Results from the tandem mirror experiment are described. The configuration of axial density and potential profiles are created and sustained by neutral-beam injection and gas fueling. Plasma confinement in the center cell is shown to be improved by the end plugs by as much as a factor of 9. The electron temperature is higher than that achieved in our earlier 2XIIB single-cell mirror experiment.PACS numbers: 52.55. Mg, 52.55.Ke This Letter reports the first results obtained from the tandem mirror experiment (TMX) at the Lawrence Livermore Laboratory. Steady-state tandem-mirror plasmas have been produced and an electrostatic barrier that improves plasma confinement has been observed. The tandem-mirror configuration 1 ' 2 can enhance the performance of a magnetic-mirror thermonuclear reactor. Such a reactor would produce power in a cylindrical, high-/3, magnetic solenoid. End losses from this center cell are reduced by electrostatic endplug barriers of positive potential, which turn back those low-energy ions which escape through the magnetic mirror. These potential barriers are established on both ends of the center cell by high-density, high-temperature, mirror-confined plasmas, which have a larger ambipolar potential than does the center-cell plasma.Earlier tandem-mirror experiments, 3 in which plasma guns were used to establish end-plug densities larger than those in the center cell, have produced potential wells. Langmuir-probe measurements indicated that the magnitude and scaling of the potential-well depth is consistent with theoretical predictions. Our results demonstrate that we can produce and sustain a tandem-mirror plasma configuration by use of neutral beams to fuel the end plugs and gas to fuel the center cell. This method can be extrapolated to continuously operated systems. Our experiments further demonCee coil Baseball coilSolenoid coils Octupole coil -Plasma flux tube 1132 Neutral beam injectors Startup plasma guns FIG. 1. Schematic diagram of TMX magnet and neutral-beam system.

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.

A study of the impurity emissions (300-1600 A) in the Tandem Mirror Experiment (TMX) is described. Brightness measurements obtained with three absolutely calibrated monochromators, each viewing a separate section of TMX, showed that the dominant impurities were oxygen, nitrogen, carbon, and titanium. In the end cells, oxygen was the most abundant impurity with a concentration of ^2%; the neutral beams were a significant source of oxygen as in the 2XIIB experiment. In the central cell, the impurity concentrations were low: <0.4% for each species. Emissions from carbon and oxygen in this region were reduced by titanium gettering of the vacuum chamber walls. Total radiative power losses due to impurities were <10% of the net trapped neutralbeam power.

Impurities in the Tandem Mirror Experiment (TMX) have been studied using extreme ultraviolet spectroscopy. Three time-resolving absolutely-calibrated normal-incidence monochromators, one on each section of TMX, were used to study the impurity emissions in the wavelength range of 300 A -1600 A. The instruments on the east end cell and central cell were each capable of obtaining spatially-resolved profiles from 22 chords of the plasma simultaneously while the instrument on the west end cell monitored the central chord. The impurities identified in TMX were carbon, nitrogen, oxygen, and titanium. Emphasis was placed upon determining the impurity densities and radiated power losses of the central cell; results indicate that the impurity concentrations were low-less than 0.4% for each species-and that less than 10% of the total net trapped neutral beam power was lost to radiation. The use of titanium gettering on the central cell walls was observed to decrease the brightnesses of singly-and doubly-ionized carbon and oxygen in the central cell plasma. In the end cells, oxygen was the main impurity with a concentration of about 1.5% and was injected by the neutral beams; the other impurities had concentrations of about 0.5%. Radiated losses from the end cells were negligible.Evidence has been obtained which indicates that impurities in the central cell are subject to a radially outward transport. The density ratios of 0 V to 0 VI indicate a short (0.3-3 ms) central cell confinement time for 0 VI ions, even though calculations predict that axial confinement times should be longer than the TMX shot duration. Impurity puffing experiments performed in the central cell of TMX support this conclusion. Oxygen and neon puffed into the central cell during the TMX shot were observed to accumulate in the outer boundary of the plasma, while only a small fraction penetrated into the central regions. None of the injected impurity was observed in the end cell plasmas; instrumental sensitivity puts an upper limit of 0.03 on the ratio of injected impurity density in the end cells to that in the central cell. Oxygen puffed before the TMX shot was observed to decay from all regions of the plasma during the shot. These results provide an explanation for the observed low concentrations of impurities in the central cell of TMX.

Impurity sources and confinement times in the central cell of TMX were examined by studying the extreme ultraviolet (EUV) emissions from intrinsic and injected impurities. Impurity ions from gases puffed radially toward the central-cell plasma showed very little radial penetration into the plasma. The confinement times of highly ionized intrinsic and injected impurities in the central cell were of the order of or less than the deuterium ion confinement times; the ambient density ratios of O V to O VI indicate overall O VI confinement times near the axis of 0.3 to 3 ms. Impurities introduced into the central-cell plasma were not detected in either end-cell plasma, indicating that axial losses of impurities from the central cell were small. This result agrees with theoretical calculations of the axial confinement times and suggests that most of the losses were radial. The radial flux of impurity ions determined from the measured radial profiles of the ambient impurity ion densities is primarily outward; the magnitude of these fluxes also implies a short confinement time for impurity ions.