Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session TI3: Non-neutral Plasmas, Fusion, and Beams: The Legacy of Ron DavidsonInvited
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Chair: Mike Mauel, Columbia University Room: 210 ABEF |
Thursday, November 3, 2016 9:30AM - 10:00AM |
TI3.00001: A perspective on the contributions of Ronald C. Davidson to plasma physics Invited Speaker: Jonathan S. Wurtele Starting in the 1960s and continuing for half a century, Ronald C. Davidson made fundamental theoretical contributions to a wide range of areas of pure and applied plasma physics. Davidson was one of the founders of nonneutral plasma physics and a pioneer in developing and applying kinetic theory and nonlinear stability theorems to collective interaction processes and nonlinear dynamics of nonneutral plasmas and intense charged particle beams. His textbooks on nonneutral plasmas are the classic references for the field and educated generations of graduate students. Davidson was a strong advocate for applying the ideas of plasma theory to develop techniques that benefit other branches of science. For example, one of the major derivative fields enabled by nonneutral plasmas is the study of antimatter plasmas and the synthesis of antihydrogen. This talk will review a few highlights of Ronald Davidson's impact on plasma physics and related fields of science. [Preview Abstract] |
Thursday, November 3, 2016 10:00AM - 10:30AM |
TI3.00002: How do classical particle-field systems become unstable? -- The last physics problem that Ronald Davidson studied. Invited Speaker: Hong Qin Many of the classical particle-field systems in (neutral and nonneutral) plasma physics and accelerator physics become unstable when the system parameters vary. How do these instabilities happen? It turns out, very interestingly, that all conservative systems become unstable by the same mechanism, i.e, the resonance between a positive- and a negative-action modes. And this is the only route that a stable system can become unstable. In this talk, I will use several examples in plasma physics and accelerator physics with finite and infinite degrees of freedom to illustrate the basic physical picture and the rigorous theoretical structure of the process. The features at the transition between stable and unstable regions in the parameter space are the fundamental characteristics of the underlying real Hamiltonian system and complex G-Hamiltonian system. The resonance between a positive- and a negative-action modes at the transition is the Krein collision well-known to mathematicians. [Preview Abstract] |
Thursday, November 3, 2016 10:30AM - 11:00AM |
TI3.00003: Evolution of an electron plasma vortex in a strain flow. Invited Speaker: J. R. Danielson Coherent vortex structures are ubiquitous in fluids and plasmas and are examples of self-organized structures in nonlinear dynamical systems. The fate of these structures in strain and shear flows is an important issue in many physical systems, including geophysical fluids\footnote{D.~G.~Dritschel and B.~Legras, {\it Phys. Today} {\bf 46}, 44 (1993).} and shear suppression of turbulence in plasmas.\footnote{P.~W.~Terry, {\it Rev. Mod. Phys.} {\bf 72}, 1 (2000).} In two-dimensions, an inviscid, incompressible, ideal fluid can be modeled with the Euler equations, which is perhaps the simplest system that supports vortices. The Drift-Poisson equations for pure electron plasmas in a strong, uniform magnetic field are isomorphic to the Euler equations, and so electron plasmas are an excellent test bed for the study of 2D vortex dynamics.\footnote{C.~F.~Driscoll, D.~Z. Jin, D.~A.~Schecter, D.~H.~E.~Dubin, {\it Physica C} {\bf 369}, 21 (2002)} This talk will describe results from a new experiment using pure electron plasmas in a specially designed Penning-Malmberg (PM) trap to study the evolution of an initially axisymmetric 2D vortex subject to externally imposed strains. Complementary vortex-in-cell simulations are conducted to validate the 2D nature of the experimental results and to extend the parameter range of these studies. Data for vortex destruction using both instantaneously applied and time dependent strains with flat (constant vorticity) and extended radial profiles will be presented. The role of vortex self-organization will be discussed. A simple 2D model\footnote{S.~Kida, {\it J. Phys. Soc. Japan} {\bf 50}, 3517 (1981).} works well for flat vorticity profiles. However, extended profiles exhibit more complicated behavior, such as filamentation and stripping; and these effects and their consequences will be discussed. [Preview Abstract] |
Thursday, November 3, 2016 11:00AM - 11:30AM |
TI3.00004: Impact of physics and technology innovations on compact tokamak fusion pilot plants Invited Speaker: Jonathan Menard For magnetic fusion to be economically attractive and have near-term impact on the world energy scene it is important to focus on key physics and technology innovations that could enable net electricity production at reduced size and cost. The tokamak is presently closest to achieving the fusion conditions necessary for net electricity at acceptable device size, although sustaining high-performance scenarios free of disruptions remains a significant challenge for the tokamak approach. Previous pilot plant studies have shown that electricity gain is proportional to the product of the fusion gain, blanket thermal conversion efficiency, and auxiliary heating wall-plug efficiency. In this work, the impact of several innovations is assessed with respect to maximizing fusion gain. At fixed bootstrap current fraction, fusion gain varies approximately as the square of the confinement multiplier, normalized beta, and major radius, and varies as the toroidal field and elongation both to the third power. For example, REBCO high-temperature superconductors (HTS) offer the potential to operate at much higher toroidal field than present fusion magnets, but HTS cables are also beginning to access winding pack current densities up to an order of magnitude higher than present technology, and smaller HTS TF magnet sizes make low-aspect-ratio HTS tokamaks potentially attractive by leveraging naturally higher normalized beta and elongation. Further, advances in kinetic stabilization and feedback control of resistive wall modes could also enable significant increases in normalized beta and fusion gain. Significant reductions in pilot plant size will also likely require increased plasma energy confinement, and control of turbulence and/or low edge recycling (for example using lithium walls) would have major impact on fusion gain. Reduced device size could also exacerbate divertor heat loads, and the impact of novel divertor solutions on pilot plant configurations is addressed. For missions including tritium breeding, high-thermal-efficiency liquid metal breeding blankets are attractive, and novel immersion blankets offer the potential for simplified fabrication and maintenance and reduced cost. Lastly, the optimal aspect ratio for a tokamak pilot plant is likely a function of the device mission and associated cost, with low aspect ratio favored for minimizing TF magnet mass and higher aspect ratio favored for minimizing blanket mass. The interplay between a range of physics and technology innovations for enabling compact pilot plants will be described. [Preview Abstract] |
Thursday, November 3, 2016 11:30AM - 12:00PM |
TI3.00005: Nonlinear Whistler Wave Physics in the Radiation Belts Invited Speaker: Chris Crabtree Wave particle interactions between electrons and whistler waves are a dominant mechanism for controlling the dynamics of energetic electrons in the radiation belts. They are responsible for loss, via pitch-angle scattering of electrons into the loss cone, and energization to millions of electron volts. It has previously been theorized that large amplitude waves on the whistler branch may scatter their wave-vector nonlinearly via nonlinear Landau damping leading to important consequences for the global distribution of whistler wave energy density and hence the energetic electrons. It can dramatically reduce the lifetime of energetic electrons in the radiation belts by increasing the pitch angle scattering rate. The fundamental building block of this theory has now been confirmed through laboratory experiments. Here we report on in situ observations of wave electro-magnetic fields from the EMFISIS instrument on board NASA's Van Allen Probes that show the signatures of nonlinear scattering of whistler waves in the inner radiation belts. In the outer radiation belts, whistler mode chorus is believed to be responsible for the energization of electrons from 10s of Kev to MeV energies. Chorus is characterized by bursty large amplitude whistler mode waves with frequencies that change as a function of time on timescales corresponding to their growth. Theories explaining the chirping have been developed for decades based on electron trapping dynamics in a coherent wave. New high time resolution wave data from the Van Allen probes and advanced spectral techniques are revealing that the wave dynamics is highly structured, with sub-elements consisting of multiple chirping waves with discrete frequency hops between sub-elements. Laboratory experiments with energetic electron beams are currently reproducing the complex frequency vs time dynamics of whistler waves and in addition revealing signatures of wave-wave and beat-wave nonlinear wave-particle interactions. These new data suggest that these weak turbulence processes may be playing a role in saturating the nonlinear instability. [Preview Abstract] |
Thursday, November 3, 2016 12:00PM - 12:36PM |
TI3.00006: A mechanistic interpretation of the wave-particle interaction of Landau Invited Speaker: Thomas M. O'Neil There are two halves to the wave-particle interaction: first there is the effect of the wave on the resonant particles and second the effect of the resonant particles back on the wave. This presentation will focus on the second half of the interaction, which is usually described through Poisson’s equation, or equivalently, through a dispersion relation obtained from Poisson’s equation. For example, for the case of a Langmuir wave with phase velocity on the tail of a Maxwellian velocity distribution, the resonant electrons make a small imaginary contribution to the wave dispersion relation, which yields a small imaginary contribution to the wave frequency, implying wave damping. An alternate, more mechanical interpretation, starts from the observation that the wave-induced displacement of the non-resonant electrons satisfies an oscillator equation that is driven by the bare electric field from the resonant electrons. This field drives the oscillator resonantly since the resonant electrons travel at the wave phase velocity. From this perspective, the wave damping simply results from the drive of the bare electric field from the resonant electrons back on the wave oscillator. The resonant wave-particle interaction also occurs in waves that are governed by \textbf{ExB} drift dynamics, such as diocotron waves that are excited on a nonneutral plasma column. The column undergoes an \textbf{ExB} drift rotation, and at a resonant radius, the rotational flow matches the azimuthal phase speed of the wave, yielding a wave-particle resonance. Again a mechanical interpretation of the wave damping is possible. The bare electric field from the resonant particles produces \textbf{ExB} drift motion that symmetrizes the plasma column, that is, damps the wave. This mechanistic interpretation also works for the case of Landau growth and for the case where nonlinear effects, such as trapping, play a role in the resonant particle dynamics. \\ \\In collaboration with Chi Yung Chim. [Preview Abstract] |
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