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 UI1: Basic Plasma Experiments |
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Chair: Thomas Sunn Pedersen, Columbia University Room: Landmark A |
Thursday, November 20, 2008 2:00PM - 2:30PM |
UI1.00001: Experimental Realization of Nearly Steady-State Toroidal Electron Plasmas Invited Speaker: M.R. Stoneking Non-neutral plasmas are routinely confined in the uniform magnetic field of a Penning-Malmberg trap for arbitrarily long times and approach thermal equilibrium. Theory predicts that dynamically stable and therefore long-lived equilibria exist for non-neutral plasmas confined in the curved, non-uniform field of a \emph{toroidal} trap, but that ultimately thermal equilibrium states do not exist. On long timescales, the poloidal $\mathbf{E}\times \mathbf{B}$ rotation through the non-uniform toroidal magnetic field leads to magnetic pumping transport. A new experiment has, for the first time, demonstrated the existence of a stable, long-lived (\emph{i.e.} nearly steady-state) toroidal equilibrium for pure electron plasmas and is poised to observe the magnetic pumping transport mechanism.\footnote{J.P. Marler and M.R. Stoneking, Phys. Rev. Lett. \textbf{100}, 155001 (2008).} Electron plasmas with densities of order $10^6$ cm$^{-3}$ are trapped in the Lawrence Non-neutral Torus II for several seconds. LNT II is a high aspect ratio ($R_o/a \approx 10$), partially toroidal trap (a 270$^\circ$ arc with $B_o=670$ G). The $m=1$ diocotron mode is launched and detected using isolated segments of a fully-sectored conducting boundary and its frequency is used to determine the total trapped charge as a function of time. The observed confinement time ($\approx 3$ s) approaches the theoretical limit ($\approx 6$ s) set by the magnetic pumping transport mechanism of Crooks and O'Neil.\footnote{S.M. Crooks and T.M. O'Neil, Phys Plamas \textbf{3}, 2533 (1996).} We also present equilibrium modeling and numerical simulation of the toroidal $m=1$ mode constrained by experimental data. Future work includes the identification of the dominant transport mechanisms via confinement scaling experiments and measurement of the $m=2$ mode frequency, and development of a strategy for making a transition to fully toroidal confinement. [Preview Abstract] |
Thursday, November 20, 2008 2:30PM - 3:00PM |
UI1.00002: Observations of a Parallel Force Balance Breaking Instability in Non-neutral Plasmas Confined on Magnetic Surfaces Invited Speaker: Quinn Marksteiner The Columbia Non-neutral Torus (CNT) is a simple stellarator devoted to the study of non-neutral plasmas confined on magnetic surfaces. At low neutral pressures pure electron plasmas in CNT are stable [Physical Review Letters \textbf{97}, p. 095003 (2006)], and have a long confinement time (up to 190 ms), but the plasma goes unstable in the presence of a finite ion fraction (n$_{i}$/n$_{e} \quad \sim $ 0.04). The equilibrium of pure electron plasmas in CNT is determined by parallel force balance between the pressure and the electrostatic field in each magnetic surface, and it was thought that this force balance condition would prevent low frequency oscillations from developing in the plasma, unless the perturbation had a mode structure that resonated with a rational surface of the magnetic field. However, the instability has a measured poloidal mode number of m = 1 [Phys. Rev. Letters \textbf{100}, p. 065002 (2008)], which does not correspond to any rational surfaces in CNT. It is likely that this violation of parallel force balance comes about because CNT has a large (65{\%}) fraction of electrons that are mirror trapped. The frequency scaling and the ion density threshold of this instability are similar to an instability found in pure-toroidal non-neutral plasmas [Phys. Fluids \textbf{12}, 2616 (1969)]; however, the magnitude of the instability in CNT saturates at a level where electron confinement is only weakly reduced. We present an overview of the CNT experiment and a detailed look at key experimental observations related to the instability, including the dependence on neutral pressure, magnetic field strength, and ion species. This work was supported by the NSF-DOE Partnership in Basic Plasma Science, Grant No. NSF-PHY-06-13662, and the NSF CAREER program, Grant No. NSF-PHY-04-49813. [Preview Abstract] |
Thursday, November 20, 2008 3:00PM - 3:30PM |
UI1.00003: Electron Acoustic Waves in Pure Ion Plasmas Invited Speaker: Francois Anderegg Electron Acoustic Waves (EAWs) are the low frequency branch of electrostatic plasma waves; these waves exist in neutralized plasmas,\footnote{D.S. Montgomery {\it et al.}, Phys. Rev. Lett. {\bf 87}, 155001 (2001).} pure electrons,\footnote{A.A. Kabantsev, F. Valentini, and C.F. Driscoll, AIP Conf. Proc. {\bf 862}, 13 (2006).} and pure ion plasmas. The EAWs typically have a phase velocity $V_{\mathrm{phase}} / V_{\mathrm{th}} \sim 1.4$, quite low compared to typical plasma waves. Linear Landau damping would suggest that such slow phase velocity waves are strongly damped; but at finite wave amplitudes, trapping of particles at the phase velocity effectively flattens the distribution function, resulting in a ``BGK-like'' state with weak damping. Our experiments on standing $m_z = 1$, $m_\theta = 0$ waves show that the small-amplitude dispersion relation for both fast Trivelpiece-Gould (TG) and slow (EAW) plasma modes is in close agreement with the ``thumb-shaped'' dispersion relation predicted by kinetic theory neglecting damping.\footnote{J.P. Holloway and J.J. Dorning, Phys. Rev. A {\bf 44}, 3856 (1991).}$^,$\footnote{F. Valentini, T.M. O'Neil and D.H.E. Dubin, Phys. Plas. {\bf 13}, 052303 (2006).} However, the surprise here is that a moderate amplitude ``off-resonant'' drive readily modifies the velocity distribution so as to make the plasma mode resonant with the drive frequency. We have observed the plasma adjusting its velocity distribution so as to become resonant with a 100 cycle drive ranging from 10 kHz to 30 kHz. With a chirped frequency drive,\footnote{W. Bertsche, J. Fajans, L. Friedland, Phys. Rev. Lett. {\bf 91}, 265003 (2003); F. Peinetti {\it et al.}, Phys. Plas. {\bf 12}, 062112 (2005).} the particle velocity distribution suffers extreme distortion, and the resulting plasma wave is almost undamped with $\gamma / \omega \sim 10^{-5}$. Laser-Induced-Fluorescence measurements of the wave-coherent particle distribution ${f} (\mathrm{v}_z , t)$, clearly show particle trapping in the EAW, with trapping widths as expected from theory considering two non-interacting traveling waves forming the standing wave. The coherent $ f (\mathrm{v}_z , t )$ measurement also shows that particles slower than the wave phase velocity $\mathrm v_{\mathrm{ph}}$ oscillate in phase with the wave, contrasting with the 180$^\circ$ out-of-phase response of the particles moving faster than $\mathrm v_{\mathrm{ph}}$. The differing response of the fast and slow particles results in a small net fluid velocity, because the electrostatic restoring force is almost totally balanced by the kinetic pressure, consistent with the low frequency nature of EAW. [Preview Abstract] |
Thursday, November 20, 2008 3:30PM - 4:00PM |
UI1.00004: Global and Local Characterization of Turbulent and Chaotic Structures in a Dipole-Confined Plasma Invited Speaker: B.A. Grierson When the plasma density increases sufficiently, plasma confined by a strong dipole magnetic field exhibit a dramatic transition to a confined state with complex turbulent behaviors. Recent experiments using the Collisionless Terrella Experiment (CTX) used statistical tools and fast imaging to understand this turbulent state with respect to both local and global paradigms. Locally, multi-point and multiple-time autocorrelation and bispectral analyses are computed and used to estimate the linear dispersion and nonlinear structure coupling of a broad band of interacting fluctuations. Globally, the whole-plasma dynamics is observed using a unique high-speed imaging diagnostic that views the time-varying spatial structure of the polar current density. The bi-orthogonal decomposition for multiple space-time points is used to decompose the measured plasma dynamics into spatial and temporal mode functions. The dominant spatial modes are found to be long wavelength and radially broad; however, the amplitudes of these global modes are chaotic and impulsive. In all cases, the fluctuations appear to be interchange-like and consistent with a model for two-dimensional electrostatic interchange mixing. To the best of our knowledge, this is the first time when both local and global dynamics of turbulent structures have been simultaneously measured and compared in hot magnetized plasma. Our measurements are sufficient to compare and contrast two competing paradigms of plasma turbulence: (i) a nonlinear mode-mode structure coupling and cascade derived from a statistical treatment of measurements from closely-spaced probes, and (ii) a chaotic evolution of a few dominant and relatively long-wavelength modes that generate an equivalent local spectrum due to the impulsive amplitudes and time-varying frequencies of the global modes. [Preview Abstract] |
Thursday, November 20, 2008 4:00PM - 4:30PM |
UI1.00005: Basic Physics of Fast Ions and Shear Alfv\'en Waves Invited Speaker: Yang Zhang A campaign is underway at the Large Plasma Device (LAPD) to study fast-ion physics issues of relevance to magnetic fusion. So far, two basic physics experiments have been completed. In the first [1], a multiple magnetic mirror array creates periodic axial variations in the index of refraction of shear Alfv\'en waves. Waves are launched by antennas inserted in the LAPD plasma and diagnosed by B-dot probes at many axial locations. As in fusion devices and other periodic media, spectral gaps are formed due to the Bragg effect. The measured width of the propagation gap increases with the modulation amplitude as expected theoretically. Simulations with a 2D finite-difference code resemble the observed spectra. In the second experiment, a Li$^+$ source [2] launches a population of nearly monoenergetic fast ions in a helium plasma. A loop antenna launches shear Alfv\'en waves at frequencies $\omega$ below the helium cyclotron frequency $\omega_{ci}$ ($\omega/\omega_{ci}=0.3$-0.8). The fast ions interact with the waves through the Doppler-shifted cyclotron resonance, $\omega - k_zv_z=\Omega_f$. (Here $k_z$ is the axial wavenumber, $v_z$ is the fast-ion axial speed and $\Omega_f$ is the fast-ion cyclotron frequency.) A collimated energy analyzer measures the non-classical spreading of the beam, which is proportional to the resonance with the wave. To compare with theory, a Monte Carlo Lorentz code launches fast ions with an initial spread in real/velocity space and random phases relative to the wave fields, which are derived from measured magnetic field data. Both the magnitude and frequency dependence of the calculated beam-spreading agree with experiment. Planned experiments include a study of fast-ion transport by turbulent fluctuations and the nonlinear interaction of fast ions with larger amplitude Alfv\'en waves. In addition to these test-particle experiments, an intense fast-ion source is under development. [1] Yang Zhang et al., Phys. Plasmas 15 (2008) 012103. [2] Y. Zhang et al., Rev. Sci. Instrum. 78 (2007) 013302. [Preview Abstract] |
Thursday, November 20, 2008 4:30PM - 5:00PM |
UI1.00006: Simulations of L and H confinement regimes in the simple magnetized plasma TORPEX and comparisons with experiments Invited Speaker: Paolo Ricci The basic plasma experiment TORPEX contributes to bridging the gap between experimental observations and simulations in the field of plasma turbulence and related transport. TORPEX is a toroidal device, in which a vertical and a toroidal magnetic field create open helical field lines. Similarly to the Scrape-Off Layer (SOL) of magnetic fusion devices, the turbulence driven by magnetic curvature and plasma gradients causes the plasma to be transported in the radial direction, while it is lost due to flows along the field lines. TORPEX facilitates the experimental study of low frequency instabilities and related turbulence, as it allows more detailed diagnostics and wider parameter scans than usually possible in magnetic fusion devices. The relative simplicity of TORPEX provides a useful testbed in which to explore transport physics in the SOL of more complex geometries. Recently, a fluid model has been developed to follow the TORPEX plasma dynamics. The model takes into account plasma sources, parallel losses, and perpendicular transport due to plasma turbulence. Simulations show that, by increasing the plasma source strength, reducing the vertical magnetic field, or increasing the ion mass, a transition from a low (L) to a high (H) confinement mode occurs. In the H-mode, a strong ExB shear limits the perpendicular transport, leading to steeper gradients and larger peak values of electron density and temperature. The TORPEX device is used to explore the accessibility of the H-mode by varying the vertical magnetic field, ion mass and microwave source power. By using the same data analysis techniques for experimental and simulation results, we discuss how the trends predicted by the theory compare with the measured quantities. [Preview Abstract] |
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