Bumpy Torus Plasma Research Articles

Heavy Ion Beam Study of Negative and Positive Potential Formation in Bumpy Torus Plasma

Potential formation is studied in Nagoya Bumpy Torus (NBT-1M) by the use of a heavy ion beam probe. The concentric negative potential well and its positive rim are observed in the standard operation. The position of the rim moves with the second harmonic ECRH zone at the midplane of each mirror section. It is confirmed that the core electron heating is essential for the potential well formation. A stable positive potential is observed near T-M transition with a smooth change from the negative profile.

A simple power deposition model for microwave heating in EBT-I

A simple model is presented for the production and absorption of ordinary- and extraordinary-mode energy in various regions of the ELMO Bumpy Torus plasma. The plasma is divided into two regions: (I) the low-magnetic-field side of the extraordinary-mode cutoff and (II) the high-field side of the cut-off. Energy balance equations are written for the sources (injection, mode conversion, and tunnelling) and sinks (mode conversion, absorption, and tunnelling) in each region, and simplified models are introduced to account for each of these processes. Since a typical ray makes several reflections from cavity wall surfaces before being absorbed, additional simplifying assumptions are made that the wave fields are an isotropic incoherent superposition of plane waves and that the energy density of each mode is uniform in a given region. – It is found that conversion between eigenmodes upon wall reflection and absorption of the extraordinary mode at the fundamental cyclotron resonance are the most rapid processes in EBT-I. The relative fractions of the injected power deposited in the various plasma components are typically 25% to the annulus, 22% to the core plasma, and 53% to the surface plasma. The partitioning of energy between the three plasma components is determined largely by geometric characteristics of the walls, the plasma, and the fundamental cyclotron resonant surface. The results are comparatively insensitive to other parameters of the model and in rough agreement with estimates of power deposition based on experimental data.

Electromagnetic emission and anomalous resistivity from equal and oppositely directed electron beams

The experimental observation and theoretical description of a previously unrecognized mechanism for generating electromagnetic emissions and anomalous resistivity from a beam-plasma interaction which is characteristic of Penning discharges is reported. Very clear emissions were observed from electric field-dominated bumpy torus plasma near the geometric mean plasma frequency. These emissions are inconsistent with familiar physical mechanisms, and occurred in excellent agreement with the theoretical frequency ωgm = 0.537(ωpeωpi)1/2. This frequency was derived from a theory based on the interaction of two opposite, interpenetrating electron beams with a heavy ion plasma. The anomalous resistivity associated with this interaction is predicted to several hundred times that of the familiar Buneman or ’’turbulent’’ resistivity. A perturbation procedure is used to obtain analytic expressions for the emission frequency and the anomalous resistivity in the zero temperature case. These results are extended to include the effects of finite ion temperatures on the dispersive properties of the plasma.

Fluctuations and turbulence in an electric field bumpy torus plasma

In this experiment, strong radial electric fields were imposed along the minor radius of a steady-state bumpy torus plasma by biasing it with electrodes maintained at kilovolt potentials. Digitally implemented spectral analysis techniques were used to experimentally investigate the characteristics of the fluctuations of plasma number density and electrostatic potential below the ion plasma and ion cyclotron frequencies. The dispersion relation between ω and k for azimuthally propagating waves was determined experimentally, and, when linear, was consistent with the E/B drift velocity. Above 100 kHz, the power spectra of both density and potential fluctuations obeyed a power law of the form P(ω) α ω−p. The spectral index p of this plasma turbulence assumed values which varied with operating conditions. The fluctuations of density and potential ranged from highly coherent rotating spokes, to highly turbulent conditions with a Gaussian distribution in amplitude.

The effect of a weak vertical magnetic field on fluctuation-induced transport in a Bumpy-Torus plasma

Digital spectral analysis techniques are used to observe that, under the appropriate experimental conditions, the radial fluctuation-induced transport in the NASA Lewis Bumpy Torus plasma may be reversed from outward to inward by applying a weak vertical magnetic field. This reversal in the transport direction corresponds to a reversal in the sign of the phase angle between the density and potential fluctuations and results in an enhancement of the plasma density by a factor of five or more.

New Mechanism for Electromagnetic Emission near the Geometric Mean Plasma Frequency

We report the experimental observation and theoretical description of a previously unrecognized mechanism for electromagnetic emission from plasmas near the geometric mean plasma frequency. Very clear emissions were observed from the electric-field-dominated NASA Lewis bumpy torus plasma at a frequency which is inconsistent with familiar mechanisms of plasma emission. A theory based on a Penning-discharge-like beam-plasma interaction predicts a frequency in excellent agreement with the observations.

Effects of Applied DC Radial Electric Fields on Particle Transport in a Bumpy Torus Plasma

The toroidal ring of plasma contained in the NASA Lewis Bumpy Torus may be biased to positive or negative potentials approaching 50 kilovolts by applying DC voltage to twelve or fewer midplane electrode rings. The electric fields, which are responsible for raising the ions to high energies by ExB/B2 drift, then point radially outward or inward. The profiles of plasma number density are observed to be flat or triangular across the plasma diameter. The absence of a second derivative in the density profile, combined with the flat electron temperature profiles which are observed, implies that the radial transport processes are not diffusional in nature and are dominated by the strong radial electric fields which are applied to the plasma. Evidence from a paired comparison test shows that the plasma number density and confinement time can increase more than an order of magnitude if the electric field acting along the minor radius of the toroidal plasma points inward, relative to the values observed when the electric field points radially outward. Some characteristic data taken under nonoptimized conditions yielded the highest plasma number density (2.7 × 1011/cc on axis) and the longest particle containment times (1.9 milliseconds) observed so far in this experiment.

Characteristics of the NASA Lewis Bumpy Torus plasma generated with high positive or negative applied potentials

The toroidal ring of plasma contained in the NASA Lewis bumpy-torus superconducting magnet facility may be biased to positive or negative potentials approaching 50 kilovolts by applying direct-current voltages of the respective polarity to 12 or fewer of the midplane electrode rings. The electric fields which are responsible for heating the ions by E/B drift then point radially outward or inward. The low-frequency fluctuations below the ion cyclotron frequency appeared to be dominated by rotating spokes.

Characteristics of the NASA Lewis Bumpy Torus Plasma Generated with Positive Applied Potentials

Experimental observations have been made during steady-state operation of the NASA Lewis Bumpy Torus experiment at input powers up to 150 kW in deuterium and helium gas, and with positive potentials applied to the midplane electrodes. This steady-state ion heating method utilizes a modified Penning discharge operated in a bumpy torus confinement geometry such that the plasma is acted upon by a combination of strong electric and magnetic fields. Experimental investigation of a deuterium plasma revealed electron temperatures from 14 to 140 eV and ion kinetic temperatures from 160 to 1785 eV. At least two distinct modes of operation exist, each of which is associated with a characteristic range of background pressure and electron temperature. Experimental data show that the average ion residence time in the plasma is virtually independent of the magnetic field strength.