Tiffany Desjardins

The HelCat basic plasma science device

The Hel icon- Cat hode(HelCat) device is a medium-size linear experiment suitable for a wide range of basic plasma science experiments in areas such as electrostatic turbulence and transport, magnetic relaxation, and high power microwave (HPM)-plasma interactions. The HelCat device is based on dual plasma sources located at opposite ends of the 4 m long vacuum chamber – an RF helicon source at one end and a thermionic cathode at the other. Thirteen coils provide an axial magnetic field B ⩾ 0.220 T that can be configured individually to give various magnetic configurations (e.g. solenoid, mirror, cusp). Additional plasma sources, such as a compact coaxial plasma gun, are also utilized in some experiments, and can be located either along the chamber for perpendicular (to the background magnetic field) plasma injection, or at one of the ends for parallel injection. Using the multiple plasma sources, a wide range of plasma parameters can be obtained. Here, the HelCat device is described in detail and some examples of results from previous and ongoing experiments are given. Additionally, examples of planned experiments and device modifications are also discussed.

Dynamics of flows, fluctuations, and global instability under electrode biasing in a linear plasma device

Grid biasing is utilized in a large-scale helicon plasma to modify an existing instability. It is shown both experimentally and with a linear stability analysis to be a hybrid drift-Kelvin–Helmholtz mode. At low magnetic field strengths, coherent fluctuations are present, while at high magnetic field strengths, the plasma is broad-band turbulent. Grid biasing is used to drive the once-coherent fluctuations to a broad-band turbulent state, as well as to suppress them. There is a corresponding change in the flow shear. When a high positive bias (10Te) is applied to the grid electrode, a large-scale ( n/n≈50%) is excited. This mode has been identified as the potential relaxation instability.

The potential relaxation instability in a helicon plasma

High positive grid biasing ( >10Te) in a large-scale helicon plasma at the University of New Mexico has led to the excitation of large fluctuations (>50%) in ion saturation current and floating potential at a low frequency (100–300u2009Hz). These fluctuations have been identified as an instability and have been studied under a variety of conditions, including variations in the bias voltage, magnetic field strength, and the length of the plasma column. A series of measurements with Langmuir probes has been used to analyze the characteristics of the fluctuations. The instability has been found to travel predominately along the axis with a speed of 1–3cs, where cs is the ion sound speed. The frequency of the mode is found to be dependent on the bias voltage. Increasing the grid bias leads to an increase in the frequency until a second critical voltage causes the fluctuations to disappear. The magnetic field and plasma length are found to affect the amplitude of the mode and the voltage range over which it exists. This mode has been identified as the potential relaxation instability.High positive grid biasing ( >10Te) in a large-scale helicon plasma at the University of New Mexico has led to the excitation of large fluctuations (>50%) in ion saturation current and floating potential at a low frequency (100–300u2009Hz). These fluctuations have been identified as an instability and have been studied under a variety of conditions, including variations in the bias voltage, magnetic field strength, and the length of the plasma column. A series of measurements with Langmuir probes has been used to analyze the characteristics of the fluctuations. The instability has been found to travel predominately along the axis with a speed of 1–3cs, where cs is the ion sound speed. The frequency of the mode is found to be dependent on the bias voltage. Increasing the grid bias leads to an increase in the frequency until a second critical voltage causes the fluctuations to disappear. The magnetic field and plasma length are found to affect the amplitude of the mode and the voltage range over which it exists...

ArI laser induced fluorescence system for measurement of neutral dynamics in a large scale helicon plasma

Neutral particles can play a significant role in the dynamics of plasma instabilities and flows through momentum transfer via ion-neutral collisions. When neutral and ion densities are spatially nonuniform, neutral-ion collisions can also exert a zero-order torque on a magnetized plasma column via the FxB force, where F is the force exerted on ions by neutrals (a neutral wind force). In order to investigate the role of neutral dynamics in helicon discharges in the HelCat (Helicon-Cathode) basic plasma science device at U. New Mexico, an ArI Laser Induced Fluorescence (LIF) system is being developed. Previous passive spectroscopic measurements of ArI and ArII lines indicate that the neutral density profile is hollow (higher nn at larger radius). Additionally, we have not been able to reconcile azimuthal flows measured by Mach probes with those expected from ExB and diamagnetic torques. It is hypothesized that neutrals play an important role in the plasma flow. The LIF system is based on a > 250 mW, tunable diode-pumped solid state laser. The laser will pump the metastable (2P03/2)4s2 level to the (2P01/2)4p2 level using 696.543 nm light, and observe fluorescence radiation from decay to the (2P01/2)4s2 level at 772.42 nm. The system design and initial results will be presented.