Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session TI1: Low Temperature Plasmas and Technology |
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Chair: Igor Kaganovich, Princeton Plasma Physics Laboratory Room: Landmark A |
Thursday, November 20, 2008 9:30AM - 10:00AM |
TI1.00001: Permanent-magnet helicon sources and arrays: a new type of RF plasma Invited Speaker: Among radiofrequency (rf) plasma sources used for materials processing in industry, helicon sources are well known for their high density but seldom used because they require a large dc magnetic field, making the source larger, heavier, more complex, and costlier than other available sources. Placing the plasma inside an annular permanent magnet (PM) does not work because the field lines carry the plasma into the wall before it can be ejected towards a substrate. However, a ring magnet has a stagnation point below which the field is weaker but almost straight. Use of a ``low-field peak'' partly compensates for the weak field by spacing a back plate so that the reflected wave constructively interferes. Strong PM helicon discharges were produced in a proof-of principle experiment.\footnote{F.F. Chen and H. Torreblanca, Plasma Phys. Control. Fusion 49, A81 (2007).} The discharge tube was optimized using the HELIC code,\footnote{D. Arnush, Phys. Plasmas 7, 3042 (2000).} resulting in 2'' diam by 2'' high, with a three-turn m = 0 antenna at the bottom end. The NeFeB magnet is 3'' ID x 5'' OD by 1'' high. To cover large substrates, an 8-tube array was constructed with 7'' between tubes. Array sources have three problems: 1) the power must be distributed equally, 2) all tubes cannot be the same distance from the matching circuit, and 3) the transmission lines have to handle the voltage at startup and the current in CW operation. These have been solved in the Medusa 2 experiment which is in a ``sweet spot'' in which the small antennas have the right inductance for the rf system. With 3kW total @ 13.56 MHz, at 7'' below the sources, the density is $\sim $5 x 10$^{11}$ cm$^{-3}$ at 1.3 eV in 15 mTorr of argon, uniform to 3{\%} over the area covered by the tubes. Possible applications are to optical coating, roll-to-roll web processing, flexible and OLED displays, solar cells, and ``smart windows'' with organic solar cells. [Preview Abstract] |
Thursday, November 20, 2008 10:00AM - 10:30AM |
TI1.00002: Development of High-Density Helicon Plasma Source and Its Applications Invited Speaker: High-density helicon plasmas can serve for various applications, including electric propulsion and basic research. Recently, we have developed new sources: the very large, 75-cm-diam and 486-cm-long [1, 2], and the very small, 2.5-cm-diam and 4.7-cm-long [3] ones. We discuss characteristics of these sources, new plasma acceleration schemes, and some underlying physics. The large source uses a multi-turn planar antenna located behind the end-window, i.e., an excitation scheme not widely used and, thus, not well comprehended. With the length reduced down to 81 cm, this source has shown a unique capability to support the discharge with only 1-W input power and to provide in the high-density mode an excellent production efficiency, $\sim $ 10$^{14}$ electrons/W. Profiles of plasma density and wave field depend on the magnetic field and antenna configurations. In particular, we have found the excitation of higher (more than the second order) radial wave modes that do not occur normally in standard helicon sources. Successful high-density plasma production with axial length less than 15 cm was also executed. In the small helicon source, we examined new plasma acceleration schemes with application of the rotating or sawtooth-shaped electric fields with the divergent magnetic field. These methods were found to result in increase of the exhaust velocity up to several km/s and, thus, to have potentiality for electric propulsion. [1] S. Shinohara and T. Tanikawa, Rev. Sci. Instrum. \textbf{75}, 1941 (2004) {\&} Phys. Plasmas \textbf{12}, 044502 (2005), [2] T. Tanikawa and S. Shinohara, Thin Solid Films \textbf{506-507}, 559 (2006), [3] T. Toki \textit{et al.}, IEPC 03-1168, the 28th Int. Electric Propulsion Conf., 2003 {\&} Thin Solid Films \textbf{506-507}, 597 (2006). [Preview Abstract] |
Thursday, November 20, 2008 10:30AM - 11:00AM |
TI1.00003: Non-self-sustained regimes of Hall thruster discharges Invited Speaker: Interesting discharge phenomena are observed between magnetized Hall thruster plasma and the neutralizing cathode, which are highly correlated with the thruster performance. In a typical Hall thruster, a steady-state cross-field discharge is self-sustained between the anode and a hollow cathode placed outside the thruster channel. It is commonly assumed that the thruster discharge current is limited by the ionization of the working gas, wall losses and electron transport across the magnetic field, and not by the supply of electrons from the cathode. We report that for Hall thrusters operating with a high ionization of working gas, the enhancement of the cathode electron emission can lead to a dramatic (up to 20-30{\%} at 50-300 W) narrowing of the plasma plume and a nearly twofold increase in the fraction of high-energy ions [1]. The measured variations of the plasma properties suggest that the electron emission from the cathode can affect the electron cross-field transport and the ionization in the thruster discharge, including generation of multicharged ions. An apparent cooling of plasma electrons observed in these regimes may support recent theoretical predictions [2] of electron kinetic effects in E x B rotating thruster plasmas. As the voltage drop between the cathode and the near-field plasma plume reaches an apparent threshold, these effects on the discharge current and plasma plume parameters finally reach saturation. Thus, it now appears that the maximum available supply of electrons from the cathode to the thruster discharge and the plasma plume can limit efficient generation of the focused plasma flow in Hall thrusters, especially at low powers. [1] Y. Raitses, A. Smirnov and N. J. Fisch, Appl. Phys. Lett. , 221502 (2007). [2] N. J. Fisch et al., in preparation (2008). [Preview Abstract] |
Thursday, November 20, 2008 11:00AM - 11:30AM |
TI1.00004: Pulsed Plasma Electron Sources Invited Speaker: Pulsed ($\sim $10$^{-7}$ s) electron beams with high current density ($>$10$^{2}$ A/cm$^{2})$ are generated in diodes with electric field of $E >$ 10$^{6}$ V/cm. The source of electrons in these diodes is explosive emission plasma, which limits pulse duration; in the case $E <$ 10$^{5}$ V/cm this plasma is not uniform and there is a time delay in its formation. Thus, there is a continuous interest in research of electron sources which can be used for generation of uniform electron beams produced at $E \le $ 10$^{5}$ V/cm. In the present report, several types of plasma electron source (PES) will be considered. The first type of PES is fiber-based cathodes, with and without CsI coating. The operation of these cathodes is governed by the formation of the flashover plasma\footnote{Ya. E. Krasik, J. Z. Gleizer, D. Yarmolich, A. Krokhmal, V. Ts. Gurovich, S.Efimov, J. Felsteiner V. Bernshtam, and Yu. M. Saveliev, J. Appl. Phys. \textbf{98}, 093308 (2005).} which serves as a source of electrons. The second type of PES is the ferroelectric plasma source (FPS).\footnote{Ya. E. Krasik, A. Dunaevsky, and J. Felsteiner, Phys. Plasmas \textbf{8}, 2466 (2001).} The operation of FPS, characterized by the formation of dense surface flashover plasma is accompanied also by the generation of fast microparticles and energetic neutrals.\footnote{D. Yarmolich, V. Vekselman, V. Tz. Gurovich, and Ya. E. Krasik, Phys. Rev. Lett. \textbf{100}, 075004 (2008).} The latter was explained by Coulomb micro-explosions of the ferroelectric surface due to an large time-varying electric field at the front of the expanding plasma. A short review of recent achievements in the operation of a multi-FPS-assisted hollow anode to generate a large area electron beam will be presented as well. Finally, parameters of the plasma produced by a multi-capillary cathode with FPS and velvet igniters\footnote{J. Z. Gleizer, Y. Hadas and Ya. E. Krasik, Europhysics Lett. \textbf{82}, 55001 (2008).} will be discussed. [Preview Abstract] |
Thursday, November 20, 2008 11:30AM - 12:00PM |
TI1.00005: Plasma Tools for Physics with Antimatter Invited Speaker: Research is described that exploits nonneutral plasma techniques to develop new tools to accumulate, manipulate, and store positrons and to make cold, bright antimatter beams. Experiments are described that use test electron plasmas (for enhanced data rate) confined in a Penning-Malmberg trap using a 4.8 T magnetic field to provide strong cyclotron cooling. Recent progress in two areas is discussed. New results are presented for radial compression of plasmas using rotating electric fields [the ``rotating wall'' (RW) technique] in a novel, strong-drive regime.\footnote{J. R. Danielson, C. M. Surko, and T. M. O'Neil, $\it{Phys. Rev. Lett.}$ $\bf{99}$, 135005 (2007).} It is characterized by rigidly rotating plasmas with the density set by the RW drive frequency. The criteria for accessing this regime, a model of the compression process, and possible limits of this technique will be discussed. Second, experiments and theory are described for the extraction of beams with small transverse spatial extent from the center of trapped plasmas.\footnote{J. R. Danielson, T. R. Weber, and C. M. Surko, $\it{Appl. Phys. Lett.}$ $\bf{90}$, 081503 2007).}$^,$\footnote{T. R. Weber, J. R. Danielson, and C. M. Surko, $\it{Phys. Plasmas}$ $\bf{13}$, 123502 (2008).} For small-amplitude pulses, the radial beam profile is Gaussian with a minimum beam radius of 2 Debye lengths. The limits of this technique are identified, and model beam profiles for larger beams are discussed. Applications of these tools and challenges for the future will be discussed. [Preview Abstract] |
Thursday, November 20, 2008 12:00PM - 12:30PM |
TI1.00006: Positron transport and thermalization - the plasma-gas interface Invited Speaker: Low energy positrons are now used in many fields including atomic physics, material science and medicine [1]. Plasma physics is providing new tools for this research, including Penning-Malmberg buffer-gas traps to accumulate positrons and the use of rotating electric fields (the ``rotating wall'' technique) to compress positrons radially and create tailored beams [1]. These devices (now available commercially), which rely in key instances on positron-neutral interactions, are a convenient way to create plasmas and beams for a variety of applications. A deeper understanding of the relevant cooling and loss mechanisms is required to take full advantage of this technology. This talk focuses on a recent study of positrons in such a tenuous gaseous environment in the presence of an applied electric field [2]. Energy-resolved collision cross sections and a Monte Carlo code modified to include positrionium (Ps) formation are used to obtain transport coefficients and the thermalization and Ps-formation rates. A markedly different type of negative differential conductivity is observed (i.e., not seen in electron systems), due to the non-conservative nature of the Ps-formation process. It is particularly prominent in gases with large, highly energy dependent Ps-formation cross sections. The relevance of these calculations to other positron applications will also be discussed, including a currently planned study of positrons in gaseous water. It is hoped that these calculations will inspire a new generation of positron transport experiments.\\ *Work done in collaboration with Z.Lj. Petrovi\'c, A. Bankovi\'c, M. \v{S}uvakov, G. Malovi\'c, S. Dujko, S.J. Buckman. \\ 1. C. M. Surko and R. G. Greaves, \textit{Phys. Plasmas} \textbf{11}, 2333-2348 (2004).\\ 2. A. Bankovi\'c, J. P. Marler, M. \v{S}uvakov, G. Malovi\'c, and Z. Lj. Petrovi\'c, \textit{Nucl. Instrum. and Meth. in Phys. Res. B} \textbf{266}, 462-465 (2008). [Preview Abstract] |
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