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.

Making axisymmetric mirrors magnetohydrodynamically (MHD) stable opens up exciting opportunities for using mirror devices as neutron sources, fusion-fission hybrids, and pure-fusion reactors. This is also of interest from a general physics standpoint (as it seemingly contradicts well-established criteria of curvature-driven instabilities). The axial symmetry allows for much simpler and more reliable designs of mirror-based fusion facilities than the well-known quadrupole mirror configurations. In this tutorial, after a summary of classical results, several techniques for achieving MHD stabilization of the axisymmetric mirrors are considered, in particular: (1) employing the favorable field-line curvature in the end tanks; (2) using the line-tying effect; (3) controlling the radial potential distribution; (4) imposing a divertor configuration on the solenoidal magnetic field; and (5) affecting the plasma dynamics by the ponderomotive force. Some illuminative theoretical approaches for understanding axisymmetric mirror stability are described. The applicability of the various stabilization techniques to axisymmetric mirrors as neutron sources, hybrids, and pure-fusion reactors are discussed; and the constraints on the plasma parameters are formulated.

We report results of plasma confinement experiments with an auxiliary warm-plasma component flowing along magnetic field lines to suppress ion-cyclotron instabilities. The reduced plasma losses, with the lower fluctuation amplitude, permits neutral-beam buildup of a 13-keV deuterium plasma to densities as high as 4 x 10 13 cm" 3 corresponding to peak beta values of 0.4. Variation of the beam energy demonstrates that longer confinement times are achieved at higher ion energies.

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