Session QO1: Education and Outreach and Plasma Technology

Plasma Science and Applications at the Intel

The Coalition for Plasma Science (CPS) has established a plasma prize at the annual Intel International Science and Engineering Fair (ISEF). The 2006 prize was awarded for a project that investigated the correlation of GPS errors with various measures of near-earth plasma activity. The CPS is a broadly-based group of institutions and individuals whose goal is to increase the understanding of plasmas for non-technical audiences. In addition to the ISEF plasma award, CPS activities include maintaining a website, http://www.plasmacoalition.org; developing educational literature; organizing educational luncheon presentations for Members of Congress and their staffs; and responding to questions about plasmas. In addition, the CPS has begun as effort to examine the plasma content of state education standards with the goal of promoting the adoption of standards with appropriate plasma conten; e.g. are there three or four states of matter. The success of this and other activities depend on the voluntary labor of CPS members and associates. Please send an e-mail to the CPS at CPS@plasmacoalition.org for information if you would like to become involved in spreading the good word about plasmas.   

A Narrative Approach to Citizen Participation in Fusion Energy

This paper examines how to increase citizen participation in fusion energy research. An increased sense of participation in fusion energy progress is essential for the acceptance of fusion energy by the general public. The obstacle this paper acknowledges is that fusion energy research needs to be in the hands of experts. The challenge is how to increase the sense of participation in fusion energy research without being able to greatly increase direct participation on the part of citizens. Individuals can cultivate a sense of participation in far off events by placing news of events, such as a war or political campaign, into a larger narrative structure. Individuals can have a sense of control over the larger narrative structure and how news of events fit into their own sense of the narrative. If citizens have a grasp of major past fusion energy events, the current status of fusion energy and what the future might hold for fusion energy research, then citizens will have a fundamental narrative structure in which to fit news of future fusion energy research events. This paper explores ways fusion energy educators can take a narrative approach to foster a sense of participation for a large number of citizens. How can educators help citizens internalize a fusion energy meta-narrative? How can educators help citizens fit news of progress in fusion energy into their internalized narratives?   

Plasma and Thruster Characteristics of a Ferroelectric Plasma Thruster

The ferroelectric plasma thruster (FEPT) is a new micropropulsion concept. The FEPT is a ferroelectric plasma source driven by oscillating high voltage at RF frequencies. The ferroelectric plasma source produces plasma at a surface partially covered by an electrode when the spontaneous polarization vector is reversed by application of an oscillating electric field. Thrust is generated by the FEPT by acceleration of ions from the surface plasma via the ponderomotive force. At the University of Missouri-Columbia, experiments are being conducted on optimizing a FEPT for microspacecraft propulsion. This includes measurements of total emitted electron and ion current, energy distribution of the emitted ions, and spectra of plasma light emission.   

Mini-Helicon Thruster Experiment at MIT

A mini-Helicon Thruster Experiment (mHTX) is in operation at MIT for under a year. The compact helicon plasma source has an electrical propulsion-oriented design and already demonstrated unusually strong collimated plume production when operating with Ar, N$_{2}$, Xe, Ne gases and mixtures including air. The RF power-to-plasma coupling as well as gas-to-plasma conversion has exceeded 90{\%} in optimal magnetic field and antenna configurations. We report recent mHTX experimental and theoretical results that include i)~thrust force measurement, ii)~plasma diagnostics, iii)~power balance analysis, and iv)~numerical simulation of the mini-helicon discharge.   

Neutral Depletion in Collisional and Collisionless Plasmas

Neutral depletion can affect dramatically the steady-state of low-temperature plasmas. For a plasma and neutral gas in pressure balance we showed that because of the inherent coupling of ionization and transport, an increase of the energy invested in ionization can nonlinearly enhance the transport process.\footnote{A. Fruchtman, G. Makrinich, P. Chabert, and J. M. Rax, Phys. Rev. Lett. \textbf{95}, 115002 (2005).} Such an enhancement of the plasma transport due to neutral depletion was shown to result in an unexpected \textit{decrease }of the plasma density when power is \textit{increased, }despite the \textit{increase }of the flux of generated plasma. The effect of neutral depletion on collisionless plasma and neutrals, that are not in pressure balance and that are coupled through ionization only, will be analyzed. Implications of the collisionless case to helicon plasma sources and to the ionization region in the Hall thruster will be discussed. The effect of neutral gas heating will also be analyzed. It will be shown that this neutral gas heating can affect the plasma steady-state even if the plasma pressure is much lower than the neutral gas pressure.   

Positive to negative sheath transition at the metallic anode of a Hall thruster

Both the anode sheath and the near-anode quasineutral regions are considered. The near anode region is studied with a simplified set of fluid equations that retain the central elements of the problem under consideration. A generalized singular / sonic condition is found in the quasineutral edge in terms of the particle fluxes of the two species. As the relative flux ratio is modified, the resulting near-singular electric field changes sign, indicating the transition to a different type of sheath. Maxwellian-type distribution functions are taken to solve the sheath. The parameters of these distributions are determined by forcing continuity of the species densities, particles fluxes and energy fluxes at the two sides of the sheath edge. This kinetic/fluid formulation provides well-matched solutions in practically the whole range of fluxes ratio.   

Characterization of a non-thermal plasma system at atmospheric pressure

Recently, the authors have developed a non-thermal plasma system for the removal of harmful particulate matter from diesel engines. In this paper, the characterization of such a non-thermal plasma system (typically including a dielectric barrier discharge (DBD) reactor and a pulse power supply) is carried out electrically and optically using a discharge waveform measurement system of a voltage probe, two current transformers, and an oscilloscope, a high dynamic range streak camera combined with a spectroscope, and a high speed camera. The pulse voltage is applied to the DBD reactor at a fixed value and a frequency of 100 Hz. The rise time, positive width, and peak value of the pulse voltage are, respectively, 3.5 $\mu $s, 6 $\mu $s, and 13.2 kV. The discharge time duration is 14 $\mu $s. The optical emission from the discharge gap lasts for 14 $\mu $s. The emission lines are found within 300 - 410 nm and 620- 770 nm. The discharges are generally of an individual micro-discharge property as reported elsewhere.   

Discharge Kinetics of N$_{2}-$O$_{2}$ Laser Generated Plasma Channels

We study both experimentally and theoretically the effects of recombination and attachment of different N$_{2}$--O$_{2}$ gas compositions upon the plasma channel dynamics at one atmosphere. The time-dependent DC electrical conductivity technique [1] has been applied to determine the electron density decay. The rate constants have been determined from the electron balance equation by curve fitting of voltage-time measurements over the decay time interval. The measured electron density decay time for air is hundreds of nanoseconds while for pure nitrogen it is much longer, about 2 $\mu $s. For air, we derive an attachment rate $\eta _{air} =1.39\times 10^7\;\mbox{s}^{-1}$ and an electron-ion recombination rate $\beta _{air} =1.02\times 10^{-8}\;\mbox{cm}^\mbox{3}{\kern 1pt}\mbox{s}^{-1}$. In order to investigate in details the kinetics of N$_{2}$--O$_{2}$ gas mixture we developed a time-dependent collisional-radiative model based upon the numerical solution of the electron Boltzmann equation for the electron energy distribution function. It is coupled with the time-dependent balance equations of electrons, atomic and molecular ions under consideration, and various nitrogen and oxygen species in ground and excited states. The validity of the model was verified by comparison of the swarm parameters derived from the model with experimental parameters for pure oxygen, pure nitrogen, and air. \newline [1] H.D. Ladouceur at al., Optics Communications, \textbf{189} (2001) 107 \newline * NRL-NRC Postdoc   

Modeling of grain surface modification in low pressure plasma via PIC algorithm

In recent years, material processing technologies have been remarkably improved by the ever-growing research interest and efforts in nano-science along with plasma processing techniques. In particular, low-pressure plasma has been widely employed in the coating industry due to its capability of efficiently generating active species, responsible for layer deposition, in a low temperature environment. Here, we focus our attention on the deposition process on a submicron particle submerged in a CH$_{4}$/H$_{2}$ plasma. A pre-existing PIC code, which performs the particle charging calculations, is extended to account for plasma species scattering and charge-exchange processes via a Monte Carlo Collision (MCC) method, for soft-sphere interactions between non-charged species governed by the Lennard-Jones law, and for plasma/surface and gas/surface interactions via a Monte Carlo method. The last takes advantage of the immerse boundary method which allows a localized surface reaction process and layer growth as a function of super-particle densities and sticking probability coefficients.   

Laser produced nanocavities in dielectrics

Tight focusing of the laser pulses opens a possibility to modify the properties of transparent materials and create high-density memories and photonic crystals. It was demonstrated recently [1,2] a formation of sub-wavelength holes by focusing a 100 ns, 100 fs laser pulses inside samples of sapphir and glass. This paper presents the results of numerical simulations of the interaction of sub-ps laser pulses with transparent dielectrics. It contains two parts : a 2D model of the laser energy deposition, based on the solution of full Maxwell equations coupled to a ionisation model accounting for the multiphotonic and avalanche ionisation, and the 3-body recombination. The second part presents the 2D hydrodynamic simulations of the shock wave propagation and the cavity formation with initial conditions provided by 2D absorption model. It is shown that the cavity parameters are very sensitive to the properties of the EOS in the phase transition domain. \mbox{}\\\mbox{} [1] E.G. Gamaly, S. Juodkazis, K. Nishimura, H. Misawa, B. Luther-Davies, L. Hallo, Ph. Nicola\"{\i}, V. T. Tikhonchuk, Phys. Rev. B 73, 214101 (2006). [2] S. Juodkazis, K. Nishimura, S. Tanaka, H. Misawa, E.G. Gamaly, B. Luther-Davies, L. Hallo, Ph. Nicola\"{\i}, V. T. Tikhonchuk, Phys. Rev. Lett. 96, 166101 (2006).   

Copper activation signatures for ablation rate measurements in inertial confinement fusion targets

Current NIF ignition designs employ an xray-driven Be(Cu) ablator to accelerate and compress the DT fuel. Ignition specifications require that the mass of unablated Be(Cu) be known to within 1{\%} of the initial mass when the fuel reaches peak velocity. We describe here a diagnostic technique to measure the remaining unablated mass to this specification. The technique relies on the activation of Cu by neutrons escaping the imploded core. The fraction of activated Cu is proportional to the areal mass of Cu at the time of neutron yield. By capturing and analyzing debris from the implosion, we may measure the Cu activation fraction and thus the areal mass. This areal mass can then be used to infer the mass of remaining ablator. We will compare the performance of this measurement with other techniques for measuring ablation rate.   

Carbon Hybridization Dynamics Inside the Bombarded Carbon Surface

We study the density and hybridization depth profiles of amorphous carbon and graphite, bombarded by D and D$_{2}$. Cumulative impacts produce modifications in the substrate structure and the sputter yields.\textbf{ }For graphite samples, the surface continues to evolve up to the highest fluences studied, of 1.9$\times $10m, while for the deuterated amorphous carbon surface, however, the sample reaches a steady state at fluences of less than 3$\times $10m. At this point the structure of the surface and sputtering properties of lighter hydrocarbons change slowly within statistical fluctuations. The steady-state surface structure is examined in detail and is found to be very different from the initial surface: It is highly supersaturated with deuterium, with enrichment in sp$^{3}$ bonds. The yields of the light hydrocarbons sputtered from these surfaces are found to be directly related to the hybridization structural properties and the depth profiles. Our simulations indicate that it is necessary to perform sputtering simulations on surfaces that have been dynamically created by impacts, rather than equilibrium surfaces, to obtain a good agreement with experiments.   

What Can We Learn on the Above-Surface Neutralization Process from the Chemical Sputtering Yields of D, D$_{2}$ AND D$_{3}$ at Carbon Surface?

We perform molecular dynamics simulations of chemical sputtering of methane and acetylene from deuterated amorphous carbon surface, by impact of deuterium, as well as of vibrationally excited D$_{2}$ and dissociating D$_{3}$ molecules in range of 5-30 eV/D impact energies. We show that the chemical sputtering yields at the lowest impact energies are very sensitive to the vibrational state of the molecules. Our sputtering yields are compared with those obtained experimentally [L.I. Vergara et al, J. Nucl. Mat. (2006), in press] by impact of D, D and D. Analyzing possible processes of the above-surface neutralization of the impinging ions in the experiment we conclude that the leading mechanism is the charge transfer from the surface-bound deuterium atoms onto vibrationally excited states of the molecules.