W.E. Nexsen

Field-reversal experiments in a neutral-beam-injected mirror machine

Data on field-reversal experiments in the neutral-beam-injected 2XIIB mirror machine are reported. The best result is an estimated field-reversal parameter ζ = ΔB/Bvac = 0.9 ± 0.2 with vacuum field strength Bvac = 4.35 kG. Experiments at higher field strength Bvac = 6.7 kG achieved ζ = 0.6 ± 0.1. Ion energy confinement nτEi for the Bvac = 6.7 kG experiment is less than that predicted by classical Spitzer electron drag. Ion-cyclotron oscillations increasing with injected neutral-beam current suggest that ion-cyclotron losses are present and that ΔB/Bvac could be increased by improving stabilization of the ion-cyclotron oscillations.

Ambipolar potential formation and axial confinement in TMX

TMX experimental data on ambipolar potential control and on the accompanying electrostatic confinement are reported. In the radial core of the central cell, measurements of electrostatic potentials of 150 V which augment axial ion confinement are in agreement with predictions using the Maxwell-Boltzmann result. Central-cell ion confinement was observed to scale according to electrostatic potential theory up to average enhancement factors of eight times over mirror confinement alone.

Production of large-radius, high-beta, confined mirror plasmas

This paper reports results of experiments in which mirror-confined plasmas with radii as high as 7 ion gyro-radii are produced and maintained by neutral-beam injection. In these plasmas, betas as high as 0.45 were achieved and limited only by the available neutral-beam power. Electron temperature and ion-energy confinement increased with larger plasma size.

Multichannel neutral‐particle analyzer system

A multichannel neutral-analyzer system developed for the analysis of charge-exchange flux from magnetically confined plasma is described. The system uses tandem magnetic-electrostatic deflection of ions produced from neutrals stripped in a gas cell to obtain the energy spectra of specific charge-to-mass-ratio species. The analyzer is collimated with a spatial resolution of 2 cm FWHM at the plasma and is movable so that radial scans of charge-exchange flux can be made. Data are recorded digitally, allowing frequency response of fluctuations in charge-exchange flux up to 50 kHz. The calibration procedure employs an auxiliary single-channel analyzer calibrated over the full energy range of the multichannel instrument with an atomic-beam setup. Typical data obtained from the 2XIIB neutral-beam-injected mirror machine are briefly presented.

Radial transport in the central cell of the tandem mirror experiment

An experimental study of radial transport in the Tandem Mirror Experiment is reported here. Plasma parameters were measured in a series of well‐diagnosed plasma discharges. A negative electric current (80±40 A within a 30‐cm radius) flowed to the end wall, implying an equal radial loss of plasma ions. The axial losses of plasma ions were 100 A from the same volume. The nonambipolar radial ion flux was of the same order as the flux resulting from resonant‐neoclassical and ion‐neutral transport, but the uncertainties are large. The ambipolar radial transport (of both ions and electrons) was investigated by comparing the observed end losses with calculations of the plasma fueling by gas penetration and neutral beams. The ambipolar radial losses are probably smaller than the loses through other processes and may be as small as the classical losses resulting from Coulomb collisions.

Plasma Injection into a Magnetic Field of Cusped Geometry

A plasma stream was directed from a field‐free region along the axis of symmetry into a magnetic field of biconical cusped geometry. No evidence was found to support the hypothesis that a directed plasma stream can not penetrate a magnetic field whose value exceeds Bc2 = 12πρv2, where ρ is the plasma density and v its directed velocity. As the plasma penetrated the magnetic field, the plasma and field were found to intermix. Large quantities of the plasma which entered the containment region through one point cusp were found to leave promptly through the second point cusp and through the line cusp. Bombardment of the vacuum chamber walls in the vicinity of the line cusp generated sufficient secondary ions to mask any small scale plasma trapping. However, there was no evidence of gross trapping of the injected plasma.

The effect of end-cell stability on the confinement of the central-cell plasma in TMX

In the Tandem Mirror Experiment (TMX), the central-cell losses provide the warm unconfined plasma necessary to stabilize the drift-cyclotron loss-cone instability in the end cells. This places a theoretical limit on central-cell confinement, which is expressed as a limit on the end-cell to central-cell density ratio. As this density ratio increases in a TMX experiment, large increases of end-cell ion-cyclotron-frequency plasma fluctuations are observed. These fluctuations cause the central-cell confinement to decrease, in agreement with a theoretical model.

A neutral-potassium-beam measurement of plasma density

A 6-keV beam of potassium atoms has been used to measure the integrated line density of a deuterium plasma in which the electron temperature was in the range of a few hundred eV. and the mean ion energy was in the range of 5–10 keV. Line densities from 5 × 1011 to 1014 particles per cm2 can be effectively measured. The various contributions to the attenuation of the neutral beam have been examined in sufficient detail to permit an evaluation of this measurement to a wide range of plasma conditions.

Boundary Condition Effects in a Magnetic Mirror Compression Experiment

The transition from unstable operation to quasi‐stable plasma containment in a plasma‐injected magnetic compression experiment has been found to be dependent upon the density and lifetime of plasma external to the confinement chamber. Transverse motion of the confined plasma is limited and the magnetic compression proceeds nearly adiabatically, provided the density of the external plasma is greater than ∼0.1% of the confined plasma. If the electrical connection to external conducting surfaces is lost after the magnetic compression is well under way (later than 50 μsec after beginning of compression for a magnetic field rise time of 120 μsec), the trapped plasma suddenly moves to the wall between the magnetic mirrors giving dramatic evidence of instability. In any case, during the magnetic compression the shifting external plasma‐wall contact results in intermittent particle and/or energy losses from the hot plasma. Later than 100 μsec, the untrapped portion of the injected plasma which has been cooled, neutralized, and contaminated through its interaction with vacuum chamber walls diffuses into the containment chamber. Thus, at all times, there are loss processes in competition with the magnetic compression.

Energy confinement studies in the tandem mirror experiment (TMX): Power balance

The power balance in the Tandem Mirror Experiment (TMX) is studied for several days of operation. Between them, these days typified the operating range of TMX. Examining the power balance on axis, it is found that 60% to 100% of the power is carried to the end walls by escaping central‐cell ions. Globally, these calculations account for 70% to 100% of the input power on each of the days studied. Based upon the power balance, the energy confinement times of the particle species are calculated. The end‐cell ion energy confinement time is similar to that achieved in the 2XIIB single‐cell magnetic mirror experiment, whereas the electron energy confinement in TMX was 10 to 100 times better. The central‐cell ion energy confinement in the central flux tube was determined by axial particle loss. At the central‐cell plasma‐edge radial particle transport and charge exchange with the fueling gas are important processes.