Influence of Small Transverse Fields and Impurities on θ-Pinch Stability and Confinement

The stability and confinement of θ-pinch plasmas in the relatively slow Pharos discharge, where the duration of the current pulse is ∼20–30 μsec, was investigated. The heating rates (∼100 eV/μsec early in the discharge) are modest because of the relatively low voltage on the coil (∼15 kV). Care was taken to reduce field nonuniformities in the coil-collector assembly so that the compressed plasma would not drift toward the wall. It is shown that small transverse fields (<1% of the maximum Bz) cause rotational instability: in particular, a pulsed Ioffe-hexapole field induces rotational instability if the field is present in the plasma before the start of the main compression. The time constant for the growth of the instability depends on the electron temperature and can be altered with 0.1% added impurity. The plasma is lost out the ends in 10–15 μsec without an initial bias field. The confinement time can be extended to ∼30 μsec with a reverse bias field. In the latter case the plasma contracts axially to a length of 60–80 cm with a 1.8-m coil; this central plasma column is connected to the ends by a narrow thread of plasma with a radius of a few millimeters. Axial studies show that instabilities, when they occur, grow faster at points away from the midplane. The origin of the rotation is discussed in terms of a model in which axial Hall currents, due to transverse fields, lead to a torque.