Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session GI3: Fundamental Plasma Physics I |
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Chair: Robert Merlino, University of Iowa Room: Ballroom BC |
Tuesday, October 30, 2012 9:30AM - 10:00AM |
GI3.00001: Lyman-alpha radiation of a probing metastable hydrogen beam to measure electric fields in diluted fluids and plasmas Invited Speaker: Fabrice Doveil The interaction between a metastable H(2s) atomic hydrogen beam and an external electric field leads to the emission of the Lyman-$\alpha $ line. It originates in the Stark mixing of the near-degenerate 2$s_{1/2}$ and 2$p_{1/2}$ levels separated by the Lamb shift [1]. The quenched radiation proportional to the square of the electric field amplitude is recovered in vacuum by using such an atomic probe beam. For larger electric field, saturation is observed and related to the beam finite transit time. We also observe the strong enhancement of the signal when the field is oscillating at the Lamb shift frequency. This technique is applied in a plasma, offering an alternative way to measure weak electric fields by direct and non-intrusive means [2]. \\[4pt] This work was inspired by late Prof. R.A. Stern to whom it is dedicated. It was done in collaboration with L. Ch\'{e}rigier-Kovacic. It was the subject of A. Lejeune's PhD thesis and was supported by a grant from Minist\`{e}re de la Recherche. The author acknowledges the help of G. Bachet and G. Prasad for the conception and construction of the experimental set-up. \\[4pt] [1] W.E. Lamb, Jr., Rep. Prog. Phys. 14, 19 (1951)\\[0pt] [2] A. Lejeune, L. Ch\'{e}rigier-Kovacic, F. Doveil, Appl. Phys. Lett. 99, 181502 (2011) [Preview Abstract] |
Tuesday, October 30, 2012 10:00AM - 10:30AM |
GI3.00002: The structure of a magnetic field propagating through low-resistivity, multi-ion species plasma Invited Speaker: Ramy Doron The study of low-resistivity plasmas interacting with transient electromagnetic fields is important for the understanding of a variety of fundamental phenomena. Previous research revealed rich physics and intriguing phenomena, including the long-standing problem of rapid magnetic-field penetration into low-resistivity plasmas. Inconsistencies between observations and theories invoked the possibility that unexplored processes, occurring in scales that are beyond the experimental resolution, play a significant role in the interaction. In the present study, significantly improved diagnostic capabilities of the plasma and magnetic-fields are developed and employed. In the configuration studied, a pulsed current (rise-time 350 ns) generating the magnetic field ($\sim $ 1 T), is driven through a plasma that prefills the volume between two electrodes. The structure of the propagating magnetic-field front is reconstructed and its width is used for estimating the plasma conductivity. The magnetic-field front structure and velocity are found to remain nearly constant when the field propagates over a length scale of the order of the front width. This observation allows for treating the magnetic-field front as an electric potential hill in the moving frame of the field. Using the properties of the potential hill, derived are the details of the ion dynamics according to their charge-to-mass ratios (Z/m). Ions of relatively low Z/m are penetrated by the magnetic-field, whereas ions of high Z/m are reflected off the magnetic-field at different field magnitudes. The inferred ion dynamics are used to predict the electron density evolution, which is found to agree with the observed density evolution. [Preview Abstract] |
Tuesday, October 30, 2012 10:30AM - 11:00AM |
GI3.00003: Dust cluster explosion Invited Speaker: Khare Avinash A model for the dust cluster explosion where micron/sub-micron sized particles are accelerated at the expense of plasma thermal energy, in the afterglow phase of a complex plasma discharge is proposed. The model is tested by MD simulations of dust particles in a confining potential. The nature of the explosion (caused by switching off the discharge) and the concomitant dust acceleration is found to depend critically on the pressure of the back ground neutral gas. At low gas pressure, the explosion is due to unshielded Coulomb repulsion between dust particles and yields maximum acceleration while in the high pressure regime it is due to shielded Yukawa repulsion and yields much feebler acceleration. These results are in agreement with recent experimental findings. Our simulations also confirm a recently proposed electrostatic (ES) isothermal scaling relation, $P_E \propto V_d^{-2} $(where $P_E $ is the ES pressure of the dust particles and $V_d$ is the confining volume). [Preview Abstract] |
Tuesday, October 30, 2012 11:00AM - 11:30AM |
GI3.00004: Effective magnetization of the dust particles in a complex plasma Invited Speaker: Hanno K\"ahlert The large mass and size of the dust particles in a complex plasma has several advantages, including low characteristic frequencies on the order of a few Hz and the ability to record their motion with video cameras. However, these properties pose major difficulties for achieving strong magnetization. While the light electrons and ions can be magnetized by (superconducting) magnets, magnetizing the heavy dust component is extremely challenging. Instead of further increasing the magnetic field strengths or decreasing the particle size, we use the analogy between the Lorentz force and the Coriolis force experienced by particles in a rotating reference frame to create ``effective magnetic fields'' which is a well-established technique in the field of trapped quantum gases~[1]. To induce rotation in a complex plasma, we take advantage of the neutral drag force, which allows to transmit the motion of a rotating neutral gas to the dust particles~[2]. The equations of motion in the rotating frame agree with those in a stationary gas except for the additional centrifugal and Coriolis forces~[3]. Due to the slow rotation frequencies ($\sim$ Hz) and contrary to the situation in a strong magnetic field, only the properties of the heavy dust particles are notably affected. Experiments with a rotating electrode realize the desired velocity profile for the neutral gas and allow us to verify the efficiency of the concept~[3].\\[4pt] This work was performed in collaboration with J. Carstensen, M. Bonitz, H. L\"owen, F. Greiner, and A. Piel.\\[4pt] [1] A. L. Fetter, Rev. Mod. Phys. {\bf 81}, 647 (2009)\\[0pt] [2] J. Carstensen, F. Greiner, L.-J. Hou, H. Maurer, and A. Piel, Phys. Plasmas~{\bf 16}, 013702 (2009)\\[0pt] [3] H. K\"ahlert, J. Carstensen, M. Bonitz, H. L\"owen, F. Greiner, and A. Piel, submitted for publication, arXiv:1206.5073 [Preview Abstract] |
Tuesday, October 30, 2012 11:30AM - 12:00PM |
GI3.00005: Electron Acceleration by High Power Radio Waves in the Ionosphere Invited Speaker: Paul Bernhardt At the highest ERP of the High Altitude Auroral Research Program (HAARP) facility in Alaska, high frequency (HF) electromagnetic (EM) waves in the ionosphere produce artificial aurora and electron-ion plasma layers. Using HAARP, electrons are accelerated by high power electrostatic (ES) waves to energies $>$100 times the thermal temperature of the ambient plasma. These ES waves are driven by decay of the pump EM wave tuned to plasma resonances. The most efficient acceleration process occurs near the harmonics of the electron cyclotron frequency in earth's magnetic field. Mode conversion plays a role in transforming the ES waves into EM signals that are recorded with ground receivers. These diagnostic waves, called stimulated EM emissions (SEE), show unique resonant signatures of the strongest electron acceleration. This SEE also provides clues about the ES waves responsible for electron acceleration. The electron gas is accelerated by high frequency modes including Langmuir (electron plasma), upper hybrid, and electron Bernstein waves. All of these waves have been identified in the scattered EM spectra as downshifted sidebands of the EM pump frequency. Parametric decay is responsible low frequency companion modes such as ion acoustic, lower hybrid, and ion Bernstein waves. The temporal evolution of the scattered EM spectrum indicates development of field aligned irregularities that aid the mode conversion process. The onset of certain spectral features is strongly correlated with glow plasma discharge structures that are both visible with the unaided eye and detectable using radio backscatter techniques at HF and UHF frequencies. The primary goals are to understand natural plasma layers, to study basic plasma physics in a unique ``laboratory with walls,'' and to create artificial plasma structures that can aid radio communications. [Preview Abstract] |
Tuesday, October 30, 2012 12:00PM - 12:30PM |
GI3.00006: Excitation of Transverse Dipole and Quadrupole Modes in a Pure Ion Plasma in a Linear Paul Trap to Study Collective Processes in Intense Beams Invited Speaker: Erik Gilson Transverse dipole and quadrupole modes have been excited in a one-component cesium ion plasma trapped in the Paul Trap Simulator Experiment (PTSX) in order to characterize their properties, and understand the effect of their excitation on equivalent long-distance beam propagation. The PTSX device is a compact laboratory Paul trap that simulates the transverse dynamics of a long, intense charge bunch propagating through an alternating-gradient transport system by putting the physicist in the beam's frame of reference. A pair of arbitrary function generators was used to apply trapping voltage waveform perturbations with a range of frequencies and, by changing which electrodes were driven with the perturbation, with either a dipole or quadrupole spatial structure. The results presented in this paper explore the dependence of the perturbation voltage's effect on the amount of trapped charge, the perturbation duration and amplitude. Perturbations were also applied that simulate the effect of random lattice errors that exist in an accelerator with quadrupole magnets that are misaligned or have variance in their field strength. The experimental results quantify the growth in the equivalent transverse beam emittance that occurs due to the applied noise and demonstrate that the random lattice errors interact with the trapped plasma through the plasma's internal collective modes. Coherent periodic perturbations were applied to simulate the effects of magnet errors in circular machines such as storage rings. The trapped one component plasma is strongly affected when the perturbation frequency is commensurate with a plasma mode frequency. The experimental results, which help to understand the physics of quiescent intense beam propagation over large distances, are compared with analytic models and particle-in-cell simulations. [Preview Abstract] |
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