Investigations on the Drift of a Plasma Ring in a Toroidal Magnetic Field

A plasma ring confined by a toroidal magnetic field drifts outward in a radial direction. This drift is caused by an acceleration proportional to an average plasma temperature. Measuring this temperature spectroscopically using the Hβ line intensity and the intensity of a continuum interval allows us to calculate the acceleration. Integrating this acceleration twice with respect to time yields the motion of the meridional center of mass of the plasma. The drift motion obtained in this way is compared with the motion directly observed by smear photographs. Within the margins of error of the temperature determination both results give the same drift motion. As a consequence of successive compression and decompression of the plasma ring, magnetohydrodynamic theory shows an oscillating motion of the plasma center in the meridional plane in addition to the gross drift motion. This oscillation can be observed and agrees well in phase but not in amplitude with the theoretical results. The investigations described here were conducted on a toroidal θ-pinch plasma in a tube with a major diameter of 50 cm and a minor diameter of 6 cm. The preionized plasma is compressed and heated by a 4-kJ capacitor bank. The average temperature rises to 7 eV. At a time 2.5 μsec after the onset of the compression, the plasma touches the outer wall of the glass vessel.