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 JM5: Mini-conference on Energetic Ions and Electrons in Helicon Sources |
Hide Abstracts |
Chair: Rod Boswell, Australian National University Room: Rosen Centre Hotel Salon 11/12 |
Tuesday, November 13, 2007 2:00PM - 2:20PM |
JM5.00001: PIC Modeling of Argon Plasma Flow in MNX Samuel Cohen, Adam Sefkow A linear helicon-heated plasma device - the Magnetic Nozzle Experiment (MNX) at the Princeton Plasma Physics Laboratory - is used for studies of the formation of strong electrostatic double layers near mechanical and magnetic apertures and the acceleration of plasma ions into supersonic directed beams. In order to characterize the role of the aperture and its involvement with ion acceleration, detailed particle-in-cell simulations are employed to study the effects of the surrounding boundary geometry on the plasma dynamics near the aperture region, within which the transition from a collisional to collisionless regime occurs. The presence of a small superthermal electron population is examined, and the model includes a background neutral population which can be ionized by energetic electrons. By self-consistently evaluating the temporal evolution of the plasma in the vicinity of the aperture, the formation mechanism of the double layer is investigated. [Preview Abstract] |
Tuesday, November 13, 2007 2:20PM - 2:40PM |
JM5.00002: Double layers in the Chi-Kung helicon source Christine Charles A current-free double layer (DL) spontaneously forms near the exit of the Chi-Kung helicon source at low pressure with an applied diverging magnetic field for electropositive and electronegative gases. Ion energy distribution functions measured with an energy analyser show a low divergence, accelerated ion beam downstream of the DL. The glass plate terminating the source presents some positive charging and its sheath potential, measured with a planar wall probe, is found to the greater than the DL potential drop. The energy distributions of the trapped and free electrons are measured using a rf compensated probe. Upstream of the DL the EEDF shows a very clear change in slope at energies corresponding to the double layer potential drop. Electrons with lower energy are Maxwellian with a temperature of 8 eV whereas those with higher energy have a temperature of 5 eV. The EEDF in the downstream plasma also has a temperature of 5 eV, suggesting that the downstream electrons come from upstream electrons that have sufficient energy to overcome the potential of the double layer, and that only a single upstream plasma source is required to maintain this phenomenon. Results on the transition from a non-DL plasma to a DL plasma are also presented. [Preview Abstract] |
Tuesday, November 13, 2007 2:40PM - 3:00PM |
JM5.00003: Argon neutral LIF measurements are consistent with no energetic electron population Amy Keesee, Earl Scime Most studies in plasma physics are devoted to studying the ions and electrons that make up plasma. However, any plasma that is not 100{\%} ionized will interact with the neutral gas present in the experiment. Understanding these neutrals can help us better understand plasma characteristics and how neutrals affect the plasma. Measurement of the neutral atoms is available with spectroscopic diagnostics such as laser-induced fluorescence (LIF) and passive emission spectroscopy of neutral lines. However, these measurements apply to an excited neutral atom state, rather than the entire neutral population. A collisional-radiative model describes the relationship between densities of the excited states, given electron densities and energy distributions. Using electron data obtained via Langmuir probe measurements, a collisional-radiative model code is used to compare radial profiles of theoretical excited state densities to those measured experimentally with LIF and passive emission spectroscopy in a helicon source with argon gas. The CR model radial neutral density and electron distribution function profiles can be varied to obtain the best comparison with experimental data. For plasma in helicon mode, the model results best match the experimental data when the radial neutral profile is hollow and electron population consists of a single Maxwellian electron distribution with no energetic electron population. [Preview Abstract] |
Tuesday, November 13, 2007 3:00PM - 3:20PM |
JM5.00004: Excited Ar II Emission Characteristics in Helicon Plasmas John Scharer, Christopher Denning, Matt Wiebold, Alex Degeling, Rod Boswell Wave field, peak Ar II 443 nm emission, and antenna code modeling have shown phase velocities in helicon sources that are in close agreement for moderate density and magnetic field (n$_{e}$ =1.2-4 x 10$^{12}$ /cc and 100-200 G) experiments at the Australian National University and University of Wisconsin. The phase velocities correspond to parallel electron energies in the range of 16-46 eV and the peak electron density occurs 10-15 cm from the end of the antennas indicating that fast accelerated electrons may play a role in helicon source operation. Higher plasma density experiments ($>$2-8x10$^{12}$ cm$^{-3}$ and higher magnetic fields) on the Australian National University and University of Wisconsin helicon facilities have both shown a peak excited Ar II 443 nm rf modulated emission signal that is in phase along the direction parallel to the magnetic field when the plasma is in the dense helicon (or ``blue'') mode. Interpretations of these observations and recent measurements will be presented. [Preview Abstract] |
Tuesday, November 13, 2007 3:20PM - 3:40PM |
JM5.00005: Double layers in electronegative plasmas Albert Meige, N. Plihon, P. Chabert, G.J.M. Hagelaar, J.-P. Boeuf, R.W. Boswell, M.A. Lieberman, A. Lichtenberg Current-free double layers observed in Helicon sources have attracted much interest, both due to their potential applications in space propulsion for example and because of their fundamental properties. In the case of electropositive plasmas, double layers that have been reported are always static and their amplitude is a decreasing function of pressure (within their range of existence), while in the case of electronegative plasmas, they have also been observed to propagate and their amplitude is essentially independent of pressure. In the present paper, focus is put on the static and propagating double layers that have been observed in a Helicon-type reactor filled up with a low-pressure mixture of Ar/SF6: the most significant experimental results are reviewed, an analytical model describing the static double layer is presented and a fully self- consistent hybrid simulation is developed to shed light on the propagating double layer. From this, a formation mechanism is proposed. [Preview Abstract] |
Tuesday, November 13, 2007 3:40PM - 4:00PM |
JM5.00006: Ion Acceleration in a Compact Helicon Source with Various Permanent Magnet Configurations Konstantin Shamrai, Valery Virko, Yury Virko The parameters of plasma and emergent ion beam were examined in a 4.5-cm-diam compact helicon source excited by a double-turn $m$ = 0 antenna and equipped with a multi-component permanent magnet system. The basic magnetic configuration was formed by a radially magnetized cylindrical assembly of the ferrite bars. It could be enhanced by an axially magnetized annular ferrite that was installed near the source outlet to create the magnetic nozzle. The magnetic configuration was found to be a crucial point for production of accelerated ions. At Ar pressure below 1 mTorr and rf power of 600 W, plasma potential in the discharge chamber was 100-120 V, which is by 50-60 V higher than in the drift chamber, but the emergent beam of accelerated ions arose only in the presence of the magnetic nozzle. This implies that electrostatics might not be the only driver for ion acceleration. The beam of accelerated ions had energies up to 120 eV, relative to ground, and the current up to 40 mA, whereas the electron temperature in the discharge chamber was 10-12 eV. The source parameters were optimized by installing various ferrite assemblies and the outlet ferrite shield, and also by testing various driving antennas and frequencies. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700