
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 â largescale (i.e., tens to thousands of
thermal ion gyroradii), transient (~510 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 KennelEngelmann 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: APSDPP 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 iontoelectron 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 iontoelectron 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 7X 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 pitchangle 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 SanchezVizuet, 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 nonstandard family of orthogonal polynomials called Maxwell polynomials. Taking a onedimensional equation describing energy diffusion due to FokkerPlanck collisions with a MaxwellBoltzmann 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 highdimensional gyrokinetic simulations. However, we also show that Maxwell based schemes are subject to a nonmodal 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


