Session BO6: Surfaces, Helicons, Thrusters, Atmospheric Pressure, and Diagnostics

Light emission from zinc oxide nano-forest

Zinc oxide nanorods were grown on patterned highly doped silicon microstructures ($\sim $ 2-10 $\mu $m long) through a chemical bath deposition technique. Homogeneous and uniform coverage was obtained, with the length of the nanorods of $\sim $ 2 $\mu $m and diameter of $\sim $ 150 nm. When microsecond pulses (up to 50V) were applied across the microstructures, very bright and intense flashes of blue-white light were obtained. The output spectra, acquired with a high sensitivity spectrometer ($\approx $ 1 nm FWHM), showed sharp peaks in the UV range (330-334 nm) and high-intensity peaks in the blue range (467-480 nm), matching Zn atomic transitions, and broad emission from 450 nm to 800 nm, suggesting formation of a plasma. The voltage generated by the emitted light on a photosensitive PIN diode placed close to the sample and the electrical current through the sample were captured simultaneously using an oscilloscope to correlate the electrical and optical responses and determine the times of the plasma's generation and power dissipation.   

Wetting Properties of Liquid Lithium on Stainless Steel and Enhanced Stainless Steel Surfaces

Research into lithium as a first wall material has proven its ability to effectively getter impurities and reduce recycling of hydrogen ions at the wall. Current schemes for introducing lithium into a fusion device consist of lithium evaporators, however, as these devices evolve from pulsed to steady state, new methods will need to be employed such as the LIMIT concept of UIUC, or thin flowing film lithium walls. Critical to their implementation is understanding the interactions of liquid lithium with various surfaces. One such interaction is the wetting of materials by lithium, which may be characterized by the contact angle between the lithium and the surface. Experiments have been performed at UIUC into the contact angle of liquid lithium with a given surface, as well as methods to increase it.~To reduce the oxidation rate of the droplets, the experiments were performed in vacuum, using a lithium injector to deposit drops on each surface. Among the materials investigated are stainless steel, both untreated and coated with a diamond like carbon (DLC) layer,~molybdenum, and boronized molybdenum. The contact angle and its dependence on temperature is measured.   

Low-energy He-ion-induced surface-morphology changes of a hot W target

We report on measurements of interactions of low-energy (50 -- 200 eV) He ions with polished tungsten surfaces heated to $\sim $1250K performed at the ORNL Multi-charged Ion Research Facility (MIRF). Goal of the measurements was to investigate the initial phases of nano-fuzz formation recently reported by M.J. Baldwin and R.P. Doerner [Nucl. Fusion 48, 035001 (2008)]. For accumulated fluences of $\sim $10$^{19}$ cm$^{-2}$, small surface-grain features were observed, which extend to He-ion-energy-dependent depths of 100 to 200 nm, as determined by FIB/SEM cross section scans. We have recently installed a deceleration module capable of using the full He ion intensity extracted from our ECR ion source, which is expected to increase achievable fluences by at least an order of magnitude. XPS, SEM, and FIB/SEM/TEM analyses of He-ion exposed hot tungsten samples irradiated at the larger fluences will be presented at the conference. The samples to be studied include both virgin W, and W that has been damaged by 30 keV self-ion irradiation to damage doses in excess of 100 dpa prior to the He-ion exposure, in order to assess the effects of high radiation damage on the initial phases of nano-fuzz formation.   

Simulation of High-Voltage DC Breakdown for Angled Dielectric Insulators including Space-Charge and Gas-Collision Effects

We report on 2D Particle-In-Cell (PIC) simulations of a semi-infinite, angled-dielectric Bergeron geometry with steady-state fields in background gas. The goal of this work is to develop the tools to predict and control breakdown under a wide range of parameters. We extend results in [1] with an improved PIC model [2], which includes the effects of space charge and particle distributions, enhanced secondary-emission modeling from metals and dielectrics [3], multiple electrodes, triple-point emission [4], and dielectric-surface outgassing. Breakdown voltage as a function of dielectric angle will be presented taking care to distinguish dominant effects in specific pressure regimes. Very low pressures (vacuum thru $\sim$100 mTorr) are dominated by multipactor avalanche while ionization and surface-charging at increased pressure ($>$1 Torr) drive space-charge-coupled oscillations.\\[4pt] [1] Jordan, N.M., et al., J. Appl. Phys., 102, 2007.\\[0pt] [2] Taverniers, S., et al., ICOPS 2009 Proceedings, 2009.\\[0pt] [3] Vaughan, J.R.M., IEEE Trans. Electron Dev., Vol. 36, No. 9, 1989, pp. 1963-1967.\\[0pt] [4] L. Sch\"{a}chter, Appl. Phys. Lett., Vol. 72, No. 4, pp. 421-423, 1998.   

Ion Heating Arising from the Damping of Short Wavelength Fluctuations at the Edge of a Helicon Plasma Source

In previous studies, the appearance of substantial ion heating at the specific combinations of driving antenna frequency and magnetic field strength that result in the equivalence of the driving antenna frequency with the lower hybrid frequency provided strong, but indirect, evidence of the damping of short wavelength, ``slow'' wave fluctuations in the edge of helicon sources. For typical helicon source parameters, the driving antenna can couple to two plasma modes; the weakly damped ``helicon'' wave, and the strongly damped, slow wave. Internal magnetic field measurements routinely demonstrate the existence of wave fields consistent with fast waves in helicon sources. However, measurement of the slow wave is considerably more difficult given its extremely short wavelength and evanescent nature. Here we present two direct measurements of spatially localized, few hundred kHz, short wavelength fluctuations that are parametrically driven by the primary antenna. The short wavelength fluctuations appear for plasma source parameters such that the driving frequency is approximately equal to the lower hybrid frequency. Measurements of the time evolution of the ion temperature and fluctuation profiles provide additional confirmation of the ion heating through wave damping hypothesis.   

Absolute neutral helium density measurements in helicon source plasmas

Laser induced fluorescence (LIF) measurements of the plasma opacity are used as a novel diagnostic to determine the absolute density of a metastable state of neutral helium atoms in a helicon plasma. The absorption scale length at a wavelength of 587.725 nm (vacuum) is determined from measurements of fluorescence intensity as a function of distance along the laser path. With a collisional-radiative model of the state populations, the absolute ground state neutral helium density is estimated from the metastable state density measurement. This work expands upon previous work through measurements of neutral density, temperature, and flow at different radial positions. The measured neutral density decreases by two orders of magnitude from the edge of the plasma to the axis of the plasma source. Furthermore, the measurements are extended to a case that the helicon source was operated in a static mode in which the helium gas was not continuously fed into the chamber. As a result, the on-axis measurements show 42{\%} increase for the plasma density and 69{\%} decrease for the neutral density, when compared to the traditionally active gas feeding of the helicon sources; yielding an ionization fraction of approximately 90{\%}.   

Two photon absorption laser induced fluorescence measurements of neutral density profiles in a hydrogen helicon plasma

Neutral particles play a critical role in all plasma experiments. We report direct ground state neutral density measurements in hydrogenic helicon plasmas using a two-photon absorption laser induced fluorescence (TALIF) system. Specifically, we report the dependence of the neutral density on rf power, source magnetic field, driving frequency and equilibrium fill pressure. Neutral density was found to increase strongly with increasing rf power and fill pressure due to increased dissociation, and decrease weakly with magnetic field strength due to increased ionization. No significant dependence was found with varying driving frequency. Careful examination of the neutral production was made near to and far from the lower hybrid resonance by scanning the resonance from 0.15$f_{rf}$ to 60$f_{rf}$. No significant change in plasma or neutral production was found on or off resonance, counter to previous results in heavy ion helicon sources. Neutral density profiles were measured in the various parameter configurations. In some cases the profile was hollow, indicating possible neutral pumping. Profiles varied significantly from discharge to discharge indicating a sensitivity to wall conditions. We present a controlled study of the effect of wall conditioning on neutral density.   

Helicon Plasma Injection into an IEC Thruster

Helicon plasma injection into an Inertial Electrostatic Confinement (IEC) thruster stage is under experimental study. Helicons are RF plasma sources using helicon waves, or low frequency whistler waves. Such inductively coupled plasmas (ICPs) produce plasma of higher density than other field-free ICPs. Permanent magnet helicon sources have been proposed for plasma generation to provide a high downstream plasma density of about 1018 m$^{-3}$ for argon [1]. The IEC stage then provides plasma acceleration. An IEC plasma is produced as a dc glow discharge, giving a ``star'' or a ``jet'' mode (current mode of interest) on chamber pressure. To decouple the chamber pressure and the IEC cathode grid accelerating voltage, the plasma generation and plasma acceleration stages are decoupled by helicon plasma injection. Normally a symmetric cathode grid produces the star mode at lower pressures while a slightly higher pressure gives the jet mode. In this experiment an asymmetric cathode grid is used in the IEC to produce a plasma jet at pressures as low as 1.0 mTorr. This mode can be used as an advanced electric propulsion system where thrust and specific impulse are decoupled, providing variable specific impulse that enables complicated orbital maneuvers and challenging space missions. \\[4pt] [1] F. F. Chen and H. Torreblanca, ``Large-area helicon plasma source with permanent magnets,'' Phys. Plasmas, 2007.   

Increased thrust-over-power in a plasma thruster due to ion-neutral collisions

We have recently shown, in experiments conducted in our Radial Plasma Source (RPS), that the thrust per power from a plasma thruster can be increased due to ion-neutral collisions [1, 2]. The plasma in the RPS flows across a magnetic field so that the magnetic field pressure is the source of the thrust. Theory shows that the thrust increase is proportional to the square root of the number of ion-neutral collisions in the acceleration region [1, 2]. We present experimental measurements for three different gases of the thrust increase, results that confirm those theoretical predictions. The gases are argon, nitrogen, and helium. Another experimental finding is that the electron cross field transport is reduced when the cross section of the channel is made narrower. The reduced cross-field transport results in an improved efficiency.\\[4pt] [1] G. Makrinich and A. Fruchtman, Appl. Phys. Lett. \textbf{95}, 181504 (2009).\\[0pt] [2] G. Makrinich and A. Fruchtman, Phys. Plasmas \textbf{16}, 043507 (2009).   

Study and Assessment of Supersonic Plasma Jets as a Standoff Compression Driver

Spherically imploding plasma liners formed by merging high Mach number plasma jets are a proposed standoff driver for magneto-inertial fusion. The Plasma Liner Experiment (PLX) is currently exploring single jet propagation and two jet merging of supersonic argon plasma jets to assess their potential for use in MIF-relevant plasma liners. Key physics issues include assessing jet densities, jet expansion and cooling during propagation, and potential merging effects such as shock heating. An 8 chord interferometer using a 561 nm diode-pumped solid state laser is being used to make time-resolved density profile measurements of the plasma jets. The interferometer phase shift is sensitive to electron, ion, and neutral atoms and thus is dependent on both plasma ionization fraction, $f$, and total atomic density. Interferometry measurements coupled with spectroscopy and synthetic diagnostic data allow us to infer physics such as plasma density range (10$^{16}$ -- 10$^{17}$ cm$^{-3})$, jet propagation velocity ($\sim $50 km/s), and radial and axial expansion.   

Study on the Feasibility of Direct Fusion Energy Conversion for Deep-Space Propulsion

A significant change in the current space mission capabilities can be achieved with a highly efficient integration of a fusion energy source with an advanced space propulsion thruster, both with low specific mass. With aneutronic nuclear fusion as the high-density primary energy source, this study considers first electric energy extraction from the fusion reaction products via direct energy conversion to recirculate power as required for the operation of the fusion core. Then the beam of remaining reaction products is conditioned to achieve the optimal thrust and specific impulse for the mission. The research is specifically focused on two key issues: (i) Efficiency improvement of a Traveling Wave Direct Energy Converter (TWDEC, [1]) by achieving a higher ion beam density and optimization of the electrode coupling and of the neutralizing electron flow. (ii) A fast-particle kinetic energy-to-thrust conversion process based on collective interaction between ion bunches well separated in space [2]. Computer simulation results and a design for a basic physics experiment currently under development are reported. \\[4pt] [1] H. Momota \textit{et al., }Fus. Tech., 35, 60(1999)\\[0pt] [2] A. G. Tarditi \textit{et al. }Proc. NETS 2012 Conf., Woodlands, TX (2012)   

Development of a New Time-Resolved Laser-Induced Fluorescence Technique

We are developing a time-resolved laser-induced fluorescence (LIF) technique to interrogate the ion velocity distribution function (VDF) of EP thruster plumes down to the microsecond time scale. Better measurements of dynamic plasma processes will lead to improvements in simulation and prediction of thruster operation and erosion. We present the development of the new technique and results of initial tests. Signal-to-noise ratio (SNR) is often a challenge for LIF studies, and it is only more challenging for time-resolved measurements since a lock-in amplifier cannot be used with a long time constant. The new system uses laser modulation on the order of MHz, which enables the use of electronic filtering and phase-sensitive detection to improve SNR while preserving time-resolved information. Statistical averaging over many cycles to further improve SNR is done in the frequency domain. This technique can have significant advantages, including (1) larger spatial maps enabled by shorter data acquisition time and (2) the ability to average data without creating a phase reference by modifying the thruster operating condition with a periodic cutoff in discharge current, which can modify the ion velocity distribution.   

Influence of the gaseous form on the precursor heating layer of a laser-supported detonation wave using half self-emission half shadowgraph visualization

After breakdown one of the possible mechanisms of occurrence of laser-produced plasma is noted as laser-supported detonation (LSD) wave. This wave consisting of the shock wave and the beam absorbing plasma travels at 1-10 km/s along the beam channel in the direction opposite to the laser incidence. The laser heating structure is recognized as the ZND model of chemical detonation. However, Shimamura et. al, showed that the plasma proceeds the shock wave during LSD regime. The role of shock compression is relatively smaller than preheating by laser. The conventional model is inconsistent with our paper. To investigate the heating structure of a LSD wave, half self-emission half shadowgraph (HSHS) methods provides the self-emission image from the plasma on the top half and the shadowgraph image of the induced shock wave on the bottom half simultaneously. A TEA CO$_{2}$ laser was used at 10 J incident energy. The locations of both wave fronts were detected from the brightness distribution of the HSHS images. As a result, the propagation of ionization front precedes that of shock wave front by the order of 10$^{-4}$ m in air and N$_{2}$. Preheating layer of N$_{2}$ is shorter than that of air because O$_{2}$ in air has the lowest ionization energy. Thus, a characteristic of preionization layer depends on the ionization properties because photoionization by the UV radiation generate the seed electrons ahead of shock wave.   

Modeling of oblique ionization front propagation in high power millimeter wave field

High power millimeter-wave discharge in atmospheric air is characterized by filamentary structure and a supersonic propagation of the ionization front, driving a shockwave. In this research, the filamentary plasma structure was studied experimentally, using a 170 GHz gyrotron at power range of 200 kW to 600 kW, and numerically. About the ionization front propagation process, it is important to investigate steady plasma formation process, in a filamentary form, through millimeter wave. Each filamentary element is formed by granular plasmoids not propagating along or perpendicularly to the electric field, but obliquely to the field. To solve this mechanism, 2-dimensional numerical analysis was conducted using plasma fluid model. In dozens of times the size of plasma element scale, the steady plasma structure formation was simulated, and the calculation results were compared with previous experimental results. The calculated formation patterns were in good qualitative agreement with experiments. However, the ionization front velocity in the simulation is much higher than the experimental result. Thus, appropriate ionization model for the simulation is needed to get a better agreement. Moreover, for a quantitative agreement, not only the ionization model but also consideration of 3-dimensional effects is necessary, since 2-dimensional simulation cannot estimate accurate wave reflection and interaction with the filamentary plasma.