Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session PM9: Mini-Conference: Nonthermal Ions in Space and Laboratory Plasmas |
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Chair: Nikolai Pogorelov, University of Alabama in Huntsville Room: Salon ABC |
Wednesday, October 29, 2014 2:00PM - 2:15PM |
PM9.00001: A tutorial on neutral beam injection into tokamaks W.W. Heidbrink Neutral beam injection heats most magnetic fusion experiments. A typical source injects 2 MW of 80 keV deuterons. Deposition is governed by electron impact ionization and charge exchange with thermal ions. A ``halo'' cloud of thermal neutrals surrounds the ``footprint'' of injected neutrals. After ionizing, the energetic ions are confined by the magnetic field, eventually forming an axisymmetric fast-ion population. Fast ions that orbit through the beam footprint sometimes reneutralize. Escaping neutrals and light emitted by reneutralized fast ions is used to diagnose the fast-ion population. The initial beam deposition and halo cloud are also measured optically. [Preview Abstract] |
Wednesday, October 29, 2014 2:15PM - 2:30PM |
PM9.00002: Superthermal Ion Transport and Acceleration in Multiple Contracting and Reconnecting Inertial-scale Flux Ropes in the Solar Wind Jakobus le Roux, Gary Zank, Gary Webb MHD turbulence simulations with a strong large-scale magnetic field show that the turbulence is filled with quasi-2D inertial-scale flux ropes that intermittently reconnect. Solar wind observations indicate that the statistical properties of the turbulence agree well with the MHD turbulence simulations, while particle simulations stress how ions can be efficiently accelerated to produce power law spectra when traversing multiple flux ropes. Recent observations show the presence of different size inertial-scale magnetic islands in the slow solar wind near the heliospheric current sheet, evidence of island merging, and of heating of ions and electrons in the vicinity. We will present a new statistical transport theory designed to model the acceleration and transport of superthermal ions traversing multiple contracting and reconnecting inertial-scale quasi-2D flux ropes in the supersonic slow solar wind. A steady-state solution for the accelerated particle spectrum in a radially expanding solar wind will discussed, showing that the theory potentially can explain naturally the existence of superthermal power-law spectra observed during quiet solar wind conditions. [Preview Abstract] |
Wednesday, October 29, 2014 2:30PM - 2:45PM |
PM9.00003: A hybrid model for computing nonthermal ion distributions in a long mean-free-path plasma Xianzhu Tang, Chris McDevitt, Zehua Guo, Herb Berk Non-thermal ions, especially the suprathermal ones, are known to make a dominant contribution to a number of important physics such as the fusion reactivity in controlled fusion, the ion heat flux, and in the case of a tokamak, the ion bootstrap current. Evaluating the deviation from a local Maxwellian distribution of these non-thermal ions can be a challenging task in the context of a global plasma fluid model that evolves the plasma density, flow, and temperature. Here we describe a hybrid model for coupling such constrained kinetic calculation to global plasma fluid models. The key ingredient is a non-perturbative treatment of the tail ions where the ion Knudsen number approaches or surpasses order unity. This can be sharply constrasted with the standard Chapman-Enskog approach which relies on a perturbative treatment that is frequently invalidated. The accuracy of our coupling scheme is controlled by the precise criteria for matching the non-perturbative kinetic model to perturbative solutions in both configuration space and velocity space. Although our specific application examples will be drawn from laboratory controlled fusion experiments, the general approach is applicable to space and astrophysical plasmas as well. [Preview Abstract] |
Wednesday, October 29, 2014 2:45PM - 3:00PM |
PM9.00004: Acceleration of pickup ions by intermittent compressive plasma waves in interplanetary space Ming Zhang Pickup ions are created in interplanetary space when penetrated interstellar neutral atoms are ionized by the solar wind or UV. Their initial speed in the plasma frame is equal to the solar wind speed $V_{sw}$ and they are the dominant supra-thermal population. Their distribution function is basically flat below $V_{sw}$, and is often accompanied by a power-law high-energy tail that sometimes can extend much above $V_{sw}$. More interestingly, power-law slope index is often very close to -5. Fisk and Gloeckler (2008, ApJ, v686, p1466) suggested this spectrum is produced by stochastic acceleration in compressive plasma turbulence or waves. We have studied the behaviors of particle acceleration by compressive wave trains. It is found that the waves can efficiently accelerate pickup ions with an exponential increase of pressure. This pressure may moderate the wave amplitude, so that the system eventually establishes equilibrium. At the point, a $p^{-5}$ distribution is automatically fulfilled by balancing the effect of acceleration with any particle loss mechanisms. This situation is more easily achieved with intensive intermittent compressive waves that only occupy a limited volume of space. We hope these phenomena will be verified in laboratory plasma experiments. [Preview Abstract] |
Wednesday, October 29, 2014 3:00PM - 3:15PM |
PM9.00005: The coupling between pick-up ions and energetic neutral atoms in the heliosphere Jacob Heerikhuisen, Eric Zirnstein, Gary Zank, Nikolai Pogorelov The expansion of the solar wind into the interstellar medium creates the heliosphere. While the ionized components of the solar wind and interstellar medium don't mix, neutrals from interstellar space enter the heliosphere where they may experience charge-exchange collisions with ions. The charge-exchange process creates a new non-thermal ion known as a pick-up ion. Such ions tend to experience different dynamic processes at the solar wind termination shock, or indeed any other shocks, and hence give rise to a broad non-thermal population of ions in the heliosheath. Charge-exchange of non-thermal ions gives rise to a population of energetic neutrals which in turn form a seed population for pick-up ions in the interstellar medium near the heliosphere. In this talk we discuss the coupling of various pick-up ion and energetic neutral populations throughout the heliospheric interface and present our latest simulation results. [Preview Abstract] |
Wednesday, October 29, 2014 3:15PM - 3:40PM |
PM9.00006: Observations of Interstellar Pickup Ions and their Suprathermal Tails in Interplanetary Space and in the Heliosheath George Gloeckler, Len Fisk Since the invention of space-borne time-of-flight mass spectrometers in the late 1990s, distribution functions of singly charged interstellar pickup ions, produced primarily by charge exchange with the solar wind and by photoionization of the interstellar neutral gas, have been observed from 1 to $\sim$ 5 AU in interplanetary space. Here we summarize observed characteristics of pickup ion spectra (primarily of H$^{+}$ and He$^{+})$ as well as of the pickup ion tails that are readily produced in Local Acceleration Regions in space, both at 1 AU and in the heliosheath, and briefly discuss the most likely mechanisms for producing interstellar pickup ions as well as their tails that in the heliosheath extend to high ($\sim$ 10 MeV/nuc) energies. [Preview Abstract] |
Wednesday, October 29, 2014 3:40PM - 3:55PM |
PM9.00007: Kappa distributions: Founding statistical mechanics in space plasmas George Livadiotis Space plasmas are collisionless systems out of thermal equilibrium described by a single type or a more complex combination of kappa distributions. These distributions have recently received impetus, as they provide efficient modeling for observed distributions in numerous space plasmas throughout the heliosphere. Moreover, theoretical developments showed the connection of kappa distributions with non-extensive statistical mechanics, an unambiguous generalization of the classical Boltzmann-Gibbs statistics, revealing the robust physical meaning of temperature, pressure, and other thermodynamic parameters. The kappa distributions and the proven tools of non-extensive statistical mechanics have been successfully applied to a variety of space plasmas throughout the heliosphere, from the inner heliosphere, e.g., the solar wind and planetary magnetospheres, to the outer heliosphere, e.g., the inner heliosheath and beyond. These analyses led to the determination of the thermodynamic variables and the understanding of the underpinning physical processes of these plasmas, as well as to more fundamental findings, such as the new quantization constant that characterizes collisionless space plasmas. [Preview Abstract] |
Wednesday, October 29, 2014 3:55PM - 4:15PM |
PM9.00008: Non-thermal plasmas in fusion tokamak edge C.S. Chang Nonthermal plasma is an important part of the tokamak edge physics, if not dominant, and makes the fluid or thermal equilibrium physics to be limited. The non-thermal plasma in the edge region is generated by plasma loss along open magnetic field lines, wall-recycled neutral particles, sputtered impurity particles, orbit loss cone in the velocity space and strong plasma turbulence. They affect the edge plasma confinement through modification of electric field, parallel heat loss, plasma flow, and turbulence transport. Various generation mechanisms of the non-thermal plasmas, their dynamics, and their effect on the plasma transport will be discussed in detail using simulation results from the plasma kinetic code XGC in realistic diverted edge geometry. [Preview Abstract] |
Wednesday, October 29, 2014 4:15PM - 4:30PM |
PM9.00009: Modeling Non-thermal Ions in the Heliosphere Nikolai Pogorelov, Matthew Bedford, Igor Kryukov, Gary Zank Interactions of flows of partially ionized, magnetized plasma are frequently accompanied by the presence of both thermal and non-thermal components in the ion distribution function. If a non-thermal component of ions is formed due to charge exchange and collisions between the thermal ions and neutrals, it experiences the action of magnetic field, its distribution function is isotropized, and it soon acquires the bulk velocity of the ambient plasma without being thermodynamically equilibrated. This situation, e.g., takes place in the outer heliosphere -- a part of the solar plasma region beyond the solar system whose properties are determined by the solar wind (SW) interaction with the local interstellar medium (LISM). We describe a new physical model of the SW flow suitable for description of the SW--LISM interaction involving thermal and nonthermal ion components. This approach is incorporated into a suite of computer codes developed at the University of Alabama in Huntsville (MS-FLUKSS, Multi-Scale FLUid-Kinetic Simulation Suite). We compare results of our modeling with direct measurements made by the fleet of NASA's near-Earth spacecraft and Voyagers providing data from the SW region beyond the heliospheric termination shock and the heliopause. [Preview Abstract] |
Wednesday, October 29, 2014 4:30PM - 4:45PM |
PM9.00010: Knudsen and inverse Knudsen layer effect on tail ion distribution and fusion reactivity in inertial confinement fusion targets C.J. McDevitt, X.-Z Tang, Z. Guo, H.L. Berk A series of reduced models are used to study the fast ion tail in the vicinity of a transition layer between plasmas at disparate temperatures and densities, which is typical of the gas-pusher interface in inertial confinement fusion targets. Emphasis is placed on utilizing progressively more comprehensive models in order to identify the essential physics for computing the fast ion tail at energies comparable to the Gamow peak. The resulting fast ion tail distribution is subsequently used to compute the fusion reactivity as a function of collisionality and temperature. It is found that while the fast ion distribution can be significantly depleted in the hot spot, leading to a reduction of the fusion reactivity in this region, a surplus of fast ions is present in the neighboring cold region. The presence of this fast ion surplus in the neighboring cold region is shown to lead to a partial recovery of the fusion yield lost in the hot spot. [Preview Abstract] |
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