Plasma Containment in a Quadrupole Configuration

The containment times of afterglow plasmas in hydrogen, helium, and neon were measured in a linear quadrupole machine (LM-1), where ne ≲ 2 × 1012, Te ≲ 10 eV, Ti < 1 eV, mean free path λ ≲ 100 cm, connection length ≈ 20 cm, and electron-ion collision time ≈ 10−7 sec. Particle collectors showed the losses to be mainly radial. The plasmas in the ∮ dl/B stable region decayed in times of order a millisecond (about 10 times the Bohm value) which increased with IQ) the conductor current, to a roughly constant value, and decreased as Te increased. An axial magnetic field which introduced shear (θ ≥ 1/30 rad/cm for the main plasma) caused negligible change to the containment time in helium, with a slight improvement in neon. Various instabilities were observed. Pronounced variations in probe signals and plasma loss rates at different axial positions suggested a convective structure which persisted in the afterglow for some milliseconds. The origin of the radial loss inside the stable region may be due to a low-frequency flutelike mode or possibly to the convective structure in the plasma.