This collaboration of 3 UCSD faculty will address a broad range of fundamental processes in plasmas, and will emphasize problems for which precise experimental tests of theory can be obtained. The experiments will be performed on camera-diagnosed pure-electron plasmas and laser-diagnosed pure-ion plasmas. The research will include work in 6 general areas: 1) Theory work on anti-hydrogen formation and relaxation via plasma collisions will continue, ranging from O'Neil's "Guiding Center Atom" regime to the more deeply-bound chaotic regimes. 2) Experiments have now measured the Salpeter enhancement of perp-to-parallel collisions in strongly correlated plasmas, and future work will quantitatively test theory ideas and apply them to "burn-front" propagation dynamics. 3) Experiments and theory on long-range ExB drift interactions will continue, characterizing heat transport, viscosity, and particle transport limited by background shear flows, utilizing laser-tagging to image the elusive thermally-excited convection cells. 4) Experiments will quantify the wave-coherent particle distribution function in the novel "Electron Acoustic Mode", with theory support to understand the extreme frequency-variability of these nonlinear waves. 5) Experiments and theory will continue on 2D fluid vortex dynamics, inviscid wave damping, and "vortex crystals," including statistical mechanics descriptions of these novel states. 6) Experiments and theory will develop the new "chaotic scattering" regime of neoclassical transport, which occurs when theta-rippled trapping separatrices are driven by plasma rotation. This work may be "transformative", given the broad range of wave damping, nonlinear wave coupling, and particle transport effects which are being observed and analyzed.
This research environment is ideal for graduate student and post-doctoral training. The research has strong interdisciplinary connections with atomic physics, fluid dynamics, and statistical mechanics, as well as relevance to fusion plasmas. This research is also funded by DoE under a separate award under the NSF-DoE Partnership in Plasma Science and Engineering joint solicitation.
T.M. O’Neil, C.F. Driscoll, D.H.E. Dubin University of California at San Diego NSF 0903877 July 2009—June 2014 Experiments and theory were developed on a broad range of fundamental plasma effects, emphasizing problems for which precise experimental tests of theory can be obtained. Joint funding in the NSF/DoE Partnership for Basic Plasma Physics funded 3 senior faculty, 2 research scientists, and 4 graduate students. The non-neutral plasma experiments are performed on two mature, well instrumented magnetic confinement apparatuses. The laser-diagnosed ion apparatus has the singular capability to "tag" ions and follow their subsequent motion, to study processes by which plasmas escape from magnetic bottles. [Image 1: Laser system generates 2 frequency-stable ultraviolet beams for diagnostics and manipulation of ion plasmas] The camera-diagnosed electron apparatus can operate in a regime where the electrons flow across the magnetic field like a low-viscosity fluid such as water; or it can operate in regimes where individual particle motions are important. Theory provides support and interpretation for the experiments. More broadly, it develops new insights into plasma equilibration, waves, and loss processes, including the unusual regimes found in atomic physics, astrophysics, statistical physics, and fluid dynamics. Significant results were obtained in the 6 broad areas originally proposed, described in 31 journal articles and 2 Ph.D. theses, available at NNP.ucsd.edu/Publications/. A) Energetic particle collisions in the correlated (near crystalline) regime. Our experiments are the first to quantitatively test the Salpeter theory of "equlibrium shielding" in correlated plasmas, which causes enhanced collisionality. Here, the results also apply to fusion collisions in stars. B) Applications to Anti-Hydrogen and Quantum Computing. These two technologies utilize many experimental techniques developed for non-neutral plasmas, such as the "rotating wall" technique for infinite-time confinement. Theory work has described the process of positive- and negative-charge recombination constrained by strong magnetic fields, as appropriate to anti-hydrogen formation. Theory work also suggests how to make flat, almost perfect crystalline lattices of ions for possible application in quantum computing devices. C) Long-range ExB Drift Collisions. Classic plasma theory describes the interaction between charged particles when they make a close collision; but under many circumstances significant interactions occur over larger distances, as the particles "drift" in their mutual electric fields. Theory and experiments now clarify the particle and energy diffusion and loss processes which can occur, including the novel effects of "velocity caging." D) Plasma Waves New understandings have been developed on compressional plasma waves, which are analogous to sound waves in air. With laser diagnostics, we are able to accurately determine the ion velocities as they "slosh" in large amplitude waves. This sloshing modifies the character of the waves and creates nonlinear couplings to other waves. [Image 2: Contour plot of experimental data shows particle "trapping" in a moderate amplitude Langmuir wave.] Recent experiments and theory have also clarified the plasma effects which cause frequency shifts in high-frequency "cyclotron" waves, representing the coherent spiral motion of particles around magnetic field lines. The frequencies of these waves are used to accurately determinate molecular masses in the chemical and biological research communities, so understanding the frequency shifts is crucial. E) 2D Vortices Recent electron experiments and theory have characterized a novel "flux-driven" process which causes vortices to become (or remain) circular, even when the fluid viscosity is near zero. [Image 3: Camera data shows an elliptical vortex and (blue) vorticity halo as a flux of particles crosses the critical radius r_s2.] Our prior research characterized other low-viscosity damping effects which are found in large-scale geophysical flows, and the applicability of this flux-driven damping is being investigated. F) Transport across trapping barriers. Electric and magnetic trapping barriers are ubiquitous in plasma confinement devices, and these "speed bumps" cause many different types of plasma loss. Recent research has identified a "chaotic" loss process, which can occur even when the plasma collisionality is low, as is characteristic of fusion containment devices.
