J. Waksman

Measurement of energetic-particle-driven core magnetic fluctuations and induced fast-ion transport

Internal fluctuations arising from energetic-particle-driven instabilities, including both density and radial magnetic field, are measured in a reversed-field-pinch plasma. The fluctuations peak near the core where fast ions reside and shift outward along the major radius as the instability transits from the n = 5 to n = 4 mode. During this transition, strong nonlinear three-wave interaction among multiple modes accompanied by enhanced fast-ion transport is observed.

Fast ion confinement and stability in a neutral beam injected reversed field pinch

The behavior of energetic ions is fundamentally important in the study of fusion plasmas. While well-studied in tokamak, spherical torus, and stellarator plasmas, relatively little is known in reversed field pinch plasmas about the dynamics of fast ions and the effects they cause as a large population. These studies are now underway in the Madison Symmetric Torus with an intense 25 keV, 1 MW hydrogen neutral beam injector (NBI). Measurements of the time-resolved fast ion distribution via a high energy neutral particle analyzer, as well as beam-target neutron flux (when NBI fuel is doped with 3–5% D2) both demonstrate that at low concentration the fast ion population is consistent with classical slowing of the fast ions, negligible cross-field transport, and charge exchange as the dominant ion loss mechanism. A significant population of fast ions develops; simulations predict a super-Alfvenic ion density of up to 25% of the electron density with both a significant velocity space gradient and a sharp radial...

Neutral beam heating of a RFP plasma in MST

Electron temperature is observed to rise due to neutral beam injection (NBI) in the Madison Symmetric Torus (MST). Heating is observed to be 100 ± 50 eV in the core of 200 kA plasmas. This is the first definitive measurement of auxiliary heating of a reversed-field pinch (RFP). This heating is consistent with a 1D classical model which was developed. This 1D model calculates the evolving thermal conductivity and ohmic power input profiles during enhanced confinement, and can calculate NBI deposition and classical fast ion diffusion and slowing. The predicted temperature change is consistent with measured beam heating both during and after enhanced confinement, which is consistent with previous observations that fast ions are well confined and behave roughly classically in the RFP.

Physics and optimization of plasma startup in the RFP

In the tokamak and reversed-field pinch (RFP), inductively driven toroidal plasma current provides the confining poloidal magnetic field and ohmic heating power, but the magnitude and/or duration of this current is limited by the available flux swing in the poloidal field transformer. A portion of this flux is consumed during startup as the current is initiated and ramped to its final target value, and considerable effort has been devoted to understanding startup and minimizing the amount of flux consumed. Flux consumption can be reduced during startup in the RFP by increasing the toroidal magnetic field, Bti, applied to initiate the discharge, but the underlying physics is not yet entirely understood. Toward increasing this understanding, we have for the first time in the RFP employed advanced, non-invasive diagnostics on the Madison Symmetric Torus to measure the evolution of current, magnetic field, and kinetic profiles during startup. Flux consumption during startup is dominantly inductive, but we find that the inductive flux consumption drops as Bti increases. The resistive consumption of flux, while relatively small, apparently increases with Bti due to a smaller electron temperature. However, the ion temperature increases with Bti, exceeding the electron temperature and thus reflecting non-collisional heating. Magnetic fluctuations also increase with Bti, corresponding primarily to low-n modes that emerge sequentially as the safety factor profile evolves from tokamak-like to that of the RFP.

Majority ion heating by neutral beam injection and confinement of fast ions in the Madison Symmetric torus reversed field pinch

Abstract A new 1 MW neutral beam injector (START-20F) is in operation on the Madison Symmetric Torus (MST) reversed field pinch. The beam, consisting of two arc discharge plasma generators, an optimized ion optical system and an integrated neutralizer/injector tank, operates at 25kV and up to 40A of neutrals for a 20 msec pulse (compared to a typical MST pulse length of 60 msec). The injected 1 MW of hydrogen neutrals (with approximately 85% in the full energy component) is significant compared to the 3-4 MW of ohmic input power in a typical target discharge. At this beam energy and a background electron density of about 1x1019 m–3 and temperature ≤1keV, roughly 90% of the injected power is deposited within the plasma. Initial experiments with the high power NBI show a large heating of the bulk ions: the fit of the width of energy spectrum as measured by Rutherford scattering (which is generally related to core ion temperature) quickly increases from 180eV to 230eV. This apparent significant and rapid heating of bulk ions is difficult to explain by classical collisions only, as modeling predicts 75% of the injected power is deposited on electrons and 15% on ions. The confinement of the fast ions (measured by the persistence in time of fusion neutrons due to a small fraction of deuterium in the beam fuel) is much greater than the canonical 1 msec confinement of particles and energy in the MST. The fast particle confinement is measured to increase with magnetic field strength. Further recent experiments document fast particle confinement time versus direction of injection (parallel or antiparallel to central magnetic field), beam energy, and background plasma properties.