Plasma Physics Seminar
 
   
  Fall 2017 Schedule  
 
September 6, Wednesday 3:30PM
ERF 1207, Large Conference Room
Open
September 13, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Open

September 20, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Open

September 27, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Dr. Lynn Wilson, Goddard Space Flight Center
Relativistic electrons generated locally within transient ion foreshock phenomena

It has been known for years that charged particles can be accelerated by high Mach number collisionless shock waves. The accelerated particles can stream away upstream to form a foreshock region in communication with the shock. Due to differences in gyroradii, ions are more readily accelerated than electrons by collisionless shocks. These energetic, suprathermal ions stream against the incident flow providing free energy that can generate foreshock disturbances large-scale (i.e., tens to thousands of thermal ion gyroradii), transient (~5-10 per day) structures. They have recently been found to accelerate ions to energies of several keV [e.g., Wilson et al., 2013] and even produce their own mini foreshocks [e.g., Liu et al., 2016]. While the high Mach number (M > 40) Kronian bow shock can generate ~MeV electrons [e.g., Masters et al., 2013], the much weaker Earth's bow shock (1 < M < 20) cannot generate electrons beyond a few 10s of keV [e.g., Wu, 1984]. We present recent results showing evidence that electrons can be energized to relativistic energies (~500 keV) far upstream of the bow shock [Liu et al., 2017; Wilson et al., 2016]. All previous observations of energetic foreshock electrons were attributed to geomagnetic and/or solar activity but we observe no geomagnetic or solar activity associated with these enhancements. Further, given that electrons generally have much smaller gyroradii than shock ramp widths and thermal speeds that greatly exceed shock speeds, no known shock acceleration mechanism can explain the energization of thermal electrons up to these relativistic energies. These observations could provide a new paradigm for electron injection in astrophysical shocks and resolve several unanswered questions in heliospheric and astrophysical plasmas.
October 4, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Open

October 11, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Open

October 20, Friday 3:30 PM
ERF 1207, Large Conference Room
Dr. Jungypo Lee, Massachusetts Institute of Techology
Quasilinear velocity diffusion in a tokamak

RF waves are injected in a tokamak to heat plasmas or generate a plasma current. Addition to the main purposes, it can be a good tool to control some MHD and transport phenomena in the fusion reactor. In this talk, I will present the recently developed theory of the quasilinear velocity diffusion due to the RF waves. The quasilinear diffusion is used to describe the RF wave model in a kinetic theory. This talk has two parts. In the first part, I will mention the modification of the Kennel-Engelmann diffusion coefficients for the toroidal geometry. In the second part, the change of the radial electric field and ion toroidal rotation due to the quasilinear diffusion will be explored, as an application of the quasilinear theory.
October 25, Wednesday 3:30 PM
ERF 1207, Large Conference Room
No seminar: APS-DPP conference

November 1, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Dr. Yohei Kawazura, University of Oxford
Development of a hybrid gyrokinetic ion and isothermal electron fluid code and its application to turbulent heating in astrophysical plasma

Understanding the ion-to-electron temperature ratio is crucial for advancing our knowledge in astrophysics. Among the possible thermalization mechanisms, we focus on the dissipation of Alfvenic turbulence. Although several theoretical studies based on linear Alfven wave damping have estimated the dependence of heating ratio on plasma parameters, there have been no direct nonlinear simulation that has investigated the heating ratio scanning plasma parameters. Schekochihin et al. (2009) proved that the turbulent heating ratio is determined at the ion Lamor radius scale. Therefore, we do not need to resolve all the scales up to the electron dissipation scale. To investigate the ion kinetic scale effectively, we developed a new code that solves a hybrid model composed of gyrokinetic ions and an isothermal electron fluid (ITEF). The code is developed by incorporating the ITEF approximation into the gyrokinetics code AstroGK (Numata et al., 2010). Since electron kinetic effects are eliminated, the new hybrid code runs approximately 2 sqrt(mi/me) times faster than full gyrokinetics codes. We will present linear and nonlinear benchmark tests of the new code and our first result of the heating ratio sweeping the plasma beta and ion-to-electron temperature ratio.
November 8, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Dr. Albert Mollen, Max Planck Institute for Plasma Physics, Greifswald
Collisional transport in stellarator plasmas

In contrast to tokamaks where turbulence typically dominates, a substantial fraction of the radial energy and particle transport in stellarators can often be attributed to collisional processes. One of the main objectives of the new Wendelstein 7-X stellarator (start of operation December 2015) is to demonstrate that stellarators can be sufficiently well optimized for small collisional transport levels, thus making the concept a realistic candidate for a future fusion reactor. The kinetic calculation of collisional transport has for a long time relied on simplified models which use mono- approximation, the simple pitch-angle scattering collision operator and are radially local. But not all experimental observations have been satisfactorily explained, and in recent years more advanced numerical tools have appeared which relax some of the approximations. We will discuss how the field has advanced and what the open questions are.energeticthe
November 15, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Open

November 22, Wednesday 3:30 PM
ERF 1207, Large Conference Room
No seminar: Thanksgiving Break

November 29, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Dr. Tonatiuh Sanchez-Vizuet, Courant Institute, New York University
Pseudo spectral collocation with Maxwell polynomials for kinetic equations with energy diffusion

We study the approximation and stability properties of a recently popularized discretization strategy for the speed variable in kinetic equations, based on pseudo spectral collocation on a grid defined by the zeros of a non-standard family of orthogonal polynomials called Maxwell polynomials. Taking a one-dimensional equation describing energy diffusion due to Fokker-Planck collisions with a Maxwell-Boltzmann background distribution as the benchmark for the performance of the scheme, we find that Maxwell based discretizations outperform other commonly used schemes in most situations, often by orders of magnitude. This provides a strong motivation for their use in high-dimensional gyrokinetic simulations. However, we also show that Maxwell based schemes are subject to a non-modal time stepping instability in their most straightforward implementation, so that special care must be given to the discrete representation of the linear operators in order to benefit from the advantages provided by Maxwell polynomials.
December 6, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Open

December 13, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Dr. Stuart Hudson, Princeton Plasma Physics Laboratory
FOCUS: Flexible Optimized Coils Using Space curves; a new approach to stellarator coil design