Plasma Physics Seminar
  Fall 2023 Schedule  
August 30th, Wed. 3:30PM
ERF 1207, Large Conference Room
John Finn, Tibbar Plasma Technologies and LANL
Meshfree Analysis of Particle Methods for Plasma Kinetic Theory

We reconsider a meshfree approach to plasma kinetic theory, specialized to 1D electrostatic plasmas. This method uses kernel density estimation for the charge density and a related Green's function method, from Gauss's law, for the electric field. The kernel K(x-y) represents the the charge distribution within each macroparticle, both for computing the electric field E(x) and for using E(x) to compute the force on each macroparticle. This method has good conservation properties, conserving momentum and energy exactly. Similarly, the continuity equation is satisfied exactly, and this Vlasov-Gauss system is exactly equivalent to the Vlasov-Ampere and the Vlasov-Poisson systems. The use of the same kernel above leads to a symmetric and positive definite kernel, the correlation of the original kernel with itself, and allows an analog of the kernel trick in Machine Learning: a single positive definite kernel can be subsituted for this correlation. We show how the use of a non-positive definite kernel can lead to a numerical instability. This analysis uncovers a connection between kernels used for density estimation and the positive definite (reproducing) kernels. This analysis is useful for analyzing PIC codes, which do have a mesh, and for constructing meshfree codes. For the latter, I will discuss ways to deal with the major drawback of meshfree codes, the N^2 scaling, where N is the number of particles. These include low-discrepancy sampling and a Fourier formulation.
September 6th, Wed. 3:30PM
ERF 1207, Large Conference Room

September 13th, Wed. 3:30PM
ERF 1207, Large Conference Room

September 20th, Wed. 3:30PM
ERF 1207, Large Conference Room

September 27th, Wed. 3:30PM
ERF 1207, Large Conference Room

October 4th, Wed. 3:30PM
ERF 1207, Large Conference Room
Sarah Conley, Princeton University
Characterizing the Field-Particle Correlation Signatures of Electron Landau Damping

A long-standing goal in the heliophysics community is to understand the mechanisms that dissipate turbulent energy and energize particles in weakly collisional plasmas like the solar wind. One of these dissipation mechanisms is Landau damping, but the extent to which it and other mechanisms contribute remains unclear due to the inherent difficulty in observing and simulating the small length scales and fast time scales involved. However, evidence for electron Landau damping has been found in situ near 1 AU using the field-particle correlation (FPC) technique. Motivated by this observation, we apply the FPC technique to gyrokinetic simulations of strong plasma turbulence in the dissipation range at varying plasma-beta in order to develop a more thorough understanding of how to identify signatures of electron Landau damping throughout the heliosphere.
October 11th, Wed. 3:30PM
ERF 1207, Large Conference Room
Harry Arnold, APL
PIC simulations of overstretched ion scale current sheets applicable to the magnetotail

Onset of reconnection in Earth's magnetotail requires the current sheet thickness to be of the order of the ion thermal gyroradius or smaller. However, existing isotropic plasma models cannot explain the formation of such thin sheets at distances where the X-lines are typically observed. Here we reproduce such thin and long sheets in particle-in-cell simulations using a new model of their equilibria with weakly anisotropic ion species assuming quasi-adiabatic ion dynamics, which substantially modifies the current density. It is found that anisotropy/agyrotropy contributions to the force balance in such equilibria are comparable to the pressure gradient in spite of weak ion anisotropy. New equilibria whose current distributions are substantially overstretched compared to the magnetic field lines are found to be stable in spite of the fact that they are substantially longer than isotropic sheets with similar thickness.
October 18th, Wed. 3:30PM
ERF 1207, Large Conference Room
Vadim Uritsky, Catholic University
Self-similar Outflows at the Source of the Fast Solar Wind: A Smoking Gun of Turbulent Coronal Reconnection?

I present recent results of a quantitative analysis of structured plasma outflows above a polar coronal hole observed by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) spacecraft. In a 6 hr interval of continuous high-cadence SDO/AIA images, more than 2300 episodes of small-scale plasma flows in the polar corona were identified. The mean upward flow speed measured by the surfing transform technique is estimated to be 122 +/- 34 km/s, which is comparable to the local sound speed. The typical recurrence period of the flow episodes is 10-30 minutes, and the mean duration and transverse size of each episode are about 3-5 minutes and 3-4 Mm, respectively. The largest identifiable episodes last for tens of minutes and reach widths up to 40 Mm. The results show, for the first time, that the polar coronal-hole outflows obey a family of power-law probability distributions which could be indicative of an impulsive interchange magnetic reconnection driven by a turbulent photosphere. The estimated occurrence rate of the detected self-similar coronal outflows is sufficient for them to make a dominant contribution to the fast-wind mass and energy fluxes and to account for the wind's small-scale structure.
October 25th, Wed. 3:30PM
ERF 1207, Large Conference Room
Jessie Duncan, GSFC
Characterizing Hot Plasma in Solar Active Regions with NuSTAR Hard X-ray Observations

Solar active regions contain plasma across a broad range of temperatures, with the thermal distribution often observed to peak in the few millions of kelvin (MK). Characterizing this distribution in detail during a flaring interval can show how energy is released and transformed over the course of the event. Additionally, understanding the thermal distribution at non-flaring times is of interest as we work to determine the mechanisms that keep the solar corona much hotter than the photosphere (the "coronal heating problem"). Instruments observing in different energy (wavelength) ranges are variably sensitive to emission from plasma at different temperatures. Hard x-ray (HXR) observations are uniquely sensitive to the high-temperature (>5 MK) components of the corona, which are particularly interesting in understanding how heating proceeds. The Nuclear Spectroscopic Telescope ARray (NuSTAR) is a powerful HXR observatory which makes periodic observations of the Sun. Via differential emission measure (DEM) analysis, we can combine NuSTAR's HXR coverage with soft x-ray and extreme ultraviolet observations (here, from Hinode/XRT and SDO/AIA) to estimate the amount of plasma present in a given source as a function of temperature. In this work, we present a detailed DEM analysis of an active region (NOAA designation AR 12671) observed for ~5 hours on 2018 May 29. Over the course of the interval, we show how the plasma distribution evolves as the region produces several small microflares and also undergoes quiescent periods without obvious HXR transients. We discuss results from this work in tandem with future prospects for gaining further insight about active region heating using HXR DEM studies.
November 1st, Wed. 3:30PM
ERF 1207, Large Conference Room
APS DPP meeting

November 8th, Wed. 3:30PM
ERF 1207, Large Conference Room

November 15th, Wed. 3:30PM
ERF 1207, Large Conference Room
Antonio Coelho, Ecole Polytechnique Federale de Lausanne
Global fluid simulations of plasma turbulence in stellarators

We present the first 3D, global, two-fluid, flux-driven simulations of plasma turbulence in stellarators with different configurations: one with an island divertor; another one corresponding to the TJ-K stellarator; and a set of equilibria with increasing torsion and ellipticity. The simulations were carried out with the GBS code [1], which solves the two-fluid drift-reduced Braginskii equations.

The vacuum magnetic field of the island divertor configuration corresponds to a 5-field period stellarator and was constructed using the Dommaschk potentials [2]. It was found that the radial particle and heat transport is mainly driven by a field-aligned mode with low poloidal wavenumber, whose origin is investigated theoretically [3]. Transport is observed to be larger on the high-field side of the device and this is explained by means of a non-local linear theory. In contrast to tokamak simulations and experiments, but in agreement with edge measurements in W7-X [4], radial propagation of coherent filamentary structures (blobs) is not observed, revealing important differences between stellarator and tokamak edge transport mechanisms.

We further present the first validation of a simulation of plasma turbulence in a stellarator configuration against experimental measurements in the TJ-K stellarator [5]. The comparison shows that GBS retrieves the main turbulence properties observed in the device, namely the fact that transport is dominated by fluctuations with low poloidal mode number. Finally we present simulations in a set of equilibria with increasing ellipticitiy and increasing torsion generated by VMEC. The limit of zero ellipticity and zero torsion corresponds to a tokamak with circular flux surfaces, allowing to study edge turbulence in the transition between a tokamak and a stellarator. The role of ellipticity and torsion as well as of magnetic shear is discussed.

[1] P. Ricci et al., Plasma Physics and Controlled Fusion 54, 124047 (2012)
[2] W. Dommaschk, Computer Physics Communications 40, 203 (1986)
[3] A. J. Coelho et al, Nuclear Fusion 62, 074004 (2022)
[4] C. Killer et al, Plasma Physics and Controlled Fusion 62, 085003 (2020)
[5] A. J. Coelho et al, Plasma Physics and Controlled Fusion 65, 085018 (2023)
November 22nd, Wed. 3:30PM
ERF 1207, Large Conference Room
Technically open, but during UMD's Thanksgiving recess so extremely unlikely

November 29th, Wed. 3:30PM
ERF 1207, Large Conference Room
Dr. Jason Parisi, PPPL
Bifurcated kinetic-ballooning-mode constraints for tokamak pedestals across aspect ratio and plasma shaping

We find the pedestal width-height scaling [0] for multiple tokamaks using a new kinetic- ballooning-mode (KBM) gyrokinetic threshold model. At low-aspect-ratio, we reproduce NSTX's experimental linear pedestal width-height scaling for ELMy H-modes [2], overcoming previous issues with low-aspect-ratio pedestal prediction [3]. We reproduce the square root pedestal width-height scaling [0] at regular aspect ratio for previously published DIII-D discharges [4]. Our model uses EFIT-AI [5] to calculate global equilibria with self-consistent bootstrap current and can be applied to any H-mode equilibria. For ELMy NSTX discharges, kinetic rather than ideal-ballooning physics is needed to match experimental data. Combined with peeling ballooning mode (PBM) stability [6,7], our model will calculate a maximum inter- ELM pedestal width and height based on KBM and non-ideal PBM stability. Finally, we predict a bifurcation in tokamak pedestal width and height scalings that depends strongly on plasma shaping and aspect-ratio [8]. This bifurcation arises from the first and second stability properties of kinetic-ballooning-modes that yield a wide and narrow pedestal branch, opening the operating space of accessible pedestal widths and heights. The wide branch offers potential for edge- localized-mode-free pedestals with a high core pressure.

This work was supported by US Department of Energy Contract No. DE-AC02- 09CH11466.

[0] P.B. Snyder et al 2009 Phys. Plasmas 16, 056118
[1] J.F. Parisi et al 2023
[2] A. Diallo et al 2011 Nucl. Fusion 51 103031
[3] R.J. Groebner et al 2013 Nucl. Fusion 53 093024
[4] W. Guttenfelder et al 2021 Nucl. Fusion 61 056005
[5] S. Kruger et al 2022, PP11.00057, APS DPP Meeting
[6] A. Kleiner et al 2021 Nucl. Fusion 61 064002
[7] A. Kleiner et al 2022 Nucl. Fusion 62 076018
[8] J.F. Parisi et al 2023 In Prep.
December 6th, Wed. 3:30PM
ERF 1207, Large Conference Room
Richard Nies, PPPL
Turbulence saturation via oscillating zonal flows and grand critical balance

The energy confinement of tokamaks is limited by turbulent transport, making the understanding of turbulence saturation crucial. In previous work [1], tokamak turbulence was modelled using the critical balance conjecture [2], which posits that the turbulence parallel and decorrelation timescales are comparable. The theory does not however predict the radial length scale of the turbulence - which is required to obtain its saturation amplitude. Reference [1] made the assumption of perpendicular spatial isotropy to obtain a solution. We show using gyrokinetic simulations that the turbulence is in fact anisotropic in the radial and binormal directions, and follows the experimentally inferred grand critical balance [3]. The radial length scale is thus determined by the balance of the nonlinear, parallel, and magnetic drift timescales. Grand critical balance requires a revision of the scalings of [1], e.g. the heat flux scales linearly in the temperature gradient instead of cubically, which we show to be satisfied in gyrokinetic simulations using the GS2 [4], stella [5] and GX [6] codes.

We posit that the validity of grand critical balance is predicated on the existence of a new oscillating zonal flow mode. We characterise this mode through a generalised theory of the secondary instability [7], which considers the growth of zonal flows due to a streamer (the primary mode) driven by microinstabilities. Our generalised theory includes the effects of parallel streaming and magnetic drifts, and describes two new modes, the oscillating toroidal secondary and the purely growing neoclassical secondary, in addition to the classical Rogers-Dorland secondary [7]. The latter is found to only be relevant at large primary amplitudes, above the values seen in gyrokinetic simulations or expected from grand critical balance. At relevant primary amplitudes, the zonal flow behaviour is instead governed by the neoclassical secondary, which relies on the transport of momentum in banana orbits, and the toroidal secondary, which involves up-down asymmetric fluxes balancing the compressibility of zonal flows induced by toroidicity. Good agreement is demonstrated between the analytical theory, gyrokinetic simulations of the secondary, and the zonal flow behaviour in fully nonlinear simulations.

[1] Barnes, M et al. (2011). Phys. Rev. Lett. 107, 115003
[2] Goldreich, P & Sridhar, S. (1995). ApJ 438, 763
[3] Ghim, Y-c. et al. (2013). Phys. Rev. Lett. 110, 145002
[4] Dorland, W et al. (2000). Phys. Rev. Lett. 85, 5579-5582
[5] Barnes, M et al. (2019). Journal of Computational Physics 391, 365-380
[6] Mandell, NR et al. (2022). arXiv:2209.06731
[7] Rogers, BN et al. (2000). Phys. Rev. Lett. 85, 5336-5339

This work was supported by US Department of Energy Contract No. DE-AC02-09CH11466.