Investigations of the magnetic structure and the decay of a plasma-gun-generated compact torus

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The results of a series of experimental measurements of compact toroidal (CT) plasmas produced by a magnetized coaxial plasma gun injecting into a flux-conserving metallic liner are reported. The experiments were performed on the Beta II facility at Lawrence Livermore National Laboratory. The magnetic equilibria are well described by a force-free eigenmode structure that results from an extension of Taylor’s theory of the reversed-field pinch. Consideration of helicity conservation during relaxation of the composite plasma-gun flux-conserver system to the final state equilibrium yields theoretical expressions that are compared with the experiment. In particular the CT poloidal flux (ψpol) and the overall electrical efficiency for producing the CT are predicted to be functions of the plasma gun inner-electrode flux (ψgun) and the volt-seconds input to the gun discharge (∫∞0 V dt). Away from a cutoff at too low values of ∫∞0 V dt or too high values, ψgun ,ψpol scales linearly with the square root of the product of ψgun and ∫∞0 V dt, whereas the electrical efficiency equals about 13% for ∫∞0 V dt/ψgun ≊10. For an electrical energy input Win =45 kJ, CT’s are produced with poloidal plus toroidal field energy up to WB =8 kJ and toroidal plasma current Itor =330 kA. The chord-averaged plasma density is 2–4×1014 cm−3, and the plasma volume equals 150 liters. The radius of the flux conserver is 37.5 cm, and the axial length is 40 cm. If a bias flux ψb is superimposed on the flux conserver, n=1 tilting is observed when ψb/ψpol exceeds a ratio of about 0.20 to 0.25. Impurity radiation measured by a pyroelectric detector accounts for all of the plasma magnetic energy if uniform volume emission of radiation is assumed. The dominant impurities observed are carbon and oxygen. Helium-like lines are not observed, indicating that the plasma has not ‘‘burned through’’ the low electron temperature radiation maxima. The experimentally observed decay times (defined by the e-folding time of plasma magnetic fields) are 80 to 160 μsec—consistent with Zeff =2 and Te in the range 5–10 eV if classical resistivity is assumed. A zero-dimensional rate equation model of impurity radiation loss gives a reasonably good account of the experimental observations and predicts that the carbon concentration must be reduced to the level of a few percent to allow burnthrough of the low-Te carbon radiation barrier. Glow discharge cleaning of the gun electrodes and flux conserver resulted in a 20% increase of the e-folding time of plasma magnetic fields (from an average value 115 to 140 μsec). The CT plasma density was observed to scale linearly with the electrical energy input to the gun discharge and to be only weakly dependent on the filling pressure and timing of pulsed deuterium gas valves. It seems likely that further improvements in increasing plasma lifetime can be made by improving the vacuum conditions and discharge cleaning methods and experimenting with the gun electrode materials.