Study of the Diffusion of High-β Plasma in a Theta Pinch

Abstract

A direct measurement is described of the perpendicular diffusion coefficient in a theta pinch operating in the low‐pressure regime with zero bias field. The diffusion was studied by comparing the measured time variation of the radial electron density distribution with computations assuming classical and other resistivities. A coil 8 m long is used in order to delay end effects and measurements confirm theoretical predictions that the plasma in the midplane is uninfluenced by the ends and free of energy losses for about 25 $\mu \sec$ . The plasma temperature was varied from 90 to 160 eV and the values of density and beta on the axis are ${\text{3−4\hspace{0.17em}×\hspace{0.17em}10}}^{16}/{\mathrm{cm}}^3$ and 0.7, respectively. In the early stages (before 2 $\mu \sec$ ) diffusion at approximately the Bohm rate is found, and this is believed to be due to a streaming mechanism. At later times the measured diffusion coefficient is in good agreement with the classical value in all conditions studied, and the experimental error is such that diffusion is unlikely to exceed $\frac{1}{10}$ of the Bohm rate.