Relaxation of Ion Pressure Anisotropy and Energy Transfer Between Ions and Electrons in High-Density Plasmas

In pulsed high-density plasmas, containment times, magnetic field diffusion, thermal conduction and radiation energy losses are highly influenced by the evolution of the ion-pressure anisotropy and by the energy transfer between ions and electrons. To study these relaxation phenomena, experiments with a very fast theta-pinch (τ/4 = 0.7 μs, Bmax =75 kG, no bias field) have been carried out where the applied voltage and initial pressure could be varied over a large range. In a typical case (line density 7 x 1016 cm-1) the time development of the plasma is as follows: at the end of theinitial fast plasma implosion, a density of 1 x 1017 cm-3 is reached, the electron temperature Te and axial ion temperature Tiθ (measured by the Doppler broadening of a CV line) are low (100 eV), whereas the radial ion temperature Ti⊥ 900 eV. During the adiabatic compression phase, the anisotropy decreases, and both ion temperatures approach the value obtained from the neutron yield (Ti = 470 eV); Te reaches about 340 eV. In agreement with the estimates given by the formulae of Spitzer and Kogan, an isotropic pressure and nearly equal Te and Ti are reached during the adiabatic compression phase when the line density is larger than 2 x 1017 cm-1. At lower line densities, when the applied voltage is high, the ion-pressure distribution does not become isotropic during the observation time, and Te remains considerably below Ti. In this case, Te depends only weakly on the line density and applied voltage. A limitation of Te has been reported by other authors and was believed to result from either thermal conduction or impurity radiation losses. Our results cannot be explained by these, effects alone. (author)