Session BI2: Basic and Dusty Plasmas

Laboratory Dipole Plasma Physics

Modern laboratory studies of plasma confined by a strong dipole magnet originated twenty years ago when it was learned that planetary magnetospheres have centrally-peaked plasma pressure profiles that form naturally when solar wind drives plasma circulation and heating. Unlike other internal rings devices, like spherators and octupoles, the magnetic flux tubes of the dipole field expand rapidly with radius. Unlike plasma confinement devices that obtain stability from magnetic shear and average good curvature, like tokamaks and levitrons, the dipole-confined plasma obtains stability from plasma compressibility. These two geometric characteristics of the dipole field have profound consequences: (i) plasma can be stable with local beta exceeding unity, (ii) fluctuations can drive either heat or particles inward to create stationary profiles that are strongly peaked, and (iii) the confinement of particles and energy can decouple. During the past decade, several laboratory dipole experiments and modeling efforts have lead to new understanding of interchange, centrifugal and entropy modes, nonlinear gyrokinetics, and plasma transport. Two devices, the LDX experiment at MIT and RT-1 at the University of Tokyo, operate with levitated superconducting dipole magnets. With a levitated dipole, not only is very high-beta plasma confined in steady state but, also, levitation produces high-temperature at low input power and demonstrates that toroidal magnetic confinement of plasma does not require a toroidal field. Modeling has explained many of the processes operative in these experiments, including the observation of a strong inward particle pinch. Turbulent low-frequency fluctuations in dipole confined plasma cause adiabatic transport and form a fundamental linkage between the radial variation of flux-tube volume and the centrally peaked density and pressure profiles.   

Laboratory realization of an ion-ion hybrid Alfv\'{e}n wave resonator

In a magnetized plasma with two ion species, shear Alfv\'{e}n waves (or guided electromagnetic ion cyclotron [EMIC] waves) have zero parallel group velocity and experience a cut-off near the ion-ion hybrid frequency $\omega_{ii}$ [1]. Since the ion-ion hybrid frequency is proportional to the magnetic field, it is possible, in principle, for a magnetic well configuration to behave as an Alfv\'{e}n wave resonator in a two-ion plasma. The important role played by the wave cut-off at $\omega=\omega_{ii}$ in determining the structure of low frequency wave spectra has long been recognized in space plasma studies. For instance, Temerin and Lysak [2] identified that the narrow-banded ELF waves seen in the S3-3 satellite were generated by the auroral electron beam in a limited spatial region determined by the local value of $\omega_{ii}$ for a mix of H$^{+}$-He$^{+}$ ions. In addition to playing a key role in magnetospheric resonators, EMIC waves and the existence of multiple ion species are also important in the scattering of high-energy electrons in the earth's inner magnetosphere [3]. The present study demonstrates [4] such a resonator in a controlled laboratory experiment (in the Large Plasma Device at UCLA) using a H$^{+}$-He$^{+}$ mixture. The resonator response is investigated by launching monochromatic waves and sharp tone-bursts from a magnetic loop antenna. The topic is also investigated theoretically, and the observed frequency spectra are found to agree with predictions of a theoretical model of trapped eigenmodes. Results of the experiment and theory will also be discussed in their relation to the ion-ion resonator feature proposed for planetary magnetospheres [5-6] and to magnetic confinement devices containing multiple ion species [7].\\[4pt] [1] Vincena, S. T., G. J. Morales, and J. E. Maggs, Phys. Plasmas, 17, 052106 (2010)\\[0pt] [2] Temerin, M, and R. L. Lysak, JGR, 89, 2849 (1984)\\[0pt] [3] Meredith, N. P., R. M. Thorne, R. B. Horne, D. Summers, B. J. Fraser, and R. R. Anderson JGR 108(A6) (2003)\\[0pt] [4] Vincena, S.T., Farmer, W.A., Maggs, J.E., and Morales, G.J., GRL, 38, L11101 (2011)\\[0pt] [5] Guglielmi, V. A., A. S. Potapov, and C. T. Russell, JETP Lett., 72, 298 (2000)\\[0pt] [6] Mithaiwala, M., L. Rudakov, and G. Ganguli, JGR, 112, A09218 (2007)\\[0pt] [7] Intrator, T., M. Vukovic, A. Elfimov, P. H. Probert, and G. Winz, Phys. Plasmas 3 (1996)   

Alpha-channeling in mirror machines

Coupling suitable electromagnetic waves in the ion cyclotron frequency range to plasmas can lead to particle ejection and cooling. This effect can be exploited for extracting energy from alpha particles to sustain the fusion reaction. First suggested for tokamaks and known as alpha-channeling [1], this technique might also improve mirror machine operation. But diffusion in mirror phase space is very different from in tokamaks; the waves are different and there are phase space loss boundaries. This talk summarizes the analytical and computational tools newly developed to study the feasibility of alpha-channeling in linear traps. We identified waves suitable for alpha-channeling in practical mirror devices by optimizing the energy extraction rate with respect to the wave parameters [2]. With the optimal regime identified, we carried out a systematic search for modes with similar parameters in mirror plasmas [3]. Modes suitable for alpha particle energy extraction are then identified in several device designs [4]. Redirection of the energy extracted from alpha particles is shown to be possible in principle by either collisional relaxation of resonant minority ions [5], or by coupling the alpha-channeling mode to the ICRH waves in the tandem mirror device plugs. As a spinoff result, in the generalized extraction problem in a network of diffusion paths, we derived a fundamental theorem [6]. \\[4pt] [1] N. J. Fisch and J. M. Rax, \textit{Phys. Rev. Lett.} \textbf{69}, 612 (1992). \\[0pt] [2] A. I. Zhmoginov and N. J. Fisch, \textit{Phys. Plasmas} \textbf{15}, 042506 (2008). \\[0pt] [3] A. I. Zhmoginov and N. J. Fisch, \textit{Phys. Plasmas} \textbf{16}, 112511 (2009). \\[0pt] [4] A. I. Zhmoginov and N. J. Fisch, \textit{Fusion Sci. Tech.} \textbf{57}, 361 (2010). \\[0pt] [5] A. I. Zhmoginov and N. J. Fisch, submitted to \textit{Phys. Plasmas}. \\[0pt] [6] A. I. Zhmoginov and N. J. Fisch, \textit{Phys. Lett. A} \textbf{372}, 5534 (2008).   

Symmetry-breaking transitions in dusty plasma clusters

We consider a cluster of $n$ identical charged particles which repel each other through a Debye (i.e., a shielded Coulomb or Yukawa) potential and which are confined by a two-dimensional biharmonic well. In the strong-coupling regime, the particles' arrangement is determined by $n$, the Debye parameter $\kappa$, and the well anisotropy $\alpha$ [1-3]. For large $\alpha$, the particles lie in a one-dimensional straight-line configuration. As $\alpha$ is reduced, the cluster undergoes a transition to a two-dimensional configuration via the zigzag instability [2,3]. In the opposite limit of an isotropic well, the ground state configuration is ``circular.'' In particular, for $n=6$ or 8 particles, the isotropic configuration has a single particle in the center which is surrounded by the remaining $n-1$ particles. Since the zigzag and isotropic configurations have differing symmetries, the symmetry of the zigzag configuration must be broken in order to transition to the isotropic configuration. We have determined the symmetry-breaking mechanism by experimentally characterizing dusty plasma clusters as the anisotropy of the potential well is varied. For $n=6$ and 8 particles, we find that the zigzag configuration becomes a finite 2-chain, which zigzags a second time to produce a 4-chain thereby pushing one particle into the center of the cluster. For $n=10$ particles, the 2-to-4-chain instability pushes two particles inside the cluster, giving the expected (2,8) isotropic ground-state configuration.\\[4pt] [1] A. Melzer, Phys. Rev. E {\bf 73}, 056404 (2006).\\[0pt] [2] T. E. Sheridan and K. D. Wells, Phys. Rev. E {\bf 81}, 016404 (2010).\\[0pt] [3] T. E. Sheridan and A. L. Magyar, Phys. Plasmas {\bf 17}, 113703 (2010).   

Linear and Nonlinear Dust Acoustic Waves, Shocks and Stationary Structures in a dc-Glow-Discharge Dusty Plasma

In 1990, Rao, Shukla, and Yu (Planet. Space Sci. 38, 543) predicted the existence of the dust acoustic (DA) wave, a low-frequency ( $\sim $ few Hz), compressional dust density wave that propagates through a dusty plasma at a phase speed $\sim $ several cm/s. The DA wave was first observed by Chu et. al., (J. Phys. D: Appl. Phys. 27, 296, 1994) in an rf-produced dusty plasma, and by Barkan et. al., (Barkan et. al. Phys. Plasmas 2, 2161, 1995) who obtained video images of the DA wave trains using light scattering from a dust suspension confined in an anodic glow discharge plasma formed within a Q machine plasma. The dispersion relation for DAWs was measured by Thompson et. al., (Phys. Plasmas 4, 2331, 1997) in a dc glow discharge dusty plasma by modulating the discharge current at a set frequency. DAWs have been investigated by many groups both in weakly-coupled and strongly-coupled dusty plasmas (E. Thomas, Jr., Contrib. Plasma Phys. 49, 316, 2009). In most experiments where DA waves are present, the wave amplitude is relatively high, indicating that they are nonlinear. In this talk, results of our recent experiments on DAWs will be presented. The following experiments, performed in a dc glow-discharge dusty plasma will be described: (1) Observations of spontaneously excited nonlinear, cylindrical DAWs, which exhibit confluence of waves propagating at different speeds. (2) Investigations of self-steepening DAWs that develop into DA shocks with thicknesses comparable to the interparticle separation (Heinrich et. al., Phys. Rev. Lett. 103, 115001, 2009). (3) Measurements of the linear growth rates of DAWs excited in merging dust clouds. (4) The formation of stationary, stable dust density structures appearing as non-propagating DAWs (Heinrich et. al., Phys. Rev. E, in press, 2011). This work was performed in collaboration with S. H. Kim, J. R. Heinrich, and J. K. Meyer.   

Ion Acoustic Waves in Ultracold Neutral Plasmas

Ultracold neutral plasmas, which are created by photoionizing laser cooled atoms near the ionization threshold, have been extensively studied in order to probe strong Coulomb coupling effects, low-energy atomic processes, equilibration, and collective phenomena [1]. The experimental study of collective modes, however, has previously been limited to phenomena involving electrons. By spatially modulating the intensity pattern of the photoionizing laser, we are now able to create controlled density perturbations on the plasma, which enables study of ion collective behavior. Periodic modulation excites ion acoustic waves [2]. We have also created two distinct plasmas that stream into each other. In the hydrodynamic regime, the central gap between the two plasmas splits into two density ``holes'' that propagate away from the plasma center at the ion acoustic velocity. At lower densities and higher particle velocities, plasmas are less collisional, and we observe kinetic effects such as plasma streams penetrating each other, with a penetration depth that reflects the ion stopping power. This general technique for sculpting the density opens many new possibilities, such as investigation of non-linear phenomena, instabilities, and shock waves in the ultracold regime, and determination of the effects of strong coupling on dispersion relations. The low temperature, small size, plasma expansion, and strongly coupled nature of ultracold plasmas make these studies fundamentally interesting. They may also shed light on similar phenomena in high energy density, laser-produced plasmas that can be near the strongly coupled regime. \\[4pt] [1] T. C. Killian, T. Pattard, Thomas Pohl, and J. M. Rost, Phys. Rep., 449, 77 (2007). \\[0pt] [2] J. Castro, P. McQuillen, and T. C. Killian, Phys. Rev. Lett. 105, 065004 (2010).