Abstract: 

This talk will describe two different sets of basic plasma physics experiments being conducted in the medium-scale HelCat (Helicon-Cathode) linear plasma device at U. New Mexico.  HelCat is a 4 m long, 50 cm device with both RF helicon and thermionic cathode sources, and magnetic field up to 2.2 kG.  The first set of experiments, motivated by a need for improved understanding of tokamak edge transport, investigates the effects of electrode biasing and velocity shear on edge turbulence and turbulent transport.  Biasing is done with both concentric ring and grid electrodes, and is found to have a strong effect on the fluctuation dynamics, including both coherent and turbulent fluctuations.  Depending on bias level and operating parameters, fluctuations can be partially or fully suppressed, or driven to a chaotic state.  Shear-driven modes appear to be observed under some conditions.  At very high electrode bias levels, e.g. V_bias > 10´T_e, a large scale, nonlinear, global instability - identified as the potential relaxation instability (PRI) - is observed.  Despite a large number of detailed measurements, the azimuthal ion flow, v_q, is not yet understood.  Observed v_q is inconsistent in magnitude and direction with E´B plus diamagnetic drifts.  It was first suspected that significant Reynolds stress-driven zonal flows were at work, but these have been ruled out as a dominant factor by operating under biased conditions where fluctuations are fully suppressed.  It is now suspected that neutrals play a significant role in the rotation, but the details are not yet resolved.

      The second set of experiments seeks to understand the details of magnetic relaxation of relatively dense magnetized plasmas in the presence of background plasmas and magnetic fields.  This may have bearing, for example, on the propagation of coronal mass ejections (CME’s) through the solar wind, or on astrophysical jet/radio lobe relaxation into the intergalactic background plasma.  These experiments utilize a compact magnetized coaxial plasma gun, with gun current I ~ 100 kA, to produce a dense magnetized plasma which is injected into a background helicon discharge and/or background magnetic field.  The gun produces either a jet, or plasma bubble with closed B-field (nominally a spheromak), depending on gun operating parameters.  The bubble/jet speed is found to be both supersonic and super-Alfvénic.  Bubble dynamics appear to be affected significantly by background B-field, but only weakly (if at all) by background plasma.  Plasma behind the leading edge of the bubble appears to be turbulent.

      Details of both sets of experiments, as well as initial experiment-model comparisons, will be discussed.