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.

Design and experimental results of a new electron gun using a magnetic multipole plasma generator Rev. Sci. Instrum. 62, 761 (1991);

In thermal-barrier experiments in the tandem mirror experiment upgrade, axial confinement times of 50 to 100 ms have been achieved. During enhanced confinement we measured the thermal-barrier potential profile using a neutral-particle-beam probe. The experimental data agree qualitatively and quantitatively with the theory of thermal-barrier formation in a tandem mirror.

A neutral-beam-sustained plasma in the 2XIIB mirror machine is stabilized by hydrogen gas ionized at one mirror. _Peak plasma energy densities that exceed the energy den-sity_of the vacuum field (P=Sft D E r> /B VSiC 2 ^1) are achieved with deuterons of average energy £ D « 9 keV, peak density 6 D^1 .5xl0 14 cm" 3 , in a magnetic field B vac =0.56 T. These high /3's are sustained for the duration of gas and neutral-beam injection, with no evidence of a £ limit.We report here recent substantial increases in the plasma j3 in the 2XIIB mirror machine, due to stabilization of the neutral-beam-sustained plasma by gas injected in a mirror throat. /3 is defined by p = &nn D E D /B vac 2 , where n D is the central deuterium energy, andB vac is the applied central magnetic field. Previous experiments 1 in the 2XIIB device employed deuterium-loaded titanium-washer guns as sources of cold streaming plasma to suppress ion cyclotron fluctuations caused by the loss-cone nature of the confined ion distribution. 2 A beam-injected plasma j3 =0.4 was achieved with a single gun, limited by periodic bursts of ion cyclotron fluctuations rather than by the available neutral-beam current. 3 Increasing the amount of stabilizing plasma stream with three guns raised j3 to 0.6; again $ was limited by rf bursts. The amount of cold plasma has recently been increased still further by injecting hydrogen gas in a mirror throat. This has resulted in central hot-plasma /3 's exceeding unity, 0>1. This result has significance for mirror fusion reactors, since higher /3's will allow higher fusion power densities for a given field, and may possibly lead to a field-reversed configuration that would enhance the fusion energy balance. 4 The gas-feed system is shown in Fig. 1. A ceramic box is located just beyond one mirror throat of the minimum-B field, with its elliptical apertures conforming to the flux tube passing through a 12-cm-diam circle at the central midplane. Hydrogen or deuterium gas is injected into the box above and below the plasma fan by four pulsed gas valves. The gas is injected with a rise time of ^1 ms, and injection continues for the duration of beam injection (^5 ms) at a rate controlled by orifices at the valve openings inside the box. To start ionization of injected gas, a hot plasma of ft D~2 xl0 13 cm" 3 is initially created in a steady mirror field by beam injection into a streaming plasma target provided by guns located beyond the opposite mirror. 3 Gas neutrals are ionized in the box by electrons (T e »60 eV) coducted along the field from the beam-injected central plasma. The cold ions formed are presumably driven out of the box at the sound speed [v s = (T e /m i ) 1/2 ] by ambipolar electric fields. With sufficient density of cold plasma at the mirror, a small fraction can penetrate the hot central plasma to provide the stabilizing stream after the plasma guns are turned off. 5 During neutral-beam and gas injection, measurements with a microwave interferometer and a Langmuir probe indicate that a dense, cold plasma (n...

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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.

Experimental data are presented on the production of field-reversed deuterium plasma by a modified coaxial plasma gun. The coaxial gun is constructed with solenoid coils along the inner and outer electrodes that, together with an external guide field solenoid, form a magnetic cusp at the gun muzzle. The net flux inside the inner electrode is arranged to be opposite the external guide field and is the source offield-reversed flux trapped by the plasma. The electrode length is 145 cm, the diameter of the inner (outer) electrode is 15 cm (32 cm). The gun discharge is driven with a 232-/lF 40-k V capacitor bank. Acceleration of plasma through the magnetic cusp at the gun muzzle results in entrainment offield-reversed flux that is detected by magnetic probes 75 cm from the gun muzzle. Field-reversed plasma has been produced for a variety of experimental conditions. In one typical case, the guide magnetic field was Bo = 4.8 kG and the change in axial magnetic field .::iB, normalized to Bo was .::iB,/Bo = -3.1. Total field-reversed flux (poloidal flux) obtained by integrating .::iB, profiles is in the range 2 X 10 3 kG cm 2 • Measurement of the orthogonal field component indicates a sizable toroidal field peaked off axis at r~ 10 cm with a magnitude of roughly one-half the poloidal field component that is measured on magnetic axis. Reconnection of the poloidal field lines has not been established for the data reported in the paper and will be addressed in future experiments which attempt to trap and confine the field-reversed plasma in a magnetic mirror.

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