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.

Classical transport of particles and heat in field-reversed mirrors is discussed. The X-points (field nulls on axis) are shown to have no deleterious effect on transport; this conclusion is true for any transport model. For an elongated Hill's vortex equilibrium the classical diffusion coefficient is calculated analytically and used to construct an analytic solution to the transport equation for particles or energy ; this yields exact results for particle and energy confinement times. These life-times are roughly 3 to 6 times shorter than previous heuristic estimates. Experimentally determined life-times are within a factor of 3 to 4 of our estimates. To assess the impact of these results on reactor designs, the authors construct an analytic reactor model in which neutral-beam input balances ion heat loss. Energy loss due to synchrotron radiation is calculated analytically and shown to be negligible, even with no wall reflection. Formulas are presented which give the reactor parameters in terms of plasma temperature, energy multiplication factor Q, and allowed neutron wall loading. The effect of anomalous resistivity is incorporated heuristically by assuming an anomalous resistivity which is enhanced by a factor A over classical resistivity. For large A the minimum power of a reactor scales as A11/6. A = 50 gives a reactor design which still seems reasonable, but A = 200 leads to extremely large, high-power reactors.

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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.

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We have completed a conceptual design study of the field-reversed mirror reactor. For this reactor a reference case conceptual design was developed in some detail. The parameters of the design result partly from somewhat arbitrary physics assumptions and partly from optimization procedures. Two of the assumptions-that only 10% of the alphaparticle energy is deposited in the plasma and that particle confinement scales with the ionion collison time-may prove to be overly conservative. A number of possible start-up scenarios for the field-reversed plasmas were considered, but the choice of a specific start-up method for the conceptual design was deferred, pending experimental demonstration of one or more of the schemes in a mirror machine. Basic to our plasma model is the assumption that, once created, the plas

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