Impact of multiple-frequency heating on the formation and control of diamagnetic electron rings in an axisymmetric mirror

Abstract

High-beta, hot-electron plasmas have been produced by electron-cyclotron heating in the SM-1 axisymmetric mirror using closely-spaced multiple frequencies. The relativistic electrons produce annular distributions (ELMO rings) with as much as ten times more stored energy than with single-frequency heating. While larger frequency separations (Δf/f∼0.1) provide some control of the ring size, the dominant effects are associated with an improvement in heating efficiency which persists to very small frequency separations (Δf/f∼10−3). Details of the reconstruction of the ring distribution (both in steady state and during build-up), the influence of multiple frequency heating on fluctuations, axial electron losses, and a scaling of these effects with power are presented.

Production and study of high-beta plasma confined by a superconducting dipole magnet

“…[3][4][5][6] The radial location of the fast electron population is viewed by an x-ray camera during times when the ECRH is on ͑Fig. From this, previous experiments, 11,12 and the measured height of the x-ray images, we believe the pressure is well approximated by P Ќ / P ʈ ϳ 5. For single-frequency ECRH, the cameras indicate the pressure peak is localized on the equatorial midplane of the field lines having the fundamental cyclotron resonance at the minimum B.…”

“…6 The ability of the dipole configuration to confine a high-beta plasma without magnetic shear may decouple particle and energy confinement, avoid the accumulation of fusion reaction products, and enable the dipole fusion power concept to operate with a 3 He catalyzed D-D fuel cycle. [8][9][10][11][12] Energetic trapped electrons were first generated in the ELMO experiments 8 where harmonic cyclotron absorption created a localized "ring" of weakly relativistic electrons ͑E h ϳ 400 keV͒ within a plasma containing a larger density of cooler electrons. The pressure results from a population of energetic trapped electrons that can be maintained for many seconds of microwave heating provided sufficient neutral gas is supplied to the plasma.…”

“…7 In this article, we report the first experiments using the LDX device and describe the production of high-beta plasma confined by a dipole magnet using neutral gas fueling and electron cyclotron resonance heating ͑ECRH͒. 9 In simple axisymmetric mirrors, internal magnetic probes were able to characterize the plasma equilibrium, and, during optimal conditions, multiple-frequency ECRH 11 produced anisotropic plasmas that reached high values of local beta, ␤ Ϸ 40%, and high ratios of the perpendicular and parallel pressures, ␤ Ќ / ␤ ʈ Ϸ 4.3. A number of previous experiments also used ECRH to produce high-beta plasma.…”

Initial Results of Multi-Frequency Electron Cyclotron Heating in the Levitated Dipole Experiment

“…One of our main "knobs" to do this is electron cyclotron resonance heating (ECRH) at multiple frequencies. This has historically proven to be an effective means to create high β hot-electron plasmas in mirror machines, [7] levitrons, [8] and CTX. [5,9] There currently are two ECRH sources in use for LDX.…”

Experimental observation of the hot-electron equilibrium in a minimum-Bmirror plasma

“…Experimental measurements of mirror equilibria have been made in electron cyclotron resonance heated (ECRH) maximum-B mirrors', 2 and bumpy tori 3 . The data shows that for maximum-B hot electron plasmas, the plasma pressure is concentrated in a ring structure which is located near the second harmonic ECRH resonance.…”