- Seminars at CIPS

At NIST we confine 104 to 106 Be+ ions in a Penning trap and use lasers to cool the ions to T < 10 mK. At these extremely low temperatures the ions form Coulomb liquids or crystals and provide an excellent laboratory system in which to study strongly-coupled plasmas, soft-condensed matter, precision spectroscopy, and quantum computing. In this talk I will focus on studies of modes in these systems, including recent experiments in which wakes are generated by pushing on the crystal with a laser beam.

Intensely charged particle beams can produce a low density "halo" surrounding the core of the beam. The primary cause of this halo is a resonant interaction between individual particles and the core. This is an important issue in various applications of high intensity charged particle beams because halo particles can hit the walls of the accelerator, causing radioactivation. In this presentation we discuss the use of nonlinear focusing as a possible method to control beam halos. Results obtained from a particle-core model and from a one dimensional particle-in-cell simulation will be shown.

Recent observations by the FAST satellite have provided high-time-resolution measurements of three interrelated phenomena in the downward current region of the auroral ionosphere: intense parallel electric fields (e.g. double layers) localized to tens of Debye lengths; drifting localized bipolar field structures interpreted in terms of electron phase-space holes; and intense quasi-electrostatic whistler emissions (VLF saucers) originating on the same field lines as the bipolar structures. Numerical simulations and theoretical modeling suggest how these observations may be related. 1-D open-boundary Vlasov simulations show that a density depression in an equipotential plasma carrying a field-aligned current can produce a strong localized parallel electric field (i.e., a potential jump) characteristic of a classical double layer. The electrons accelerated by this field interact with a low-velocity population on the high-potential side to produce a series of electron phase space holes propagating away from the potential ramp. Multi-dimensional (magnetized) PIC and Vlasov simulations of the two-stream instability show that electron phase-space holes initially develop coherence perpendicular to B, thus forming "tubes" in phase space. However, these tubes later become unstable due to a resonant interaction of electrostatic whistlers (or lower-hybrid waves) with vibrational modes of the phase-space tubes. The whistler waves generated by this instability may be the source of the observed VLF saucers.

We have developed an object oriented framework for a new plasma physics simulation code, VORPAL. Through the use of recursion and template specialization VORPAL is designed to run in any number of physical dimensions without loss to performance. The dimension of the simulation is set at runtime allowing for a quick run to be done in 2D to get qualitative results, then move to 3D with the same code and nearly the same input file. VORPAL has a fully general 3D domain decomposition with message passing using MPI. This gives the VORPAL the capacity to incorporate load balancing. The VORPAL framework supports multiple models for the both the fields and the particles. Currently a Yee Mesh finite differencing scheme for the electromagnetic fields and a cold fluid representation of the particles are in place. VORPAL also has the moving-window capability for following phenomena, like laser pulse propagation, that move at a given speed. We will present simulations of laser-plasma interactions, in particular the generation of laser wakefields, using VORPAL.

A primary challenge in plasma physics is understanding how very small, low-frequency turbulent fluctuations cause energy and particle transport. Including electron physics in any detail is quite a challenge in turbulence simulations. Electrons have a very small mass, and move extremely fast relative to the phase velocities of interest, yet their dynamics is still very important. Electrons drive instabilities and the transport of electrons needs to be better understood. Here, we report on recent progress including fully kinetic electrons and their impact on transport.

The Sandia Z machine is the world's most powerful x-ray source, achieving peak powers of over 300 TW. Researchers are using these high-power x-rays for a variety of experiments, with applications to astrophysics, equation-of-state physics, and inertial confinement fusion. This talk will review recent results from these experiments and discuss some of the active areas of research in improving the performance of the Z machine.

Recent measurements on C-MOD and other fusion devices shows that (i) the density and particle flux in the scrape-off-layer (SOL) is intermittent (turbulent) in space and time, and (ii) non-diffusive transport of particles can play a major role in the SOL. The mechanism for this transport is not yet understood. One possible mechanism for fast convective plasma transport in the SOL may be related to the plasma filaments observed in simulations of turbulence and in experiments using fast cameras and probes. Previous work [1] suggested that high density plasma filaments or "blobs" (highly localized perpendicular to B but with extended structure along B) could detach from the bulk plasma, possibly as a result of turbulence, and would propagate to the outer wall due to the curvature-drift induced polarization and the associated ExB radial drift. In this talk the physical arguments of Ref. 1 will be extended by calculations using a two-field (density, potential) fluid model. I will discuss the properties of density and vorticity blobs, the dependence of the SOL density and plasma flux profiles on the size distribution of the blobs, and the role of ionization. Relevance to experiments will also be discussed.

Work in collaboration with J. R. Myra (Lodestar) and S. I. Krasheninnikov (UCSD)

Branch prediction and speculative execution consist of making probabilistic predictions about the likely near-term evolution of the near-Earth space, and distributing among the cluster machines simulations that assume each of the probabilistically predicted outcomes as initial conditions. As the near-Earth space evolves and real-time satellite data get assimilated into the algorithm, some of the speculatively executed simulations will be proved wrong. At that point the machines that were executing them will be reassigned either to new lines of speculative simulation, or to increase the processing power devoted to more promising simulations already executing. Branch prediction and speculative execution have been very successful in the design of microprocessors, allowing CPUs to attain average processing speeds much higher than linear code execution would permit. The scheme will be demonstrated using particle simulations to tune the parameters of WINDMI, a low-dimensional nonlinear dynamical model of the coupled Magnetosphere-Ionosphere system. Upgrading the scheme to be used with more demanding 3D MHD simulations will also be discussed.

I will describe my efforts in trying to understand some features of the interaction mechanisms in dusty plasmas. Two aspects will be discussed.

First, microscopic processes will be considered. Over the last few years, I have developed several simulation techniques implemented in computer codes and I have developed mathematical physics models for studying fundamental processes in dusty plasmas. I will briefly overview these efforts. The main focus of the presentation will be the results obtained in using such techniques to understand the presence of attractive forces in dusty plasmas. Attractive forces are believed to be responsible for some lattice structures observed in dusty plasma crystals as well as for some coagulation processes. Two mechanisms will be presented, discussed and analyzed quantitatively using simulation methods: plasma wakes and dipole moments induced by asymmetric charging in flowing plasmas (found in space and at the edge of sheaths in glow).

Second, global collective effects will be discussed. The simulation techniques used here differ profoundly to those considered above. Monte Carlo and Molecular Dynamics methods will be presented and applied to study macroscopic effects and transitions of state in complex fluids and in complex plasmas.

Wires carrying high (megaAmpere) currents are suspected of having a thin, hot, ionized corona surrounding a cooler, more dense core region. The details of how these wires behave is important to high-power, wire array z-pinch x-ray sources such as Sandia's Z machine. We model a these wires as a Bennett equilibrium with a skin current and radially increasing temperature profile. Using the ALEGRA-MHD code developed at Sandia, we investigate the m=0 linear growth rates for this profile and compare them to growth rates for a low-current (constant current density and temperature) wire.

The current perturbation associated with magnetic reconnection has been measured in the Madison Symmetric Torus (MST). A reversed field pinch plasma configuration such as MST normally exhibits strong magnetic field fluctuations due to tearing modes. Large amplitude, highly spatially localized perturbations in the parallel current density are expected to occur in the region of the reconnection. One such region, the reversal surface, is in the plasma edge. This region was accessed using a pair of insertable probes, each with Rogowskii coils and magnetic coils. The current perturbation's radial structure is broad, comparable to the expected island width, rather than highly localized. The magnetic fluctuation driven radial charge flux due to these perturbations was also measured. This charge flux is proportional to the flux surface average of the product of the parallel current density perturbation and the radial magnetic field perturbation. The measured charge flux, considered as a difference between ion and electron fluxes, is small compared to the total particle flux.

Solar flares, the reconnection-mediated release of magnetic energy stored in the coronal magnetic field, are physical phenomena of great interest in and of themselves, as well as for their geomagnetic consequences. In addition, according to a conjecture put forth a little over a decade by E.N. Parker, a population of very common but very small flares (the so-called nanoflares) might be responsible for coronal heating. In this talk I will first briefly review the coronal heating problem and Parker's conjecture. I will then discuss a model related to Parker's conjecture, in which flares take the form of an avalanche of small reconnection events in a complex stressed magnetic field driven to a state of self-organized criticality.

Recent initiatives in space physics have emphasized understanding of solar disturbances and the quantitative prediction of resultant magnetospheric and ionospheric phenomena. These initiatives go under the name of `space weather'. Such quantitative predictions are of practical importance but also test of our understanding of the mechanisms by which the Sun influences the Earth. Solar activity in the form of solar flares, coronal mass injections (CME), and recurrent high speed streams influence Earth's space weather by increasing ionization in the ionosphere and by producing magnetic storms and their associated electrical currents, energetic charged particle injections, and aurora. Of increasing concern as mankind relies more on satellite systems are energetic particles, which can lead to satellite failure through radiation damage. In particular the MeV electrons, also known as `killer electrons', have a deleterious impact on satellite subsystems through deep dielectric discharging. Of special concern is the radiation environment at geostationary orbit where the largest number of satellites is located.

Here I report on a new method of predicting MeV electron fluxes at geostationary orbit 1-2 days in advance using only solar wind measurements. Using this method we have achieved a prediction efficiency of 0.82 and a linear correlation of 0.84 for the two years 1995 and 1996. Using the same model parameters based on the years 1995-1996, the prediction efficiency and the linear correlation for the five year period 1995-1999 are 0.61 and 0.77, respectively. The model is based on the standard radial diffusion equation, which is solved by setting the phase space density larger at the outer boundary than the inner boundary and by making the diffusion coefficient a function of solar wind velocity and interplanetary magnetic field. This model also provides a physical explanation for some observed features of the correlation between the solar wind and the electron flux.

Non-neutral plasmas (e.g. pure-electron plasmas) confined in simple cylindrical traps provide excellent laboratory systems for the study of fluid dynamics and basic plasma transport processes. One such transport process is the viscous transport of particles and angular momentum across magnetic field lines. This type of transport brings about the confined thermal equilibrium state of rigid rotation and nearly uniform density. We have measured the coefficient of viscosity in pure-electron plasmas and find it to be as much as 8 orders of magnitude larger than what is predicted by Boltzmann collisional theory. The enhanced viscosity is due to long range "ExB drift collisions" where particles interact over distances on the order of the Debye length. (Here the Debye length is much larger than the cyclotron radius.) In this talk I will discuss these viscous transport measurements and their implications. In addition, I will provide a partial overview of the experimental and theoretical work done by the Non-neutral plasma group at U.C. San Diego.

The physics of kinetic electrons and electromagnetic fluctuations are key challenges in microturbulence simulation research. Recently, we have made progress in this area by developing a drift-kinetic electron model using both the split-weight scheme and the canonical parallel momemtum formulation of gyrokinetics in a fully nonlinear three-dimensional toroidal field-line-following simulation. This model includes magnetic field perturbations perpendicular to the equilibrium magnetic field. Numerical issues arising from the resolution of the magnetic skin depth currently limit these simulations to small beta and progress in this area will be reported. A complementary hybrid simulation with fully gyrokinetic ions and a zero-inertia electron fluid has been developed as well. The electron fluid equations are derived from moments of the drift kinetic equation and a predictor-corrector scheme for the fluid-hybrid model has been implemented in three-dimensional toroidal field-line-following geometry. This is a much simpler electron model and works well at high beta. We are currently using both models to study the effects of electron dynamics on turbulence, including particle transport (which is zero in simulations using adiabatic response), kinetic Alfven modes and modification to zonal flows due to kinetic electrons and the generation of zonal fields through including parallel vector potential. Both hybrid and the fully kinetic simulations have been carefully benchmarked with linear theory in the slab limit. Simulation results for turbulence with both trapped-electron drive and ion-temperature-gradient drive will be presented. We will report results including the fluctuation spectra and transport levels (particle and energy) for both the ions and electrons for core H-mode plasma parameters.