Ralph Kelly

The HelCat dual-source plasma device

The HelCat (Helicon-Cathode) device has been constructed to support a broad range of basic plasma science experiments relevant to the areas of solar physics, laboratory astrophysics, plasma nonlinear dynamics, and turbulence. These research topics require a relatively large plasma source capable of operating over a broad region of parameter space with a plasma duration up to at least several milliseconds. To achieve these parameters a novel dual-source system was developed utilizing both helicon and thermionic cathode sources. Plasma parameters of n(e) approximately 0.5-50 x 10(18) m(-3) and T(e) approximately 3-12 eV allow access to a wide range of collisionalities important to the research. The HelCat device and initial characterization of plasma behavior during dual-source operation are described.

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.

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

In order to investigate the role of both neutral and ion dynamics in large-scale helicon discharges, a laser induced fluorescence (LIF) system capable of measuring both ArI and ArII fluorescence using a single tunable laser is being developed. The system is based on a >250 mW solid state laser. For ArI measurements, the laser pumps the metastable (2P03/2)4s level to the (2P01/2)4p level using 696.7352 nm light, and fluorescence radiation from decay to the (2P01/2)4s level at 772.6333 nm is observed. For ArII, currently in development, the metastable (3P)3d 4F7/2 level will be pumped to the (3P)4p 4D07/2 level using 688.8511 nm, and decay fluorescence to the (3P)4s 4P5/2 level at 434.9285 nm measured. Here all wavelengths are in a vacuum.

A support vector regression method for efficiently determining neutral profiles from laser induced fluorescence data

A support vector regression (SVR) method is integrated with a collisional radiative (CR) model of helicon plasmas in the Helicon-Cathode (HelCat) linear plasma device to determine Ar i profiles based on metastable-pumped Laser Induced Fluorescence (LIF) measurements. A machine learning approach to the CR model allows for an efficient exploration of the input parameter space and can inherently incorporate probe and LIF measurement errors in profile inputs to which a CR model would normally be sensitive. A training set is created for mapping CR model outputs to Ar i input profiles using radial points as SVR input features and parameters of a sigmoidal-type function as output features. This SVR method may easily be adapted to other LIF pumping schemes and may even be used in conjunction with a CR model to validate electron temperature and density plasma profiles if neutral or ion profiles are already known.

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.

Characterization and Overview of the Helcat (Helicon-Cathode) Dual Source Linear Plasma Device

Summary form only given. The HELCAT (helicon-cathode) device is a dual-source linear plasma device that has recently begun full operation at the University of New Mexico. HELCAT is 4 m long, 50 cm diameter, with axial magnetic field < 2.2 kG. An RF helicon source of tunable frequency 10 -30 MHz and P < 5 kW, resides at one end of the device, while a thermionic BaO-Ni cathode capable of discharge currents up to 2.5 kA is located at the other end. Nominal parameters are: Te ~ 5 -10 eV, ne ~ 1012 /cc (cathode), 1013 - 1014 /cc (helicon), plasma diameter 15-20 cm. An overview of the device characteristics and initial experimental results are presented.