T.C. Simonen

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.

Measurements of ion cyclotron instability characteristics in a mirror‐confined plasma

Frequency spectrum, wavelength, and phase velocity measurements of ion cyclotron oscillations in a mirror‐confined plasma are described. The measured potential amplitudes reach, but do not exceed, the electron temperature. Other experiments in this device, not discussed here, reveal a wide range of plasma conditions where these oscillations are not detected.

Dynamic gas flow during plasma operation in TMX‐U

Control of the neutral density outside of the plasma radius is essential for proper operation of the various plasma configurations in Lawrence Livermore National Laboratory’s (LLNL) Tandem Mirror Experiment‐Upgrade (TMX‐U). TMX‐U excess‐beam, stream‐gun, gas‐box, and beam‐reflux gases are pumped internally in regions defined by 73° Ti‐gettered liners and warm Ti‐gettered plasma liners. The array of fast and slow ion gauges—a large TMX‐U diagnostic—has been used to measure the dynamic pressure in many of the liner‐defined regions on three time scales. The natural divertor action, or plasma pump effect, of mirror plasmas has been measured using the ion gauge diagnostics on a fast time scale during operation of TMX‐U with ECRH start up. Routine operation of TMX‐U is enhanced by the ability to verify the effectiveness of gettering and to locate leaks using pressure data collected on the two slow time scales. A computer code, dynavac 6, which treats TMX‐U as a set of conductance‐coupled regions with pumping an...

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.

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.

TMX-U thermal-barrier experiments

Thermal-barrier experiments in the Tandem Mirror Experiment Upgrade (TMX-U) are reported, along with progress made at the Lawrence Livermore National Laboratory in plasma confinement and central-cell heating. Thermal barriers in TMX-U improved axial confinement by two orders of magnitude over a limited range of densities, compared with confinement in single-cell mirrors at the same ion temperature. It is shown that central-cell radial nonambipolar confinement scales as neoclassical theory and can be eliminated by floating the end walls. Radial ambipolar losses can also be measured and reduced. The electron energy balance is improved in tandem mirrors to near classical, resulting in T/sub e/ up to 0.28 keV. Electron cyclotron heating (ECH) efficiencies up to 42%, with low levels of electron microinstability, were achieved when hot electrons in the thermal barrier were heated to average betas as large as 15%. The hot-electron distribution was measured from X-rays and is modeled by a Fokker-Planck code that includes heating from cavity radio-frequency (RF) fields. >

Measurement of sloshing-ion spatial profiles in end cell of tandem mirror experiment-upgrade (TMX-U)

Neutral‐beam injection is used to establish an energetic sloshing‐ion distribution in the end cells of the Tandem Mirror Experiment‐Upgrade (TMX‐U). The angular and radial distributions of these ions are measured with arrays of secondary‐emission detectors. Unfolding the angular distribution gives the axial extent of the sloshing ions. Sloshing‐ion density measurements from the charge‐exchange flux indicate a high‐density (nis≂3 to 6×1012 cm−3) but narrow (FWHM ∼16 cm) sloshing‐ion plasma.

TMX upgrade magnet system: Design characteristics and physics considerations

Summarized in this article are the design characteristics of the magnet-system configuration constructed for use in the modified Tandem Mirror Experiment (TMX Upgrade), and a description of the resulting vacuum magnetic field. Many engineering and physics considerations and limitations governed the design. Several of the physics issues are discussed here, including single-particle drift surfaces and adiabaticity, central-cell resonant radial transport, magnetohydrodynamic stability analysis, and finite-beta equilibrium. The described design procedures can be applied to other tandem-mirror experiments or reactor studies.