100 MW modular compact fusion neutron source “battery” small enough to fit inside a conventional fission blanket. ©2009 American Institute of Physics
DOI:10.1063/1.3110984
Evaluation of solar wind-magnetosphere coupling functions during geomagnetic storms with the WINDMI model
E. Spencer, A. Rao, W. Horton, M. L. Mays
Abstract
We evaluate the performance of three solar wind-magnetosphere coupling functions in training the physics-based WINDMI model on the 3–7 October 2000 geomagnetic storm and predicting the geomagnetic Dst and AL indices during the 15–24 April 2002 geomagnetic storm. The rectified solar wind electric field, a coupling function by Siscoe, and a recent formula proposed by Newell are evaluated. The Newell coupling function performed best in both the training and prediction phases for Dst prediction. The Siscoe formula performed best during the training phase in reproducing the AL faithfully and capturing storm time events. The rectified driver was discovered to be the best in overall performance during both training as well as prediction phases, even though the other two coupling functions outperform it in the training phase. The results indicate that multiple drivers need to be concurrently employed in space weather models to yield different possible levels of geomagnetic activity. ©2009 American Geophysical Union
DOI:10.1029/2008JA013530
Effect of electrostatic turbulence on magnetic islands
F. L. Waelbroeck, F. Militello, R. Fitzpatrick, W. Horton
Abstract
A numerical analysis of the interaction of resistive drift wave and interchange turbulence with a magnetic island in a two-dimensional slab is presented. The time-scale for the evolution of the island is assumed to be much longer than that for the turbulence, allowing the use of an electrostatic model. The effects of the turbulence are isolated by choosing the parameters such that only even modes are unstable. This makes it possible to compare turbulent states with quiescent states in which turbulence is suppressed by enforcing odd parity. The turbulence is found to reduce the propagation velocity of the island. Its effect is destabilizing for thin islands but becomes stabilizing for islands greater than a few times the Larmor radius. Analysis of the quiescent solutions reveals the possibility of oscillations of the island amplitude and frequency through hysteretic transitions between bistable states. © 2009 IOP Publishing Ltd
DOI: 10.1088/0741-3335/51/1/015015
A simple ideal magnetohydrodynamical model of vertical disruption events in tokamaks
R. Fitzpatrick
Abstract
A simple model of axisymmetric vertical disruption events (VDEs) in tokamaks is presented in which the halo current force exerted on the vacuum vessel is calculated directly from linear, marginally stable, ideal-magnetohydrodynamical (MHD) stability analysis. The basic premise of the model is that the halo current force modifies pressure balance at the edge of the plasma, and therefore also modifies ideal-MHD plasma stability. In order to prevent the ideal vertical instability, responsible for the VDE, from growing on the very short Alfvén time scale, the halo current force must adjust itself such that the instability is rendered marginally stable. The model predicts halo currents which are similar in magnitude to those observed experimentally. An approximate nonaxisymmetric version of the model is developed in order to calculate the toroidal peaking factor for the halo current force. ©2009 American Institute of Physics
DOI: 10.1063/1.3068467
Plume detachment from a magnetic nozzle
C. Deline, R. Bengtson, B. Breizman, M. Tushentsov, J. Jones, D. Chavers, C. Dobson, B. Schuettpelz
Abstract
High-powered electric propulsion thrusters utilizing a magnetized plasma require that plasma exhaust detach from the applied magnetic field in order to produce thrust. This paper presents experimental results demonstrating that a sufficiently energetic and flowing plasma can indeed detach from a magnetic nozzle. Microwave interferometer and probe measurements provide plume density, electron temperature, and ion flux measurements in the nozzle region. Measurements of ion flux show a low-beta plasma plume which follows applied magnetic field lines until the plasma kinetic pressure reaches the magnetic pressure and a high-beta plume expanding ballistically afterward. Several magnetic configurations were tested including a reversed field nozzle configuration. Despite the dramatic change in magnetic field profile, the reversed field configuration yielded little measurable change in plume trajectory, demonstrating the plume is detached. Numerical simulations yield density profiles in agreement with the experimental results.© 2009 American Institute of Physics
DOI: 10.1063/1.3080206
Wide-angle infrared absorber based on a negative-index plasmonic metamaterial
Y. Avitzour, Y. Urzhumov, G. Shvets
Abstract
A metamaterial-based approach in making a wide-angle absorber of infrared radiation is described. The technique is based on an anisotropic perfectly impedance-matched negative-index material (PIMNIM). It is shown analytically that a PIMNIM that is subwavelength in all three dimensions enables absorption close to 100% for incidence angles up to 45° to the normal. A specific implementation of such frequency-tunable PIMNIM based on plasmonic metamaterials is presented. Applications to infrared imaging and coherent thermal sources are described. © 2009 The American Physical Society
DOI: 10.1103/PhysRevB.79.045131