Session H11: Plasma Physics

Inconsistency between Keplerian rotation and MHD induction equation; particle-based accretion dynamo

Keplerian rotation and the ideal MHD Ohm's law are shown to represent respectively the {large gravity, small magnetic field} and the {small gravity, large magnetic field} limits of the dynamical equation governing charged particle motion. Being a different limit of the same equation, Keplerian rotation therefore cannot be used in conjunction with the ideal MHD Ohm's law. Because the MHD induction equation is a consequence of the ideal MHD Ohm's law, the induction equation also cannot be used in conjunction with the assumption of Keplerian rotation. This realization invalidates the very large number of plasma astrophysics models that combine the ideal MHD induction equation or the ideal MHD Ohm's law with Keplerian rotation. Important examples of models that are invalidated are the Keplerian accretion dynamo and the magnetorotational instability. A new, particle-based dynamo model which avoids these problems is outlined. This non-MHD dynamo involves accumulation of certain specific types of charged particles falling down a gravitational potential to create a battery-like electrostatic potential suitable for driving the axial currents associated with bipolar astrophysical jets. Analysis of the particle motion shows that the infalling particles lose all their mechanical angular momentum as they descend down the gravitational potential. Thus, `beads-on-a-string' models for astrophysical jet acceleration are also invalidated.   

``Crystal'' Magnetic Structure in Axisymmetric Plasma Accretion Disks

A general class of stationary magnetic configurations that can exist in (thin) plasma accretion disks is identified by solving analytically [1] the coupled non-linear equations that describe the radial and the vertical equilibrium conditions of the disk. These configurations have a ``crystal'' structure characterized by a sequence of pairs of toroidal current filaments that can involve nul points of the poloidal magnetic field. The obtained solutions are valid in the limit where the magnetic energy density is smaller than the thermal energy density ($\beta > 1 $). In view of studying magnetic disk configurations for which jets can emerge, and for which the limit where $\beta \sim 1$ is important, the relevant equilibrium equations are derived and their symmetries are pointed out. In the limit where the toroidal current densities are weak and a linearized approximation can be adopted, the resulting equilibrium configuration becomes that described by the presence of a marginally stable axisymmetric ballooning mode [2]. \par \noindent [1] B. Coppi, M.I.T.-LNS Report HEP 04/09, to be published in Phys. of Plasmas.\par \noindent [2] B. Coppi and P.S. Coppi, {\it Phys.Rev. Lett.} {\bf 87}, 051101 (2001) and B. Coppi and E.A. Keyes, {\it Ap. J.} {\bf 595}, 1000 (2003).   

Investigation of the Coherent Synchrotron Radiation Instability

The emission of coherent synchrotron radiation (CSR) is undesirable for the operation of bunch compressors needed to inject low emittance beams into a linear collider. On the other hand CSR may help to understand the high brightness temperature and the observed spectra emitted by radio pulsars. Analytical results obtained from solving the Vlasov-Maxwell equations for a collisionless, relativistic, finite-strength, cylindrical layer of charged particles in free space are compared to similar results obtained from MHD. We emphasize the effect of the betatron oscillations in layers with non-zero thickness which lead to a significant decoherence and a characteristic spectrum which is similar to the observed spectra of radio pulsars. The particle-in-cell code OOPIC has been used to verify some aspects of our model including the scaling of the growth rates and the emitted power as a function of Lorentz factor, number density and energy spread. The relation of the CSR instability with other instabilities will be discussed briefly.   

Plasma Generated Spherules

Z-pinch plasma simulations have been performed that indicate the production of spherules under certain experimental parameters. (A. L. Peratt, private communication) While performing experiments dealing with the impact of plasma discharges on various materials, we observed that spherules were created at the surface of some of the materials. For specific materials and conditions, spherules were always produced. Both individual spherules and joined spherules were created. The size and shapes were nearly identical to items found by the Mars rover, \textit{Opportunity}, and called ``blueberries.'' Sky {\&} Telescope, June 2004, p. 20, among other sources indicated the blueberries were gray spherules composed of hematite. The experiments produced hematite spherules identical in appearance to those found on Mars. These experiments suggest how the newly discovered blueberries were formed on Mars while providing an explanation that does not depend on the presence of water.   

Examination of Liner Stability During Magnetic Implosion Using Experiments and Simulations

The onset of Magneto-Raleigh-Taylor (MRT) instabilities in metal liners, when excessive current is applied, has been the limitation on use of these liners. In several experiments where melting of the liner was present the outside liner surface was observed to remained stable(B$\sim $0.5 to 1.3 MG). Analysis of this and other cases compared to MHD simulations enabled us to examine the effect of drive conditions on the instabilities. Variations of drive conditions, initial liner surface, and EOS show two factors that effect instability. While the nature of the instability still is fundamentally the acceleration of a fluid interface, a liquid/vapor phase change drastically accentuates growth. Observed cases that remained stable even after melting suggest there may be drive conditions that maintain the aluminum at conditions above the saturated liquid line and reduce or delay the MRT instabilities. We propose that gradient of force within the melted liner also impact these growth rates. We will present analysis, data, and simulations that examine these mechanisms that effect instability growth.   

Schmidt Numbers at an Interface between Two Plasmas

We review the concept of Schmidt number as a predictor of characteristics of the mixing of two fluids, along an interface. We then formulate Schmidt numbers for a particular type of plasma system of general interest; namely, a system made up of electrons, light ions, and partially stripped heavy ions. The light and heavy ions are considered to exist as two distinct fluid components, in contact along an interface and coupled by a permeating gas of free electrons, and self-consistent electric fields. Such a system might be descriptive of a tokomak plasma, in the vicinity of a very hot ablating wall or limiter, made from a refractory metal.   

The Columbus Concept in the Context of a Science Based Approach to Fusion Research

Exploring the physics of meaningful fusion burning plasmas by a spectrum of complementary experiments is a keystone of the “Science First” approach [1] to fusion research. The rationale for this approach is that the possible discovery of new phenomena and the understanding of known ones, in particular sawtooth oscillations, under fusion burning conditions will contribute to define the characteristics of future fusion reactors. Accordingly, we have progressed in the study of the Columbus experiment [2] that is designed to reach ignition conditions in D-T plasmas where the heating due to the produced $\alpha$-particles compensates for all energy losses. Columbus adopts all the design criteria and solutions of the Ignitor device, but has increased dimensions ($R_0 \simeq 1.5$ m is the plasma torus major radius, $a \simeq 0.53$ m and $b \simeq 0.98$ m are the minor radii) and plasma currents $I_p \simeq 12.2$ MA that is the value that the ITER concept would produce for the same stability safety factor without, however, reaching ignition. Columbus is proposed for construction in the U.S. while Ignitor has been developed in Italy. \par \noindent [1] B. Coppi, M.I.T.-RLE report PTP02/04 (2002), Presentation to the National Research Council. \par \noindent [2] B. Coppi M. Salvetti, M.I.T.-RLE report PTP02/06 (2002).   

Nuclear Diagnostics of Implosion and Fusion Burn at the National Ignition Facility"

The National Ignition Facility will be the most revolutionary scientific facility of the early 21st century. The ability to drive inertial confinement fusion implosions with over 1 MegaJoule of laser energy will lead to tremendous advances in the study of fusion burn, hydrodynamic mix, and other research areas. Diagnostics of the fusion burn will be central to these studies. Los Alamos is leading efforts to develop three main fusion product diagnostic systems for the NIF: a Neutron Imaging System using pinhole-apertures; a burn or reaction history diagnostic that measures the DT fusion gammas using a Gas Cerenkov Detector; and a set of Neutron Scintillators to measure relative fusion yield and ion temperature, possibly with time dependence. All these systems are presently under development using implosions at the OMEGA Laser at the University of Rochester. An overview of the requirements and systems proposed for NIF will be presented. This work was performed under the auspices of the U.S. Department of Energy by the Los Alamos National Laboratory under Contract No. W- 7405-ENG-36.   

Protonic Theory of Dielectric Breakdown of Water

We are investigating how electric sparks form in heavy water (D$_{2}$O) in order to test our model of dielectric breakdown in regular water (H$_{2}$0). The precursors to these sparks are called leaders, which are charged plasma filaments with densities greater than liquid water. Our model is a one dimensional plasma wave that relies on a concomitant increase in dissociation rates and proton mobility. Since there is no electronic mechanism in this model, it predicts a significant difference between breakdown in water and heavy water. We have measured both the electric field necessary and the time needed to create an electric spark in samples of heavy water and regular water that have identical electrical conductivity. The electrodes have a point to plate geometry and were examined under a scanning electron microscope before and after each discharge in order to test the leading electronic theory of Jones and Kunhardt\footnote{H.M. Jones and E.E. Kunhardt 1995 \textit{J. Phys. D: Appl. Phys.} \textbf{28} 178-188\par }, which is based on the existence of microscopic asperities. We will also show the agreement between our model and our experimental results, including preliminary calculations showing the density of a leader.