B.W. Stallard

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.

Fokker-Planck calculations of hot-electron formation by electron cyclotron resonance heating in the TMX-Upgrade tandem mirror

The paper studies cold plasma trapping and heating of hot electrons in mirror geometry using a time-dependent, bounce-averaged Fokker-Planck code with quasi-linear diffusion due to RF heating at fundamental and second harmonic frequencies. With the restriction k|| = 0, the code models the beam-controlled heating (spatially restricted electric fields) that will be used to create thermal barriers in the TMX-Upgrade tandem-mirror experiment. By spatially localizing the microwave beams, which are strongly absorbed in a single pass, the mean hot-electron energy may be controlled. Heating is away from the midplane to control anisotropy (P⊥/P|| ). For a given magnetic field geometry and cold-plasma source temperature Ts, the parameters of the hot electrons scale with the quantity χ ≡ 2/nsω, where is the electric field, ns is the cold-plasma density, and ω is the frequency.

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.

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.

Control of first-wall surface conditions in the 2XIIB magnetic mirror plasma confinement experiment

The control of first-wall surface conditions in the 2XIIB Magnetic Mirror Plasma Confinement experiment is described. Before each plasma shot, the first wall is covered with a freshly gettered titanium surface. Up to 5 MW of neutral beam power has been injected into 2XIIB, resulting in first-wall bombardment fluxes of 10

Scaling of plasma potentials in TMX-U with the end walls grounded

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Plasma wall charge-exchange interactions in the 2XIIB magnetic mirror experiment

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Wave-electron coupling in helicon source

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Stabilization of a neutral-beam-sustained, mirror-confined plasma

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