Plasma Physics Seminar
 
   
  Fall 2018 Schedule  
 
January 16, Wednesday 3:30PM
ERF 1207, Large Conference Room
Novimir Pablant, Princeton Plasma Physics Laboratory
Role of neoclassical transport and the radial electric field in Wendelstein 7-X

The role of the radial electric field in high performance ion-root plasmas on Wendelstein 7-X (W7-X) is examined and compared with neoclassical predictions. In stellarator plasmas the neoclassical radial electric field (Er) is not intrinsically ambipolar, and is instead strongly tied to the plasma profiles. The properties of the Er profile strongly influence neoclassical transport of heat, particle and impurities.

Measurements of the core radial electric field (Er) have confirmed that ion-root conditions (negative Er in the plasma core) have been achieved in W7-X with high density plasmas, central ERCH heating and temperature equilibration (Te close to Ti). This is an important achievement as these are precisely the plasma conditions for which W7-X has been optimized. These measured Er profiles agree well with the neoclassical ambipolar Er predicted by the code SFINCS. This good agreement provides confidence in the validity of neoclassical calculations in high-density ion root conditions, and enables initial studies on the role of neoclassical transport in the optimized high-density regime of W7-X.

Experimental radial electric field profiles are inferred from the perpendicular velocity, as measured by the XICS diagnostic, and available with a high time resolution of up to 10ms. These diagnostic measurements provide the detailed profile evolution of the radial electric field in response to changes to the plasma density and heating power. Profile measurements of electron temperature (Te), ion temperature (Ti) and electron density (ne) along with approximations for the average value of Zeff have been used as inputs to the SFINCS code to calculate the ambipolar Er profile along with neoclassical ion and electron heat flux profiles (Qi, Qe). Finally the total experimental energy input to the electrons and ions, from ECRH heating and collisional heat transfer, has been compared to the neoclassical heat fluxes to provide a first estimate for the fraction of transport that can be attributed to neoclassical processes in reactor relevant high-density ion-root conditions.
January 24, Thursday 3:30PM
ERF 1207, Large Conference Room
Fulvia Pucci, Princeton Plasma Physics Laboratory
Energy transfer and electron energization in collisionless magnetic reconnection for different guide-field intensities

Electron dynamics and energization are a key component of magnetic field dissipation in collisionless reconnection. In 2D reconnection, the main mechanism that limits the current density and provides an effective dissipation is most probably the electron pressure tensor term, that has been shown to break the frozen-in condition at the x-point. In addition the electron- meandering-orbit scale controls the width of the electron dissipation region, where the electron temperature is observed to increase both in recent MMS observations as well as in laboratory experiments (MRX). By means of two-dimensional, full-particle simulations in an open system (Pei et al. 2001; Ohtani and R. Horiuchi 2009), we investigate how the energy conversion and particle energization depends on the guide field intensity. We study the energy transfer from the electromagnetic field to the plasma, and the threshold guide field separating when parallel and perpendicular energy transfers dominate, confirming recent MRX results, in agreement with MMS observations. We calculate the energy partition between fields and kinetic and thermal energy of different species, from the electron scales to ion scales, showing there is no significant variation for different guide field configurations. We study electron distribution functions and self consistently evolved particles orbits for high guide field configuration, investigating possible mechanisms for electron perpendicular heating. Finally I will give an idea of our 2D simulations with plasmoids for which the setup can be easily extended to study 3D reconnection, thanks to GPU technology.
January 30, Wednesday 3:30PM
ERF 1207, Large Conference Room
Adrian Fraser, University of Wisconsin
Cancelled

February 6, Wednesday 3:30PM
ERF 1207, Large Conference Room
Open


February 13, Wednesday 3:30PM
ERF 1207, Large Conference Room
Denis St-Onge, Princeton Plasma Physics Laboratory
Turbulent Dynamo in a Collisionless Magnetized Plasma

The Universe is magnetized. While magnetic-field strengths of just ~10^{-18} G are required to achieve this both in our Galaxy and in clusters of galaxies, observations of Faraday rotation, Zeeman splitting, and synchrotron emission all make the case of ubiquitous ~microGauss fields. That these systems are not content with hosting weaker fields is surprising, at least until one realizes that the energy density of a ~microGauss field is comparable to that of the observed turbulent motions. It is then natural to attribute the amplification and sustenance of (at least the random component of) the interstellar and intracluster magnetic fields to the fluctuation (or "turbulent") dynamo. In this talk, we will explore the various ways in which plasma microphysics makes magnetic-field amplification in weakly collisional plasmas by macroscale turbulent motions possible, with application to the intracluster medium of galaxy clusters.
February 20, Wednesday 3:30PM
ERF 1207, Large Conference Room
Open


February 27, Wednesday 3:30PM
ERF 1207, Large Conference Room
Adrian Fraser, University of Wisconsin
Postponed to March 27th.

March 6, Wednesday 4:00PM
ERF 1207, Large Conference Room
Chris Smiet, Princeton Plasma Physics Laboratory
Mapping the Sawtooth

The topological changes that occur in a tokamak during a sawtooth oscillation are strongly constrained by the fact that the magnetic field constitutes a 2.5 degree of freedom Hamiltonian system. Its structure is exposed by repeatedly mapping a poloidal cross-section to itself along the magnetic field lines to produce a Poincare plot, where the topologically stable fixed points of this mapping correspond with x- and o-points. We perform 3d numerical simulations of a sawtoothing tokamak discharge and calculate the location and Greene's Residue of these fixed points. The field evolves through a series of bifurcations where fixed points are split or combined according to rules that conserve topological index and the cycle culminates in the annihilation of the magnetic axis with an x-point from the broken (1,1) rational surface. The subsequent topological changes after the crash are not caused by tearing of any intact resonant surface, but by the lowering safety factor shifting the field into renonance with present nonaxisymmetric flux. As a consequence the (1,1) rational surface need not be healed for a sawtooth cycle to occur. The developed topological framework explains the Kadomtsev-like process in the simulation but the topological constraints are applicable to any model for the sawtooth.
March 13, Wednesday 4:00PM
ERF 1207, Large Conference Room
Open


March 20, Wednesday 4:00PM
ERF 1207, Large Conference Room
No seminar. Spring break.


March 27, Wednesday 4:00PM
ERF 1207, Large Conference Room
Adrian Fraser, University of Wisconsin
Role of stable modes in shear-flow turbulence

Sheared flows are found in a variety of fusion and astrophysical systems, where they may become unstable and drive turbulence. While the effects of physical parameters on the underlying instability's threshold and growth rate are often well-understood, in many cases their effect on the resulting turbulence is much less clear. I will discuss ongoing efforts towards understanding how the Kelvin-Helmholtz (KH) instability, the canonical shear flow instabiliy, saturates and drives turbulence. Particularly, I will discuss the role played by linearly stable (damped) modes, which have been shown to be important in other fusion-relevant systems. Analytical calculations show that these stable modes, universally neglected in reduced KH models, are in fact nonlinearly pumped to significant amplitudes in saturation. At these amplitudes, they present a large-scale energy sink for turbulent fluctuations, and can change the evolution of the mean flow. Additionally, I will discuss gyrokinetic simulations of a driven, KH-unstable flow, where observations of unstable and stable mode amplitudes in the fully-developed turbulent state have informed successful reduced models for how the mean flow scales with physical parameters. These findings have motivated ongoing work to use stable modes to help understand how KH saturation in MHD is affected by equilibrium magnetic fields.
April 3, Wednesday 4:00PM
ERF 1207, Large Conference Room
Open


April 10, Wednesday 4:00PM
ERF 1207, Large Conference Room
Open


April 17, Wednesday 4:00PM
ERF 1207, Large Conference Room
Open


April 24, Wednesday 4:00PM
ERF 1207, Large Conference Room
Open


May 1, Wednesday 4:00PM
ERF 1207, Large Conference Room
Open


May 8, Wednesday 4:00PM
ERF 1207, Large Conference Room
Adelle Wright, Australian National University
Realisability of discontinuous MHD equilibria

Smooth 3D MHD equilibria with non-uniform pressure may not exist but, mathematically, there exist 3D MHD equilibria with non-uniform, stepped pressure profiles. The pressure jumps occur at surfaces with highly irrational values of rotational transform and generate singular current sheets. If physically realisable, how such states form dynamically remains to be understood. To be physically realisable states, MHD equilibria must exist for some non-trivial timescale, meaning they must be at least be ideally stable and sufficiently stable to the fastest growing resistive instabilities. This presentation will discuss recent progress towards understanding discontinuous MHD equilibria via a stability analysis of a continuous cylindrical equilibrium model with radially localised pressure gradients, which examines how the resistive stability characteristics of the model change as the localisation of pressure gradients is increased to approach a discontinuous pressure profile in the zero-width limit.
May 15, Wednesday 4:00PM
ERF 1207, Large Conference Room
Open


May 22, Wednesday 4:00PM
ERF 1207, Large Conference Room
Alan Glasser, Fusion Theory and Computation, Inc.
DCON for Stellarators

DCON is a code for determining the ideal MHD stabiliity of axisymmetric tokamaks, based on the Direct Criterion of Newcomb [Phys. Plasma 23, 072505 (2016)]. It is very fast and accurate and used at tokamak labs around the world. Fixed-boundary stability is determined by a generealization of the Newcomb crossing criterion. Free-boundary stability is computed by coupling to a vacuum code to determine whether there are modes with negative total energy. The DCON formalism has recently been extended to nonaxisymmetric equilibria with stellarator symmetry, using equilibria computed by both the VMEC code, imposing nested flux surfaces, and the SPEC code, allowing for regions of stochastic magnetic fields.