Faust 1 — A hardcore discharge stability experiment

A hardcore discharge stability experiment is described in which the final configuration is obtained by the radial inward collapse of the plasma sheath from the wall of the tube. By varying the initial conditions an axial field of arbitrary sign can be trapped within the annular plasma column. The construction and mode of operation of the apparatus is fully described.The gas currents were the order of 50 kA and rod currents were 120 kA, so that the current in the gas was less than half that in the rod at all times. The general behaviour of the discharge was examined and the effects of pressure, preionization and trapped axial field studied. A detailed examination was made of a representative set of conditions with electric, magnetic and double Langmuir probes. These measurements, together with quantities derived from them, are discussed and the derived values of electron temperature and kinetic pressure are compared with the directly measured values. The validity of the pressure balance analysis is considered in the light of these results.The discharge, without trapped axial field, showed a high degree of reproducibility and the observed fluctuations were of low amplitude (<10%) and low frequency (∼500kc/s) in both electric and magnetic field and in particle density. The last showed a well-defined annular sheath until late in the halfcycle; the radial profiles of azimuthal and axial current densities, kinetic pressure, temperature, and particle density showed good agreement. With trapped axial field, however, the discharge showed a reduction in stability, especially with trapped reverse field which introduces a null in the total axial field. Even with these, the most unstable conditions, the fluctuations were still relatively small (<20%) and of low frequency (∼1 Mc/s). The stability of the discharge is quantitatively compared with stability theory and the results discussed.