J.C. Ingraham

Energetic electron measurements in the edge of a reversed-field pinch

The edge plasma of the ZT‐40M [Fusion Technol. 8, 1571 (1985)] reversed‐field pinch has been studied using a combination of three different plasma probes: a double‐swept Langmuir probe, an electrostatic energy analyzer, and a calorimeter–Langmuir probe. The edge plasma has been measured both with and without a movable graphite tile limiter present nearby in the plasma. Without a limiter a fast nonthermal tail of electrons (T≂350 eV) is detected in the edge plasma with nearly unidirectional flow along B and having a density between 2% and 10% of the cold edge plasma (T≂20 eV). The toroidal sense of this fast electron flow is against the force of the applied electric field. A large power flux along B is measured flowing in the same direction as the fast electrons and is apparently carried by the fast electrons. With the limiter present the fast electrons are still detected in the plasma, but are strongly attenuated in the shadow of the limiter. The measured scrape‐off lengths for both the fast electrons and...

Asymmetric magnetic flux generation, m=1 activity, and edge phenomena on a reversed-field pinch

The ZT‐40M [Fusion Tech. 8, 1571 (1985)] reversed‐field pinch has been used to study magnetic flux perturbations during high‐θ [θ=Bθ(a)/〈Bφ〉>1.6] discharges. Asymmetric toroidal magnetic flux perturbations are found to be associated with magnetic flux emerging through the toroidal shell gap and with m=0 magnetic disturbances moving toroidally. Ramping current discharges, which are a special case of high‐θ operation, show the most robust self‐generation of toroidal flux. The electron density fluctuations on the inside major radius and associated m=1 and m=0 fluctuations seen on edge magnetic field probes provide a clearer picture of activity during a soft x‐ray sawtooth crash. During the sawtooth crash, significant magnetic energy is apparently converted into kinetic energy of the particles.

Oscillating field current drive experiments in a reversed field pinch

Steady‐state current sustainment by oscillating field current drive (OFCD) utilizes a technique in which the toroidal and poloidal magnetic fields at the plasma surface are modulated at audio frequencies in quadrature. Experiments on the ZT‐40M reversed field pinch [Fusion Technol. 8, 1571 (1985)] have examined OFCD over a range of modulation amplitude, frequency, and phase. For all cases examined, the magnitude of the plasma current is dependent on the phase of the modulations as predicted by theory. However, evidence of current drive has only been observed at relatively low levels of injected power. For larger modulation amplitudes, the data suggest that substantial current drive is offset by increased plasma resistance as a result of modulation enhanced plasma–wall interactions. The initial experimental results and supporting theoretical interpretations of OFCD are discussed.

Fluctuations and transport in a reversed field pinch edge plasma

Edge fluctuations are characterized and their associated transport is determined from Langmuir probe measurements in the ZT-40M reversed field pinch. It is found that the fluctuations have high normalized amplitudes and |n|/n = 0.4). There are significant contributions from magnetic perturbations acting on the equilibrium gradients. Compared to the global estimates, the fluctuation driven particle flux is large, whereas the corresponding electron energy flux is not. In the limiter shadow, the equilibrium density and electron temperature scale lengths are shorter and the fluctuation levels are higher. The fluctuation driven particle flux in the limiter shadow is 60% less than that in the plasma edge; most of the reduction is in the low frequency spectral region, which is where global MHD magnetic fluctuations are strongest.

Effects of limiters on reversed-field pinch confinement

This paper documents the effects on confinement of introducing discrete limiters into the edge plasma of the ZT‐40M reversed‐field pinch (RFP) [Fusion Technol. 8, 1571 (1985)]. RFP confinement is not significantly degraded by appropriately designed single limiters inserted to a sufficient depth for effective local vacuum vessel wall protection. Inserting limiters too deeply into the plasma results in excess limiter heating, and a consequent increase in the impurity content of the plasma. Under these conditions the plasma loop voltage increases. The heating of the limiters is observed to be asymmetric, with the majority of the heat flux in ZT‐40M being attributable to suprathermal electrons [Tsupra∼2–3×Te(0)] reaching the edge moving almost unidirectionally along magnetic‐field lines.