A. L. Hunt

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.

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.

Cooperative Effects in a Tenuous Energetic Plasma Contained by a Magnetic Mirror Field

The formation and characteristics of a steady‐state hydrogen plasma contained in a magnetic mirror field are described. The mean ion energy is 20 keV. The plasma is formed by ionizing and trapping a portion of a beam of energetic hydrogen atoms passing through the confining field. The methods of measurement used to determine the plasma properties are described. Measurements of the radial and azimuthal trapped‐ion distributions, the average ion and electron densities, and the plasma potential are compared with the predictions of simple theory, neglecting cooperative plasma effects. The observed deviations from these simple predictions show that the plasma properties are dominated by cooperative phenomena. The plasma density is found to be limited to a low value (∼4 × 107 ions/cm3) by a flute or drift instability. This instability is characterized by a low frequency rotation of the plasma at a frequency typically close to the ▿B precession frequency of a 20‐keV proton in the nonuniform mirror field. The pla...

Initial wall conditioning for the TMX‐U fusion experiment

The introduction of impurities and hydrogen from the walls and liners of the TMX‐U device can have a deleterious effect on the plasma. The area of these surfaces is large (∼107 cm2) and has a complicated shape, so cleaning is difficult. Techniques that have been used to condition these walls include (1) initial solvent cleaning; (2) baking; (3) glow discharge cleaning; (4) gettering; and (5) plasma clean up. Several diagnostic techniques, including UV and visible spectroscopy and residual gas analysis, have been used to evaluate the effectiveness of the conditioning procedures. A series of experiments was performed to optimize the glow discharge in the TMX‐U geometry; during approximately 80 h of glow discharge cleaning, low‐Z impurities were removed. Gettering along with pumping with liquid‐nitrogen‐cooled liners and cryopumps resulted in base pressures ∼10−8 Torr. This paper describes these procedures and their importance in the formation of initial plasmas in TMX‐U.

Gas pressure in the end plug regions of the TMX‐U thermal barrier experiment

We consider briefly the upper bound on background pressure (0.5 to 1.0×10−6 Torr) that is required for operation of the thermal barrier end plugs of TMX‐U and propose a rate equation for pressure buildup that includes terms for neutral beam injection and wall reflux. During early operation of TMX‐U the background pressure exceeded this upper bound; in addition, axial end plugging of plasma losses seemed limited by the excessive pressure. Modifications to the vacuum system to decrease this pressure to the desired range are described. Furthermore, wall reflux is shown to depend linearly on pressure. We conclude with an example of the dramatic increase in time duration of axial plugging of plasma losses with the reduced background pressure.

The LLNL tandem mirror experiment (TMX) upgrade vacuum system

The tandem mirror experiment (TMX) upgrade is a large, tandem, magnetic‐mirror fusion experiment with stringent requirements on base pressure (10−8 Torr), low H reflux from the first walls, and peak gas pressure (5×10−7 Torr) due to neutral beam gas during plasma operation. The 225 m3 vacuum vessel is initially evacuated by turbopumps. Cryopumps provide a continuous sink for gases other than helium, deuterium, and hydrogen. The neutral beam system introduces up to 480 l/s of H or D. The hydrogen isotopes are pumped at very high speed by titanium sublimed onto two cylindrical radially separated stainless steel quilted liners with a total surface area of 540 m2. These surfaces (when cooled to about 80 K) provide a pumping speed of 6×107 l/s for hydrogen. The titanium getter system is programmable and is used for heating as well as gettering. The inner plasma liner can be operated at elevated temperatures to enhance migration of gases away from the surfaces close to the plasma. Glow discharge cleaning is par...

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.

Summary Abstract: Control of gas input and background pressure in the end plug regions of the TMX‐U thermal barrier experiment

Rate equations for the plasma species in a thermal barrier end plug establish an upper bound on the neutral pressure (P) external to the plasma. For the Tandem Mirror Experiment-Upgrade (TMX-U), this bound is P less than or equal to 0.5 - 1.0 x 10/sup -6/ Torr. Initially TMX-U did not satisfy this criterion, and axial end plugging of plasma losses seemed limited by the excessive pressure. Subsequently, we modified the machine to improve the vacuum conditions, decreasing P to the desired range. At the same time axial end plugging of plasma losses increased to the duration of neutral beam injection and ECRH heating. Here we summarize our experimental measurements of gas input.