Session CI2: Low Temperature Plasma

Current fundamental science challenges in low temperature plasma science that impact energy security and international competitiveness

Products and consumer goods that utilize low temperature plasmas at some point in their creation touch and enrich our lives on almost a continuous basis. Examples are many but include the tremendous advances in microelectronics and the pervasive nature of the internet, advanced material coatings that increase the strength and reliability of products from turbine engines to potato chip bags, and the recent national emphasis on energy efficient lighting and compact fluorescent bulbs. Each of these products owes their contributions to energy security and international competiveness to fundamental research investments. However, it would be a mistake to believe that the great commercial success of these products implies a robust understanding of the complicated interactions inherent in plasma systems. Rather, current development of the next generation of low temperature plasma enabled products and processes is clearly exposing a new set of exciting scientific challenges that require leaps in fundamental understanding and interdisciplinary research teams. Emerging applications such as liquid-plasma systems to improve water quality and remediate hazardous chemicals, plasma-assisted combustion to increase energy efficiency and reduce emissions, and medical applications promise to improve our lives and the environment only if difficult science questions are solved. This talk will take a brief look back at the role of low temperature plasma science in enabling entirely new markets and then survey the next generation of emerging plasma applications. The emphasis will be on describing the key science questions and the opportunities for scientific cross cutting collaborations that underscore the need for increased outreach on the part of the plasma science community to improve visibility at the federal program level. This work is supported by the DOE, Office of Science for Fusion Energy Sciences, and Sandia National Laboratories, a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000   

Nanoscale control of energy and matter in plasma-surface interactions: towards energy-efficient nanotech

This presentation focuses on the plasma issues related to the solution of the grand challenge of directing energy and matter at nanoscales. This ability is critical for the renewable energy and energy-efficient technologies for sustainable future development. It will be discussed how to use environmentally and human health benign non-equilibrium plasma-solid systems and control the elementary processes of plasma-surface interactions to direct the fluxes of energy and matter at multiple temporal and spatial scales. In turn, this makes it possible to achieve the deterministic synthesis of self- organised arrays of metastable nanostructures in the size range beyond the reach of the present-day nanofabrication. Such structures have tantalising prospects to enhance performance of nanomaterials in virtually any area of human activity yet remain almost inaccessible because the Nature's energy minimisation rules allow only a small number of stable equilibrium states. By using precisely controlled and kinetically fast nanoscale transfer of energy and matter under non-equilibrium conditions and harnessing numerous plasma- specific controls of species creation, delivery to the surface, nucleation and large-scale self-organisation of nuclei and nanostructures, the arrays of metastable nanostructures can be created, arranged, stabilised, and further processed to meet the specific requirements of the envisaged applications. These approaches will eventually lead to faster, unprecedentedly- clean, human-health-friendly, and energy-efficient nanoscale synthesis and processing technologies for the next-generation renewable energy and light sources, biomedical devices, information and communication systems, as well as advanced functional materials for applications ranging from basic food, water, health and clean environment needs to national security and space missions.   

Experimental test of instability-enhanced collisional friction for determining ion loss in two ion species plasmas

Recent experiments have shown that ions in weakly collisional plasmas containing two ion species of comparable densities nearly reach a common velocity at the sheath edge equal to the bulk plasma ion sound velocity. A new theory suggests that this is a consequence of collisional friction between the two ion species enhanced by two stream instability. The theory finds that the difference in velocities at the sheath edge depends on the relative concentrations of the species being small when the concentrations are comparable and large, with each species reaching its own Bohm velocity, when the relative concentration differences are large. To test these findings, ion drift velocities were measured with Ar and Xe ion laser-induced fluorescence in Ar-Xe, He-Ar and He-Xe plasmas and combined with ion acoustic wave and plasma potential data. In addition, Te and neutral pressure were varied. The predictions were found to be in excellent agreement with the experimental data.   

Temporally-resolved ion velocity distribution measurements in a radio-frequency plasma sheath

The ion velocity distribution function (IVDF) above and within a radio-frequency (RF) biased plasma sheath is studied experimentally with a pulsed laser-induced fluorescence (LIF) diagnostic in an industrial plasma etch tool. Temporally-resolved measurements taken at 8 different phases of the 2.2 MHz bias waveform show the ion dynamics to vary dramatically throughout the RF cycle (the ratio of the average ion transit time through the sheath to the RF period is $\tau _{ion}$/$\tau _{RF}$ = 0.3). The position of the pre-sheath/sheath edge is constant throughout the RF cycle and the ion flux is conserved within the sheath region. Modeling results depict ion dynamics in agreement with experimentally observed results. The characteristic bimodal structure of the time-averaged ion distributions found in previous experiments is observed to arise from the time-dependent ion dynamics, in accord with existing theory. The large temporal variation of the IVDF has implications for the plasma chemistry and etching quality.   

Traveling wave model for laser-guided discharges

We have developed a 1-D traveling wave model for laser-guided discharges, which is easily solvable and provides insight into discharge structure, range, and propagation requirements for positive and negative discharges. The hydro, electrodynamic and chemistry equations are reduced to ODE's in retarded time $\tau \equiv $t-z/u by assuming constant propagation speed u. We show negative discharges propagate only if u$>\mu $E$_{b}$, where $\mu $ is the electron mobility and E$_{b}$ is the breakdown field, and positive discharges propagate only if channel pre-conductance exceeds 6$\times $10$^{-11}$m/ohm. At the discharge head, the axial electric field E spikes up to several$\times $E$_{b}$, and then quickly relaxes to a value $\sim $E$_{b}$ which persists as long as the gas is cold and unexcited. In this ``streamer'' stage, the current, channel conductance and potential all increase linearly with $\tau $, nearly identically in attaching and non-attaching gases. The transition to the ``leader'' stage, where E is much smaller, occurs in two steps, first excitation of vibrational and low-lying electronic states, and then gas heating. The propagation range, which scales inversely with E, increases strongly if the initial discharge radius is decreased. For given maximum driving voltage, the range is a decreasing function of the voltage rise rate. Radial expansion of the heated channel is a major contributing factor to increased range. This work was done in collaboration with R. F. Fernsler, S. P. Slinker, and D. F. Gordon.   

Review theories and experiments of improving HPM window breakdown thresholds

Dielectric window breakdown is a seriously confronting challenge in HPM transmission and radiation. Breakdown at the vacuum/dielectric interface is triggered by multipactor and finally realized by plasma avalanche in the ambient desorbed or evaporated gas layer above the dielectric [1-4]. The methods of improving breakdown thresholds become key issues of HPM system. We review the main theoretical and experimental progress, and then, we further survey the mechanisms of multipactor suppression of the periodic rectangular [5] and triangular surface profiles [6-8] by dynamic analysis and particle-in-cell simulations, and the demonstration of improving HPM thresholds by proof-of-principle experiments and multi-GW experiments. We also synthesize the theory of using magnetic field [9] to suppress multipactor.\\[4pt] [1] C. Chang, et al., POP 15, 093508, 2008.\\[0pt] [2] C. Chang, et al., POP 16, 033505, 2009.\\[0pt] [3] C. Chang, et al., POP 16, 053506, 2009.\\[0pt] [4] C. Chang, et al., POP 17, 053301, 2010.\\[0pt] [5] C. Chang, et al., JAP 105, 123305, 2009.\\[0pt] [6] C. Chang, et al., POP 16, 083501, 2009.\\[0pt] [7] C. Chang, et al., LPB 28, 185, 2010.\\[0pt] [8] C. Chang, et al., PIER 101,157, 2010.\\[0pt] [9] C. Chang, et al., APL 96, 111502, 2010.