Plasma Physics Seminar
 
   
  Spring 2017 Schedule  
 
February 3, Friday 2PM
ERF 1207, Large Conference Room
Prof. Troy Carter, UCLA
Electromagnetic turbulence and transport in increased-beta LAPD plasmas
February 8, Wednesday 4PM
ERF 1207, Large Conference Room
Open

February 15, Wednesday 4PM
ERF 1207, Large Conference Room
Open

February 22, Wednesday 4PM
ERF 1207, Large Conference Room
Dr. Alfred Mallet, University of New Hampshire
Intermittency and Anisotropy in Alfvenic Turbulence


On length scales larger than the ion gyroradius, the turbulence in a plasma with a strong mean magnetic field may be modelled using the equations of reduced magnetohydrodynamics, which describe the evolution of Alfvenic fluctuations propagating up and down the magnetic field. This (strongly nonlinear) turbulence is (i) anisotropic with respect to the direction of the local mean magnetic field - a steeper spectral index (for example) in the parallel direction compared to the perpendicular. (ii) "aligned" - vector fluctuations in the fields in the perpendicular plane point in the same direction to within a small angle. (iii) highly intermittent - the shape of the probability distributions of many (but not all!) turbulent quantities depends on scale in a non-trivial way. I will discuss the work we have performed connecting these phenomena, and the resulting statistical model for the Alfvenic turbulence we have developed.
March 1, Wednesday 4PM
ERF 1207, Large Conference Room
Open

March 8, Wednesday 4PM
ERF 1207, Large Conference Room
External Review

March 15, Wednesday 4PM
ERF 1207, Large Conference Room
Prof. Ben Maruca, University of Delaware
Thermalization of Solar-Wind Ions via Coulomb Collisions

The solar wind consists of hot, magnetized plasma that supersonically streams from the solar corona into deep space. The ions therein are rarely in thermal equilibrium because Coulomb collisions, through which particles exchange energy, act in the solar wind on timescales comparable to those of its expansion. Even the two most abundant ion-species, protons (ionized hydrogen) and alpha-particles (fully ionized helium), rarely have equal temperatures. In-situ observations from the Wind spacecraft's Faraday cups reveal that, at 1 AU from the Sun, values of the alpha-proton temperature-ratio have a complex, bimodal distribution. To better understand this, a theoretical model was developed to account for collisional and expansion effects and was then applied to the observations to backtrack them from 1 AU to the corona. Remarkably, the extrapolated alpha-proton temperature-ratios show a simple, monomodal distribution. Thus, the bimodality observed in the 1-AU distribution may simply result from the incomplete thermalization of protons and alpha-particles. This result suggests that the relative heating of different ion species in the corona may be more uniform than previously believed.
March 22, Wednesday 4PM
ERF 1207, Large Conference Room
Spring Break - No Seminar

March 29, Wednesday 4PM
ERF 1207, Large Conference Room
Eric Shi, Princeton University
Gyrokinetic continuum simulations of turbulence in open-field-line plasmas


The scrape off layer (SOL) of a tokamak is an edge region in which plasma flows freely along open magnetic field lines towards a material surface, where plasma losses are mediated by a Debye sheath. It is important to develop numerical codes to study the SOL because an accurate prediction for the SOL heat-flux-channel width in ITER is necessary to address major concerns regarding damage to plasma-facing components by the exhaust power. While the use of sophisticated gyrokinetic particle-in-cell and continuum codes for studies in the tokamak core region has become widespread, major code extensions or new codes are required to handle the additional challenges of the edge region. In this talk, I will present results from my thesis work on developing a gyrokinetic continuum code for the simulation of plasma turbulence in a model SOL. I will discuss details about our numerical approach, which is based on discontinuous Galerkin methods, and gyrokinetic sheath model boundary conditions. I will show results from our simulations of turbulence in the Large Plasma Device at UCLA, including the suppression of turbulence by applied flow shear, and results from our recent work in modeling a NSTX-like SOL to investigate turbulent heat-flux spreading.
April 5, Wednesday 4PM
ERF 1207, Large Conference Room
Dr. Colin Komar, NASA/Catholic University

April 12, Wednesday 4PM
ERF 1207, Large Conference Room
Dr. Fatima Ebrahimi, Princeton Plasma Physics Lab

April 19, Wednesday 4PM
ERF 1207, Large Conference Room
Dr. Luca Comisso, Princeton Plasma Physics Lab
Plasmoid Instability in General Current Sheets


We present the recent formulation of a general theory of the onset and development of the plasmoid instability [1]. We consider the general problem of a reconnecting current sheet that can evolve in time, rather than assuming a fixed Sweet-Parker current sheet. The new theoretical framework has lead to completely new results, which have shown that previously obtained power laws are insufficient to capture the correct properties of the plasmoid instability. The new scaling laws are shown to depend on the initial perturbation amplitude, the characteristic rate of current sheet evolution, and the Lundquist number. The detailed dynamics of the instability is also elucidated, and shown to comprise of a long period of quiescence followed by sudden growth over a short time scale.

[1] L. Comisso, M. Lingam, Y.-M. Huang, A. Bhattacharjee, Phys. Plasmas 23, 100702 (2016)
April 26, Wednesday 4PM
ERF 1207, Large Conference Room
Dr. Leila Mays, NASA/Goddard

May 4, Thursday 4PM
ERF 1207, Large Conference Room
Prof. Tünde Fülöp, Chalmers University of Technology, Sweden

May 10, Wednesday 4PM
ERF 1207, Large Conference Room
Dr. Chris Crabtree, Naval Research Lab