Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session RI2: Inertial Confinement, Plasma Etch, and Plasma Thruster Technologies |
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Chair: John Caughman, Oak Ridge National Laboratory Room: Adam's Mark Hotel Plaza Ballroom EF |
Thursday, October 27, 2005 2:00PM - 2:30PM |
RI2.00001: Forming Smooth Cryogenic Target Layers for OMEGA Direct-Drive ICF Implosions and Prospects for Direct-Drive Targets for the NIF Invited Speaker: More than 100 cryogenic D$_{2}$ target ice layers have been formed for direct-drive ICF implosion experiments at LLE. While all of these layers are smooth to several microns rms, some of them have achieved the 1-\textit{$\mu $}m rms nonuniformity required for high-yield implosions. The largest effect on the quality of a cryogenic target layer is the thermal uniformity of the target's surroundings. Temperature nonuniformities at the ice that exceed 100 \textit{$\mu $}K are observable in the layers. Control of the thermal environment determines the uniformity of the ice layer thickness and the time it takes to form the layer. Detailed evidence for this sensitivity and the importance of the thermal environment to the ice quality are presented. The initial direct-drive target design for the NIF is significantly different from the current OMEGA design with the addition of a fill tube and a refracting ``Saturn'' ring around the target equator (allows direct drive with the NIF x-ray drive beam configuration). Progress at making these targets and a strategy for creating a thermal environment capable of forming high-quality ice layers in these targets will be presented. LLE is modifying its cryogenic systems to perform DT implosions. Transitioning from pure D$_{2}$ to mixtures of D$_{2}$, DT, and T$_{2}$ adds complexity that may affect the ice layer quality. The disparate freezing temperatures of the isotopes may result in partial fractionation with the standard slow-cool protocol used to form a smooth layer. The ability to enhance the layering process using infrared heating may be affected by the inhomogeneity of the isotope concentrations in the ice. These effects are reported for a mixture of H$_{2}$, HD, and D$_{2}$ that is used as a proxy for D$_{2}$, DT, and T$_{2}$ mixtures. The status of DT cryogenic operations will be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460. Contributors: M. J. Bonino, T. Duffy, D. H. Edgell, L. M. Elasky, R. Q. Gram, D. Jacobs-Perkins, R. Janezic, S. J. Loucks, L. D. Lund, D. D. Meyerhofer, W. Seka, W. T. Shmayda, and M. D. Wittman, \textit{LLE}. [Preview Abstract] |
Thursday, October 27, 2005 2:30PM - 3:00PM |
RI2.00002: Progress Towards Fabrication of Graded Doped Beryllium and CH Capsules for the National Ignition Facility Invited Speaker: Current ignition designs require graded doped beryllium or CH capsules. In this paper, we report on the progress towards fabricating both beryllium and CH capsules that meet the current design criteria for achieving ignition on the NIF. NIF scale graded copper doped beryllium capsules have been made by sputter coating, while graded germanium doped CH capsules have been made by plasma polymer deposition. The plasma polymer deposition process has produced dense, void free graded doped CH shells that meet the ignition surface finish requirements. The sputtering process used for fabricating graded beryllium shells can lead to ablators with a high void fraction ($>$5\%) and rough surfaces ($\sim$1~$\mu$m RMS). Varying the deposition parameters can lead to several different beryllium microstructures, which can potentially be tuned to reduce the void size and fraction to within specifications. In addition, polishing of beryllium-coated shells reduces the outer surface roughness of shells to ignition specifications. Transmission electron microscopy has been used to characterize void fraction and grain structure. Layer thickness and dopant concentrations have been measured by quantitative contact radiography (non-destructive) and electron probe (destructive) techniques. Control over the dopant concentration has been demonstrated to within the desired specifications for each layer by careful control of the coating parameters. [Preview Abstract] |
Thursday, October 27, 2005 3:00PM - 3:30PM |
RI2.00003: Developing a Commercial Production Process for 500,000 Targets Per Day --- A Key Challenge for Inertial Fusion Energy Invited Speaker: As is true for current-day commercial power plants, a reliable and economic fuel supply is essential for the viability of future Inertial Fusion Energy (IFE) power plants. The ``target" is the vehicle by which the fuel is delivered to the reaction chamber. Thus the cost of the target becomes a critical issue in regard to fuel cost. Typically six targets per second, or about 500,000/day are required for a 1000~MW(e) power plant, thus the cost per target must become extremely low for commercial application of IFE. The electricity value within a typical target is about \$3, allocating 10\% for fuel cost gives only 30~cents per target as-delivered to chamber center. Complicating this goal, the target supply has many significant technical challenges - fabricating the precision fuel-containing capsule, filling with DT, cooling to cryogenic temperatures, layering the DT, characterizing the finished product, accelerating to high velocity for injection into the chamber, and tracking the target to steer the driver beams to meet it with micron-precision at chamber center.\par ~~~~The target cost of about 30~cents represents about four orders of magnitude reduction from current experimental targets. Thus, the technology development program for IFE targets becomes a critical component of any proposal to commercialize fusion energy using inertial confinement. Over the past few years, fabrication processes have been identified for every step of the IFE target supply, and a significant development program is underway to experimentally demonstrate their feasibility. In this paper we describe the target supply process steps, provide an overview of the planned development program, and assess the probability of success for this key challenge for fusion energy. [Preview Abstract] |
Thursday, October 27, 2005 3:30PM - 4:00PM |
RI2.00004: Plasma Systems for Dielectric Etch Invited Speaker: Plasma systems used to etch oxides on silicon wafers impose a complex set of simultaneous requirements on the plasma to meet performance specifications for etching present (70nm) and next-generation features (45-32 nm). Dielectric etch systems usually comprise an rf plasma source and an rf plasma bias. The function of the bias is to create a DC sheath by the rectification of the rf power that then accelerates ions into the wafer. Typically, a weakly ionized plasma in Ar/O/C$_{x}$F$_{y}$ chemistries is used in the millitorr pressure range. Because of the pressure range, the neutral/plasma collisions can substantially alter the power deposition between that needed for the sheath and the remainder that creates the plasma. Allocation of power for the ion energy distribution must also be considered in addition to the power division between bulk plasma and dc sheath. Ion energy distributions are tailored to meet etch requirements (which depend on the material etched) by using two rf frequencies in the bias system to accelerate ions. These two frequencies adjust energy from mono-energetic (when the ion sheath transit time is long compared to an rf period) to very broad (when the transit time is short relative to a period). To acquire higher etch rates and a wider dynamic range of plasma densities, a third rf frequency is used as a plasma density source. It is more advantageous to use a very low voltage rf capacitive source to generate sufficient density for the least power. We discuss the interrelated requirements, their impact on the wafer, the manner in which they are achieved, and limitations imposed by the physics of each frequency/power/pressure range. [Preview Abstract] |
Thursday, October 27, 2005 4:00PM - 4:30PM |
RI2.00005: Nonequilibrium Plasmas Confined to Microcavities: New Opportunities in Plasma Science and Its Applications* Invited Speaker: The recent development of devices in which nonequilibrium, low temperature plasmas are spatially confined to cavities having cross-sectional dimensions as small as 10 $\times $ 10 $\mu $m$^{2}$ offers new avenues of inquiry in plasma science and its applications. Characterized by plasma volumes of typically nanoliters or less and specific power loadings of the plasma of tens of kW-cm$^{-3}$ to $\ge $ 1 MW-cm$^{-3}$, these microcavity plasma devices offer access to a previously unexplored region of plasma parameter space. In particular, nonequilibrium plasmas operating continuously with a plasma frequency of 1 THz and at number densities approaching that of a supercritical fluid appear to be attainable. In this presentation, the seminal characteristics of microcavity plasmas with characteristic dimensions below 100 $\mu $m, as well as several of their photonics applications, will be discussed. The latter include the realization of microplasma arrays as large as 250,000 devices, the observation and characterization of photodetection in the ultraviolet, visible, and near-infrared with atmospheric pressure microplasma, and an optical amplifier in the blue ($\sim $460 nm) excited by a microplasma array. *Work supported by the U.S. Air Force Office of Scientific Research. [Preview Abstract] |
Thursday, October 27, 2005 4:30PM - 5:00PM |
RI2.00006: Experimental Study of an Advanced Plasma Thruster using ICRF Heating and Magnetic Nozzle Acceleration. Invited Speaker: Electric propulsion (EP) systems utilize plasma technologies and have been developed for years as one of the most promising space propulsion approaches. It is urgently required to develop high-power plasma thrusters for human expeditions to Mars and future space exploration missions. The advanced thruster is demanded to control thrust and specific impulse by adjusting the exhaust plasma density and velocity. In the VASIMR project, a combined system of efficient ion cyclotron heating and optimized magnetic nozzle design is proposed to control the ratio of specific impulse to thrust at constant power[1]. In this system a flowing plasma is heated by ICRF (ion cyclotron range of frequency) waves and the plasma thermal energy is converted to flow energy in a diverging magnetic field nozzle. We have recently performed the first demonstration of ion cyclotron resonance heating and acceleration in a magnetic nozzle by using a fast-flowing plasma with Mach number of nearly unity. Highly ionized plasma is produced by Magneto-Plasma-Dynamic thruster (MPDT). When RF power is launched by a helically-wound antenna, electromagnetic ion cyclotron waves are excited, and plasma thermal energy and ion temperature drastically increase (nearly ten-fold rise) during the RF pulse. The value of resonance magnetic field is affected by the Doppler shift due to the fast-flowing plasma. Dependences of heating efficiency on both plasma density and input RF power will be presented. Ion acceleration along the field line is also observed in a diverging magnetic field nozzle. Perpendicular component (to the magnetic field) of ion energy decreases, whereas parallel component increases along the diverging magnetic field.\newline \newline [1] F.R. Chang Diaz, ``The VASIMR Engine,'' AIAA 2004-0149. AIAA conf. (Reno,2004); Bulletin of APS (46th APS-DPP), NM2A-3, 2004. [Preview Abstract] |
Thursday, October 27, 2005 5:00PM - 5:30PM |
RI2.00007: Studies of anode sheath phenomena in a Hall-effect thruster discharge Invited Speaker: Crossed electric and magnetic fields devices (plasma thrusters, magnetrons, coaxial plasma guns, plasma opening switches, etc.) are routinely used for plasma production and in other applications. Despite these numerous applications, the fundamental anode sheath phenomena in many of these devices have received surprisingly little experimental scrutiny. We chose a Hall-effect thruster (HT) discharge for our study of the anode sheath. It has been typically assumed in most fluid models of an HT that its steady-state operation requires the presence of a negative anode fall (electron-repelling anode sheath). Such anode fall behavior, opposite to that in typical glow discharges or hollow-anode plasma sources, is the result of a relatively high degree of ionization in HTs, achieved by applying a radial magnetic field transverse to the direction of the discharge current. Our data from non-perturbing probe measurements showed for the first time that the anode fall in HTs can be either negative or positive (electron-attracting anode sheath), depending on conditions at the anode surface. The path for current closure to the anode turns out to be quite subtle in HTs. This path determines the mechanism of the anode fall formation. In varying the magnetic field topology in the channel from a more uniform to a cusp-like one, we uncover intriguing results. For cusp configurations, in which the radial magnetic field changes polarity somewhere along the channel, the anode fall is positive, whereas it is negative for a more uniform field. This polarity difference could be attributed to the decreased electron mobility across the magnetic field in the cusp-like configuration. Our theoretical modeling of the anode sheath correlates well with the experimental results in describing how the magnitude of the sheath varies with the discharge voltage and mass flow rate. [Preview Abstract] |
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