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Multipactor onset, growth, associated space charge effects, and transition to ionization breakdown due to ambient or desorbed gases represent key stages of single and multifrequency RF -driven phenomena that inhibit performance in space-based and terrestrial vacuum electronics devices. Performance degradation through space charge detuning and interference with gain is expected for medium duration pulses, and ion generation and damage for longer pulses. In this research, combined theoretical, computational, and experimental approaches are applied.

Spherically imploding plasma liners1 are a proposed low-cost, reactor-relevant magneto-inertial-fusion (MIF) driver for compressing magnetized plasma targets to fusion conditions. The Plasma Liner Experiment–ALPHA (PLX-α aims to demonstrate the formation of subscale plasma liners via dozens of merging supersonic plasma jets (with initial ion density ~ 1016 cm-3, velocity ≈50 km/s, mass ~ 1 mg, and using various gas species). In the ongoing, first set of PLX-α experiments, we plan to merge six and seven plasma jets to form a conical section of a spherically imploding plasma liner in order to assess the shock heating (and associated Mach-number degradation) and uniformity of the liner upon jet merging and during further convergence, before proceeding to fully spherical liner-formation experiments (if warranted by the conical-liner results). In this talk, we will summarize experimental findings to date on characterizing plasma jets formed by the newly designed PLX-α guns and conical-plasma-liner formation with up to seven guns. Gated fast-framing-camera images from initial shakedown experiments suggest that shock formation between adjacent merging jets is consistent with oblique-shock formation as observed in earlier two- and three-jet merging experiments.2,3

The Plasma Liner Experiment-ALPHA (PLX-α) is investigating the merging of supersonic plasma jets into a spherically imploding plasma liner as a driver for use in magneto-inertial fusion (MIF) architectures. 1 The present work is focused on characterizing the merging of six and/or seven plasma jets, converging in a cone of solid angle $0.4 \pi $ over a distance of 1.3 meters. Results from high-speed imaging, photodiode arrays, self-emission visible survey spectroscopy, and self-emission visible high-resolution spectroscopy will be presented, yielding measurements of plasma velocity, number density, electron/ ion temperatures, and mean ionization state pre- and post-merge. Anticipated plasma parameter regimes are $\mathrm {n}\sim 10 ^{15}-10 ^{17}$ cm$^{-3}$, $\mathrm {T}\sim 1-10$ eV, and $\mathrm {v}\sim 50$ km/s, with gas species varied among argon, nitrogen, neon, krypton, and xenon. Images and spectra will be compared with synthetic data generated from 3D fronttracking and smooth-particle-hydrodynamic simulations coupled with atomic physics / opacity analysis codes. Results will inform questions of liner-Mach-number and lineruniformity evolution throughout the jet-merging and subsequent liner-convergence process.

Spherically imploding plasma liners 1 are a proposed reactorfriendly magneto-inertial-fusion (MIF) driver for compressing magnetized plasma targets to fusion conditions. The Plasma Liner Experiment–ALPHA (PLX-α) is aiming to demonstrate the formation of such liners via dozens of merging supersonic plasma jets (with ion density $\sim 10 ^{16}$ cm$^{-3}$, velocity $\approx 50$ km/s, mass $\sim 1$ mg, and various species). In the ongoing, first set of PLX-α experiments, we are merging 6 and 7 plasma jets to form a conical section of a spherically imploding plasma liner in order to assess the shock heating (and associated Mach-number degradation) and uniformity of the liner upon jet merging and during further convergence. Diagnostics include single- and multi-frame fast-gated ICCD cameras, visible survey spectrometer, high-resolution spectrometer, 12-chord visible interferometer, and visible photodiodes. Presented here will be the first results of the density and electron temperature uniformity of 6- and 7-jet merging using several gas species (argon, nitrogen, neon, krypton, xenon). Several 12-chord laser interferometry configurations are fielded with both end-on and crosssectional laser chord paths to determine density distribution and evolution. Results presented here will be compared to synthetic data from 3D simulations run using both smoothedparticle-hydrodynamics and the FronTier codes.

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 n n 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 (2P0 3/2 )4s2 level to the (2P0 1/2 )4p2 level using 696.543 nm light, and observe fluorescence radiation from decay to the (2P0 1/2 )4s2 level at 772.42 nm. The system design and initial results will be presented.

The use of metamaterials (MTM's) in high power microwave (HPM) devices has been proposed as a way to improve efficiency and overall performance. However, by nature, MTM's are composed of subwavelength structures, often with sharp features, where high electric field concentrations may occur. Thus these structures may be prone to electrical breakdown. Additionally, some materials - especially dielectrics - proposed for MTM composite structures may have difficulty withstanding the levels heat loading found in typical HPM devices. In order to investigate the survivability of potential MTM structures in an HPM environment, two test stands are being constructed to characterize MTM material response to electric field stress, thermal stress, and charging. This is part of an overall, multi-university effort to develop MTM-based HPM devices with improved performance. The first of these test stands will utilize an existing HPM accelerator (Sinus-6 accelerator at UNM) that operates at ≤ 600 kV, kA current levels, and a 15 ns pulse. MTM's will be placed in close proximity to the beam, and breakdown will be characterized via fast imaging, and survey and high resolution spectroscopy. Additionally, a low current electron gun with Vbeam ≤ 50 kV, that can operate from ns pulsed to steady state, will be used to investigate effects of thermal loading and charging. Ultimately, results of this characterization will be used to develop robust MTM resonant/slow wave structures for HPM applications.

Laser triggered gas switches are critical components in many pulsed power driven systems, such as ZR at Sandia National Laboratories. Timing jitter is of concern is such systems, where power flow from multiple modules must be switched to a load simultaneously. Laser triggered gas switches utilize a laser-produced plasma channel (LPPC) to initiate breakdown between electrodes biased to ∼ 80% of breakdown voltage. The effective length of the LPPC is an important parameter affecting the breakdown timing. Backscattering of microwaves inside a waveguide by an LPPC, introduced by focusing the trigger laser through holes in the broad wall, has been used to characterize effective length of the channel. Simulations indicate that the backscattering is sensitive to the LPPC conductor length both inside and outside the waveguide. A quarter wavelength stub has therefore been introduced outside of the waveguide, to short circuit the LPPC conductor to the waveguide wall, while still allowing laser access. Theoretical, computational, and initial plasma channel experimental results, as well as comparisons with other diagnostics, are presented.

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.

Summary form only given. Many toroidal fusion devices now routinely generate edge and/or core transport barriers, where heat and particle transport are reduced far below Bohm diffusion levels. However, minimal particle transport is not necessarily desirable, since it can lead to core impurity accumulation, or alpha particle buildup. Ideally, active, stable control over the transport, rather than simple minimization, could be obtained. To this effect, research is now underway to investigate active control of particle transport. Turbulence and transport dynamics are, of course, strongly nonlinear, and apparently not deterministic. However, modern nonlinear control methods now exist, such as chaotic control and fuzzy control, which do not rely on a model of the system dynamics to affect stable control. Experiments are being conducted in the HELCAT (HELicon-CAThode) linear device at UNM. HELCAT is a 4 m long device, with B < 0.22 T, and cathode-produced densities, n ~ 1-5x10nland12 cmnland-3. Sheared ExB flows, generated via biased concentric rings, are utilized to modify the transport. Fluctuations and flux are monitored with probe arrays. Parameters, such as RF power, gas pressure and magnetic field, are investigated for their effects on plasma behavior (turbulence, blobs, profile, shear layer, fluctuations, etc.). Open loop experiments have demonstrated that drift fluctuations can be fully suppressed by simple biasing. Additionally, a bias regime between drift instability and full suppression exits where fluctuations are chaotic. Experimental results and analysis will be presented.

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.

Summary form given only. Secondary electron emission (SEE) results from bombarding materials with electrons, atoms, or ions. When studying SEE, one is usually interested in determining the secondary electron yield, defined as the number of secondary electrons produced per incident primary electron. The amount of secondary emission is well known to depend on factors such as bulk and surface properties of materials, energy of incident particles, and their angle of incidence. However, results presented here indicate large variation of yield with incident electron dose/current density in addition to the above factors. There has been little effort in literature to quantify this effect. Experiments in this thesis aim to fill this gap.

Summary form only given. The design construction and operation of two quadrature homodyne millimeter wave interferometers, one operating at 96 GHz and the second operating at 40 GHz, are described. Together with electrostatic probes, these interferometers will provide absolute electron density measurements at two axial locations in the Helcat (helicon-cathode) plasma device. Helcat is a four meter long magnetized plasma device with a thermionic cathode source at one end and a RF helicon source at the other. Plasma densities with both sources range from 1012-1014/cc, often with significant axial density gradients. The two interferometer frequencies are optimized for operation at the high and low density ends of the device respectively. The circuit, optical and mechanical design as well as initial measurements will be presented.

Summary form only given. Many toroidal fusion devices now routinely generate edge and/or core transport barriers, where heat and particle transport are reduced far below Bohm diffusion levels. However, minimal particle transport is not necessarily desirable, since it can lead to core impurity accumulation, or alpha particle buildup. Ideally, active, stable control over the transport, rather than simple minimization, could be obtained. To this effect, research is now underway to investigate active control of particle transport. Turbulence and transport dynamics are, of course, strongly nonlinear, and apparently not deterministic. However, modern nonlinear control methods now exist, such as chaotic control and fuzzy control, which do not rely on a model of the system dynamics to affect stable control. Experiments are being conducted in the new HELCAT (HELicon-CAThode) linear device at UNM. HELCAT is a 4 m long device, with B

Summary form only given. Over the last few years, there has been a great deal of interest in secondary electron emission (SEE) phenomena. SEE results from the interaction of materials with electrons, atoms, or ions. The amount of secondary emission depends on factors such as the bulk and surface properties of materials, the energy of incident particles, and their angle of incidence. When studying SEE, one is interested in determining the total secondary electron emission yield, which is defined as the number of secondary electrons produced per incident primary electron. The goal of our research is to identify novel materials (with a very low SEE yield coefficient) that will make high-power microwave devices more efficient and robust. To this end, we have employed a low-energy electron gun (5 eV to 2000 eV) to characterize different materials. These measurements have been performed in the DC regime, but pulsed mode measurements are planned for the future to supplement the data. In addition, in support of the experiments, ICEPIC (improved concurrent electromagnetic particle-in-cell) simulations of SEE will be performed. Results obtained to-date will be presented