Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session TI2: Plasma Technology, Nozzles, and the First Wall |
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Chair: David Ruzic, University of Illinois Room: Rosen Centre Hotel Salon 3/4 |
Thursday, November 15, 2007 9:30AM - 10:00AM |
TI2.00001: Industrial Plasma Antennas Invited Speaker: This presentation summarizes an extensive program on plasma antennas. Plasma antennas are just as effective as metal antennas. In addition, they can transmit, receive and reflect lower frequency signals while being transparent to higher frequency signals. When de-energized, they electrically disappear. Plasma noise does not appear to be a problem. New technology that has been developed include a method of operating at high plasma density at minimal power consumption, a novel technique of noise reduction, and a method of opening a plasma window in a plasma microwave barrier on a time scale of microseconds rather than the usual time scale of milliseconds due to plasma decay. We are at present testing an intelligent plasma antenna in which a plasma~``window'' in a circular plasma barrier surrounding an antenna rotates azimuthally, seeking a radio transmitter. When located, a computer locks onto the transmitter. When the transmitter is de-energized, the plasma window recommences scanning. Commercial interest is strong, with invited papers being presented for 4 years in succession at the SMi Stealth Conference in London, UK, an operating model on permanent exhibition at the Booze-Allen headquarters in Alexandria, VA, and strong interest from Lockheed-Martin. \newline \newline In collaboration with Ted Anderson, Haleakala R\&D Corp.; Esmaeil Farshi, Fred Dyer, Jeffrey Peck, Eric Pradeep, Nanditha Pulasani, and Naresh Karnam, University of Tennessee. [Preview Abstract] |
Thursday, November 15, 2007 10:00AM - 10:30AM |
TI2.00002: Measurements of electric field strengths in ionization fronts during breakdown Invited Speaker: The electrical field strength during the initial phase of a low pressure, pulsed discharge in Xenon has been measured as a function of spatial position and time using fluorescence-dip Stark spectroscopy. For the first time, the role of the electrical field as the driving force of electrical breakdown has been studied experimentally in detail. A moving ionization front, measured with sub-microsecond resolution, has been detected. In this ionization front, the electrical field is roughly a factor 2 larger than the average in the discharge gap. [Preview Abstract] |
Thursday, November 15, 2007 10:30AM - 11:00AM |
TI2.00003: Plasma Shield for In-Air and Under-Water Beam Processes Invited Speaker: As the name suggests, the Plasma Shield is designed to chemically and thermally shield a target object by engulfing an area subjected to beam treatment with inert plasma. The shield consists of a vortex-stabilized arc that is employed to shield beams and workpiece area of interaction from atmospheric or liquid environment. A vortex-stabilized arc is established between a beam generating device (laser, ion or electron gun) and the target object. The arc, which is composed of a pure noble gas (chemically inert), engulfs the interaction region and shields it from any surrounding liquids like water or reactive gases. The vortex is composed of a sacrificial gas or liquid that swirls around and stabilizes the arc. In current art, many industrial processes like ion material modification by ion implantation, dry etching, and micro-fabrication, as well as, electron beam processing, like electron beam machining and electron beam melting is performed exclusively in vacuum, since electron guns, ion guns, their extractors and accelerators must be kept at a reasonably high vacuum, and since chemical interactions with atmospheric gases adversely affect numerous processes. Various processes involving electron ion and laser beams can, with the Plasma Shield be performed in practically any environment. For example, electron beam and laser welding can be performed under water, as well as, in situ repair of ship and nuclear reactor components. The plasma shield results in both thermal (since the plasma is hotter than the environment) and chemical shielding. The latter feature brings about in-vacuum process purity out of vacuum, and the thermal shielding aspect results in higher production rates. Recently plasma shielded electron beam welding experiments were performed resulting in the expected high quality in-air electron beam welding. Principle of operation and experimental results are to be discussed. [Preview Abstract] |
Thursday, November 15, 2007 11:00AM - 11:30AM |
TI2.00004: Plasma Physics and Radiation Hydrodynamics in Development of EUV Light Sources for Lithography Invited Speaker: Understanding of radiation generation in laser-produced high-Z plasma (LPP) is important for inertial fusion, astrophysics and x-ray source development. Extreme ultraviolet (EUV) light of 13.5 nm wavelength is strongly desired for manufacture of next-generation microprocessors with node size less than 45 nm. A commercial EUV lithography system would require output EUV power of about 400 W into a solid angle of 2$\pi$ str within a 2{\%} bandwidth (BW). Laser-produced tin (Sn) plasma at electron temperature of 30-70 eV and ion density of 10$^{17-20}$ cm$^{-3}$ is an attractive light source due to its compactness and high conversion efficiency (CE) from laser to EUV light [1]. The critical issues for practical use are high CE and damage caused by target debris. Many 4d-4f transitions of Sn$^{8+}$ to Sn$^{13+}$ ions mainly contribute to strong emission around 13.5 nm. We first discuss the importance of satellite lines, opacity and photo excitation in radiation transport, especially in high density plasmas produced by 1 $\mu$m laser. Experiment and simulation indicate that the maximum CE of 3{\%} is limited by these effects for 1$\mu$m laser. We show that the use of a long wavelength laser, such as CO$_{2}$ laser, results in higher CE of 3-6{\%}, since the spectral efficiency, the ratio of 13.5 nm emission within 2{\%} BW to total radiation, increases with the reduction of the plasma density. We present theoretical and experimental results of the CE dependence on laser intensity, pulse duration and laser wavelength. Radiation hydrodynamic simulations agree fairly well with EUV spectra observed in the experiments. High energy ions up to 10 keV generated in LPP cause damage to a collecting mirror. We show that the maximum energy is essentially determined from the ratio of plasma radius to Debye length. We also show that the use of the long wavelength laser also reduces the ion energy. We discuss mitigation of high energy ions by a magnetic field and the stability of plasma expansion taking finite ion Larmor radius effects into account. \newline \newline [1] K. Nishihara et al., Conversion Efficiency of LPP Sources, EUV Sources for Lithography (SPIE Press, Edited by V. Bakshi, (2006)). [Preview Abstract] |
Thursday, November 15, 2007 11:30AM - 12:00PM |
TI2.00005: Magnetic Nozzle and Plasma Detachment Scenario Invited Speaker: Some plasma propulsion concepts rely on a strong magnetic field to guide the plasma flow through the thruster nozzle. The question then arises of how the magnetically controlled plasma can detach from the spacecraft. This talk presents a magnetohydrodynamic detachment scenario in which the plasma stretches the magnetic field lines to infinity [1]. Such a scenario is of particular interest for high-power thrusters. As plasma flows along the magnetic field lines, the originally sub-Alfv\'enic flow becomes super-Alfv\'enic: this transition is similar to what occurs in the solar wind [2]. In order to describe the detachment quantitatively, the ideal MHD equations have been solved analytically for a plasma flow in a slowly diverging nozzle. The solution exhibits a well-behaved transition from sub- to super- Alfv\'enic flow inside the nozzle and a rarefaction wave at the edge of the outgoing flow. The magnetic field in the detached plume is almost entirely due to the plasma currents. It is shown that efficient detachment is feasible if the nozzle is sufficiently long. In order to extend the detachment model beyond the idealizations of analytical theory, a Lagrangian fluid code has been developed to solve steady-stated MHD equations and to optimize nozzle efficiency by adjusting the magnetic coil configuration. This numerical tool enables broad parameter scan with modest computational requirements (single workstation). The code has been benchmarked against the idealized analytical picture of plasma detachment and then used to investigate more realistic nozzle configurations that are not analytically tractable. Most recently, the code has been used to interpret experimental data from the Detachment Demonstration Experiment (DDEX) [3] facility at NASA Marshall Space Flight Center. In collabotation with: M. Tushentsov, A. Arefiev, R. Bengtson, J.Meyers (University of Texas at Austin), D. Chavers, C. Dobson, J. Jones (Marshall Space Flight Center), B.Schuettpelz, (University of Alabama in Huntsville), C. Deline (University of Michigan). \newline [1] A. Arefiev and B. Breizman,Phys. Plasmas {\bf12}, 043504 (2005). \newline [2] E. N. Parker, Astrophys. J.{\bf128}, 664 (1958). \newline [3] D. Chavers et al., ``Status of Magnetic Nozzle and Plasma Detachment Experiment,'' CP813, Space Technology and Applications International Forum, pp. 465-473, AIP 2006. [Preview Abstract] |
Thursday, November 15, 2007 12:00PM - 12:30PM |
TI2.00006: Lithium Surface Coatings and Improved Plasma Performance in NSTX Invited Speaker: NSTX research on lithium-coated plasma facing components is the latest step in a decade-long, multi-institutional research program to develop lithium as a plasma-facing system that can withstand the high heat and neutron fluxes in a DT reactor. The NSTX research is also aimed towards sustaining the current non- inductively in H-mode plasmas which requires control of both wall recycling and impurity influxes. Employing several techniques to coat the plasma facing components (PFCs) with lithium, NSTX experiments have shown, for the first time, significant benefits in high-power divertor plasmas. Lithium pellet injection (LPI) uses the plasma itself to distribute lithium on the divertor or limiter surfaces. The multi-barrel LPI on NSTX can introduce either lithium pellets with masses 1 - 5 mg or powder during a discharge. This significantly lowered recycling and reduced the density in a subsequent NBI-heated, divertor plasma. Lithium coatings have also been applied with a LIThium EvaporatoR (LITER) that was installed on an upper vacuum vessel port to direct a collimated stream of lithium vapor toward the graphite tiles of the lower center stack and divertor. The lithium was evaporated either before tokamak discharges, or continuously between and during them. By evaporating lithium into the helium glow discharge that typically precedes each tokamak discharge, a coating of the entire PFC area was achieved. Lithium depositions from a few mg to 1 g have been applied between discharges. Among the effects observed in subsequent neutral-beam heated plasmas were decreases in oxygen impurities, plasma density, and the inductive flux consumption, and increases in electron temperature, ion temperature, energy confinement and DD neutron rate. In addition, a reduction in the ELM frequency, including their complete suppression, was achieved in H-mode plasmas. Additional observations, such as, the duration of the lithium coatings, increases in core metal impurity radiation, and diagnostic window depositions will also be discussed. [Preview Abstract] |
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