C. Watts

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.

The Dose Effect in Secondary Electron Emission

In this paper, total incident electron dose as an inherent parameter in secondary electron emission is experimentally demonstrated. A completely automated experimental setup allows for measuring of secondary electron yield (SEY) as a function of beam energy, angle of incidence of primary electrons, electron dose, and time. SEY data are presented for copper, plasma-sprayed boron carbide, and titanium nitride samples with principal focus on dose dependence. Experiments were conducted in the low-energy range (5-1000 eV) and direct-current regime. Experimental results have been compared with formulas in literature, and good agreement was observed. Modified empirical formulas incorporating the dose effect have also been proposed.

Effects of Laser Surface Modification on Secondary Electron Emission of Copper

The effects of surface roughness on secondary electron emission (SEE) are investigated for copper. The surface of the copper samples was modified using a high-power neodymium-doped yttrium aluminum garnet laser, where the degree of surface modification depended on the duration and intensity of the laser exposure. Four different levels of modification were tested, in addition to the unmodified sample. Minor modification resulted in the biggest effect, significantly reducing the SEE. However, this effect only holds true for very low electron dose, where dose is the incident charge per unit area. For higher dose levels, those that are commensurate with “technical materials” in applied situations, there was a negligible effect of surface roughness on SEE. The data set presented here seeks to quantify the effect of surface roughness with further gradations than what is accounted for in the semiempirical formulas in the literature.

Triple probe signal detection electronics for systems lacking a well defined ground.

Triple probes have been used to measure plasma parameters of low temperature and edge plasmas, yielding simultaneous measurements of electron temperature, ion density, and floating potential. Unlike standard Langmuir and double probe techniques, there is no requirement to sweep the probe potential relative to the plasma, thus allowing fast time resolution. However, in some plasma systems ground is not well defined with respect to a known ground, may vary strongly in time, or may be at an inconveniently high voltage. The resulting high plasma (or floating) potential requires common mode rejection before the signals can be digitized. A signal detection circuit constructed from inexpensive operational amplifiers and that makes use of a novel floating bias generation configuration is described.

Laboratory-generated Coronal Mass Ejections

We have begun a series of laboratory experiments focused on understanding how coronal mass ejections (CME) interact and evolve in the solar wind. The experiments make use of the Helicon-Cathode (HelCat) plasma facility, and the Plasma Bubble eXperiment (PBeX). PBeX can generate CME-like structures (sphereomak geometry) that propagate into the high-density, magnetized background plasma of the HelCat device. The goal of the current research is to compare CME evolution under conditions where there is sheared flow in the background plasma, versus without flow; observations suggest that CME evolution is strongly influenced by such sheared flow regions. Results of these studies will be used to validate numerical simulations of CME evolution, in particular the 3D BATS-R-US MHD code of the University of Michigan. Initial studies have characterized the plasma bubble as it evolves into the background field with and without plasma (no shear).

Design of a compact coaxial magnetized plasma gun for magnetic bubble expansion experiments

The design of a compact coaxial magnetized plasma gun and its associated hardware systems are discussed in detail. The plasma gun is used for experimental studies of magnetic bubble expansion into a lower pressure background plasma as a laboratory model for extragalactic radio lobe expansion into the interstellar medium. The gun is powered by a 120µF, 10kV ignitron-switched capacitor bank. High-pressure gas is puffed into an annular gap between inner and outer coaxial electrodes. The applied high voltage ionizes the gas and creates a radial current sheet. The ∼100kA discharge current generates toroidal flux and poloidal flux is provided by an external bias magnetic field. Axial J×B force then ejects plasma out of the gun. If the J×B force exceeds the magnetic tension of the poloidal flux by a sufficient amount then a detached magnetized plasma bubble will be formed. Using a multi-tip Langmuir probe array, a high speed camera and a B-dot probe array, the evolution of this plasma “bubble” as it interacts with pre-existing low pressure background plasma, is studied. In particular, details of the plasma bubble formation system including the main gun cap-bank power system, gas valve control system, bias flux cap-bank power system, and experimental data are provided.

Plasma diagnostics to study cathodes used to drive long-pulse magnetrons

Conventional long-pulse magnetrons use a heated cathode for thermionic emission of electrons, but short cathode lifespan and the necessity of providing a power supply to provide this heat are undesirable aspects of these devices. There is ongoing research to develop field emission cathodes with performance comparable to that of thermionic cathodes. One concern with these field emission cathodes is the possible evolution of surface plasmas over the fairly large surface areas of 10–100cm2. This presentation describes a plasma diagnostics capability we have assembled to apply to this research. A visible and UV spectrum fast CCD camera and a McPhereson 2M spectrometer capable of 2×10−3nm resolution is used to determine the temporal and spatial evolution of both anode and cathode plasma regions of a square patch of material. Specifically, the electron energy distribution, sub-eV ion temperature (through Doppler broadening), plasma composition, and impurity abundance are determined. Plasma evolution similar to that achieved by the thermionic cathode must be demonstrated in order to avoid premature plasma closure and shorting of the magnetron resonant cavity used to produce microwaves. MAGIC PIC code simulations are also used to extrapolate the capability of novel field emission cathodes to provide near space-charge-limited current flow and optimize future cathode design using the spectroscopic data obtained.

Nonlinear dynamics of fluctuations in the presence of sheared parallel and perpendicular flows in a magnetized laboratory plasma

Laboratory experiments are described which utilize a set of concentric bias rings to affect the velocity (flow) shear in the linear HELCAT (HELicon-CAThode) device at the University of New Mexico. HELCAT is 4 m long, 0.5 in diameter, with B0 les 2.2 kG, and utilizes two plasma sources: an RE helicon at one end of the device, and a thermionic cathode at the other. With increasing ring bias, relative to the vacuum chamber wall, it is found that both axial and azimuthal flow shear change by only a small amount in magnitude, but move inward to the plasma core from the wall. As bias is increased, drift waves decrease in magnitude and are eventually fully suppressed, then the Kelvin-Helmholtz (K-H) mode is destabilized. It appears that the azimuthal flow shear is mainly responsible for suppression of drift modes, while the azimuthal shear is the primary driver of the K-H instability. While bias applied to rings at any radii suppresses drift fluctuations with nearly equal effectiveness, the K-H mode is more easily excited by biasing at the plasma edge. Fluctuations show increasingly chaotic and intermittent behavior as bias increases, up to V ~ 10 kTe/e, when the chaos disappears, as indicated by a rapid drop in correlation dimension, and very bursty behavior. Additionally, detached edge blobs are observed in cathode plasmas, but appear to be absent from helicon discharges, even when other operating parameters (magnetic field, background pressure) are identical. Experimental results and comparisons with theory are described.

Design of a Compact Coaxial Magnetized Plasma Gun for Laboratory Studies of Plasma Relaxation

Summary form only given. We discuss the design of a compact coaxial magnetized plasma gun and its associated systems in detail. The plasma gun will be used for experimental studies of plasma relaxation to be conducted in the HELCAT facility at UNM. These studies will advance our knowledge of basic plasma physics in the areas of magnetic relaxation and space and astrophysical plasmas, including the evolution of active galactic jets. The gun will be powered by a 120 muF ignitron switched capacitor bank which is operated in a range of 5 ~ 10kV. High pressure gas will be puffed into an annular gap between the inner and outer coaxial electrodes. The applied high voltage ionizes the gas and creates a radial current sheet. The ~ 100kA discharge current generates toroidal flux and an external applied magnetic field provides poloidal flux. The axial JxB force ejects plasma out of the gun. If the JxB force exceeds the magnetic tension of the poloidal flux by a sufficient amount then a detached magnetized plasma will be formed. We plan to study the evolution of this plasma bubble as it interacts with a preexisting lower pressure background plasma.