Plasma Physics Seminar
  Spring 2018 Schedule  
January 24, Wednesday 3:30PM
ERF 1207, Large Conference Room

February 2, Friday 3:30 PM
ERF 1207, Large Conference Room
Thomas Sunn Pedersen, Max Planck Institute for Plasma Physics
Experimental results from first operation with a divertor in the Wendelstein 7-X stellarator

Stellarators provide a potentially attractive concept for fusion power production, owing to their intrinsic steady-state capabilities, and their lack of current-driven disruptions. This talk will introduce the Wendelstein 7-X (W7-X) experiment, a highly optimized stellarator that went into first operation in 2015-16 and was operated again in 2017. With a 30 cubic meter volume, a superconducting coil system operating at 2.5 T, and steady-state heating capability of eventually up to 10 MW, it was built to demonstrate the benefits of optimized stellarators at parameters approaching those of a fusion power plant. The W7-X mission and goals will be presented, and results will be presented from recently obtained results from operation with the full set of 10 divertor units, which are passively cooled, but nonetheless allowed discharges as long as 26 seconds. The more than 30 diagnostics allowed a detailed physics program to be conducted, and also confirmed high confinement times (of order 200 ms) with central ion temperatures of 3.5 keV.
February 7, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Dr. Carlos Romero-Talamas, UMBC
Measurements of Dusty Plasma Rotation in Filament-Free Inductively Coupled Discharges at High Magnetic Fields

I will report on recent results from magnetized dusty plasma experiments carried out in collaboration with the Magnetized Dusty Plasma Experiment (MDPX) research group at Auburn University. Using RF antennae external to a non-conducting cylindrical vacuum chamber we produced inductively coupled plasmas (ICP) free of the filamentary structures that have been previously observed in capacitively coupled plasmas (CCP) at high magnetic fields and low pressures [E. Thomas Jr., et al., Phys. Plasmas 23, 055701 (2016), and references therein]. In our experiments, silica hollow microspheres with a diameter of 50 micronsand wall diameter on the order of 100 nm were levitated in ICP at neutral pressures varying from 5 to 300 mTorr, and magnetic fields ranging from 0 to 3.25 T. Dust rotation was observed similar to previous experiments at lower magnetic fields [N. Sato et al., Phys. Plasmas 8, 1786 (2001)]. The ICP RF frequency is chosen to be 22-30 kHz, which is low compared to other experiments that use CCP and RF of 13.56 MHz. Given this low frequency, RF is cutoff close to the chamber's cylindrical wall, leading to a plasma density gradient that peaks at the wall and is minimum at the chamber's axis. We conjecture that such density gradients cause pressure gradients that are in turn responsible for dust rotation through ion momentum transfer in the grad-P x B direction, initially proposed as an explanation for the Sato experiments [P.K. Kaw et al., Phys. Plasmas 9, 387 (2002)]. However, our rotation is faster than that reported by Sato et al., albeit at much higher fields and larger dust diameter. Rotation velocity increases with B, but reaches a maximum at around 1 T, and then decreases as B is increased. I will present hypotheses to explain this previously unseen behaviour, our future experimental plans to test these ideas.
February 14, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Dr. Ben Zhu, Dartmouth College
Global Two-Fluid Study of Tokamak Edge Turbulent Transport

A flux-driven global code, Global Drift Ballooning (GDB) model, based on the drift-reduced Braginskii equations is developed to study the low frequency turbulence at the tokamak edge region. In this model, profiles of plasma density, electron and ion temperature, electric potential, magnetic flux and parallel flow are self-consistently evolved across the entire edge region: from plasma sources in the inner core to plasma sinks in the outer-most scrape-off layer (SOL). GDB has successfully simulated realistic Alcator C-Mod L- and H-like plasmas in a simple shifted circular configuration, and reproduced the $\alpha_{d}-\alpha_{mhd}$ turbulence phase space diagram, in agreement with the previous local studies. It is also used to study the interaction between turbulence, global profiles, and the spontaneous $E{\times}B$ shear flow that is not captured in the previous local studies. In particular, we find that the spontaneous formation of the $E{\times}B$ drift in the electron diamagnetic drift direction in the closed-flux region can be explained based on the quasi-steady-state ion continuity relation $\nabla {\cdot} n \vec{v}_i {\approx} 0$. Another interesting phenomenon exhibits in GDB simulations is the up-down asymmetric plasma profiles caused by the transverse heat flux. The temperature gradient driven transverse heat flux heats the ions at the bottom while cools them on the top, and vice versa for electrons. Since the electrons have a faster parallel thermal diffusion, the up-down asymmetric pattern on electron temperature is weaker than ion temperature. As a result, density profile is driven to be up-down asymmetric due to the total plasma pressure is up-down symmetric enforced by the force balance constraint. Analysis shows this effect is profound for cold, dense plasma with finite temperature gradient; it might be the explanation to the formation of strongly asymmetric density profile when discharge approaches to the density limit observed in experiments back from the 80s. Furthermore, the symmetry breaking also affects the spontaneous generated $E{\times}B$ flow, resulting two asymmetric convective cells with a net inward particle flux which is typically two orders larger than the thermal-diffusion theory predicts.
February 21, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Dr. John Dorelli, Goddard Space Flight Center
The global structure of the ion diffusion region at Earth's dayside magnetopause: What have we learned from MMS?

Magnetic reconnection is the primary mode by which the solar wind couples to Earth's magnetosphere, making possible the efficient transport of plasma and magnetic flux across the dayside magnetopause and into the magnetosphere. In the magnetotail, magnetic reconnection is responsible for the rapid release of stored magnetic energy that powers magnetic storms and the aurorae. Much progress has been made in the last two decades in understanding how ion scale physics makes fast reconnection possible in collisionless plasmas, but it is still unclear how this understanding scales up from small systems with simple boundary conditions to very large non-toroidal systems like Earth's magnetosphere. This system size problem is challenging because it is difficult to handle the enormous range of scales involved on presently available high performance computing resources. NASA's Magnetospheric Multiscale (MMS) mission has provided much needed experimental guidance, having now sampled thousands of dayside magnetopause crossings below the ion scale. While much of the focus of MMS has been on a handful of electron scale dissipation events, we argue that much of the magnetic energy dissipation at the magnetopause may occur over a much more extended ion scale dissipation region.
February 28, Wednesday 3:30 PM
ERF 1207, Large Conference Room

March 7, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Diego Del-Castillo-Negrete, Oak Ridge National Laboratory
Full-orbit and backward Monte Carlo simulation of runaway electrons

High-energy relativistic runaway electrons (RE) can be produced during magnetic disruptions due to electric fields generated during the thermal and current quench of the plasma. Understanding this problem is key for the safe operation of ITER because, if not avoided or mitigated, RE can severely damage the plasma facing components. In this presentation we report on RE simulation efforts centered in two complementary approaches: (i) Full orbit (6-D phase space) relativistic numerical simulations in general (integrable or chaotic) 3-D magnetic and electric fields, including radiation damping and collisions, using the recently developed particle-based Kinetic Orbit Runaway electron Code (KORC) and (ii) Backward Monte-Carlo (MC) simulations based on a recently developed efficient backward stochastic differential equations (BSDE) solver. Following a description of the corresponding numerical methods, we present applications to: (i) RE synchrotron radiation (SR) emission using KORC and (ii) Computation of time-dependent runaway distributions, RE production rates, expected slowing-down and runaway times using BSDE. We study the dependence of these statistical observables on the electric and magnetic field, and the ion effective charge. SR is a key energy dissipation mechanism in the high-energy regime, and it is also extensively used as an experimental diagnostic of RE. Using KORC we study full orbit effects on SR and discuss a recently developed SR synthetic diagnostic that incorporates the full angular dependence of SR, and the location and basic optics of the camera. It is shown that oversimplifying the angular dependence of SR and/or ignoring orbit effects can significantly modify the shape and overestimate the amplitude of the spectra. Applications to DIII-D RE experiments are discussed.
March 16, Friday 3:30 PM
ERF 1207, Large Conference Room
Brett Scheiner, Los Alamos National Laboratory
Fireball Onset and Steady State Properties

Low-pressure anode spots, also known as fireballs, are a discharge phenomenon that can occur at electrodes biased above the plasma potential. Although fireballs are one of the oldest know plasma phenomenon [1], and despite the fact that they have been extensively studied, the mechanism behind their formation remained unknown due to the rapidity of their onset. This talk presents particle in cell simulations, laser-based experimental observations, and a model of the fireball onset [2,3]. Simulations show that the fireball forms when enough positive space charge from electron impact ionization within the sheath is present to form an electron trapping potential well. Using these observations, a model for the spot onset and steady state properties is formulated. The predicted formation process has characteristic features that are observed in laser-based measurements of the fireball electric field and electron density.
[1] I. Langmuir, Phys. Rev. 33, 954 (1929)
[2] B. Scheiner, E. V. Barnat, S. D. Baalrud, M. M. Hopkins, B. T. Yee, Phys. Plasmas 24, 113520 (2017)
[3] B. Scheiner, E. V. Barnat, S. D. Baalrud, M. M. Hopkins, B. T. Yee, in review
March 21, Wednesday 3:30 PM
ERF 1207, Large Conference Room
No seminar: Spring break

March 28, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Dr. Caoxiang Zhu, Princeton Plasma Physics Laboratory
Development of the FOCUS code for designing stellarator coils

Finding an easy-to-build coil set has been a critical issue for stellarator design for decades. Conventional approaches assume a toroidal `winding' surface. We present a new method to design stellarator coils. The new coil design code, FOCUS, represents coils as arbitrary, closed, one-dimensional curves embedded in three-dimensional space. The target function to be minimized consists of multiple physical requirements and engineering constraints. By differentiating the first and second order derivatives of the target function with respect to coil parameters, FOCUS uses gradient-based and Hessian-based minimization algorithms to optimize the coils, fast and robustly. FOCUS has been applied to design modular/helical/RMP coils for stellarator and tokamak configurations. With analytically calculated Hessian, FOCUS can also use the eigenvalues of the Hessian matrix for determining the error field sensitivity to coil deviations. The sensitivities could provide information to avoid dominant coil misalignments and simplify coil designs for stellarators.
April 4, Wednesday 3:30 PM
ERF 1207, Large Conference Room

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

April 18, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Prof. Christine Hartzell, University of Maryland
Dust-Plasma Interactions on Asteroids

While asteroids vary significantly in size, morphology and chemical composition, there is a consistent need to understand the behavior of surface grains in order to improve understanding of the evolution of these bodies. Due to the weak gravity and lack of an atmosphere on these bodies, non-gravitational forces can dominate the interactions of grains. Electrostatic dust motion, due to the interaction of the surface dust grains with the solar wind plasma, has been hypothesized to occur on asteroids. This talk will discuss the physics and dynamics of electrostatic dust lofting and levitation on asteroids, as well as plans to look for signatures of these phenomena at Bennu. On-going experimental work to understand triboelectric charging of regolith will also be discussed.
April 26, Thursday 3:30 PM
ERF 1207, Large Conference Room
Alessandro Geraldini, University of Oxford
Kinetic treatment of ions in the magnetic presheath

Boundary layers are present in the thin region of a tokamak where the Scrape-Off Layer plasma reaches the divertor or limiter target. If the magnetic field impinges with an oblique angle on the target surface, there is a small region - called the "magnetic presheath" or "Chodura sheath" - of size a typical ion Larmor radius, in which ions may intersect the wall during an orbit. Typically this region is quasineutral and collisionless to a good approximation. In this region, ions feel electric forces (directed towards the wall) that compete with the magnetic forces, therefore the approximately periodic ion orbits are distorted. An expression for the ion density in terms of the electrostatic potential profile is obtained by exploiting an asymptotic expansion of the ion trajectories in the small angle between magnetic field and wall. The full distortion of the lowest order periodic orbits is retained. The electron density is assumed to be a Boltzmann distribution. By using an iteration scheme to impose the quasineutrality equation, the self-consistent electrostatic potential, ion density and ion flow across the magnetic presheath are numerically found with some prescribed distribution functions at the magnetic presheath entrance. The numerical solution can be obtained for any distribution function that satisfies a solvability condition at the magnetic presheath entrance. This condition is the kinetic generalization of the fluid Chodura condition, which states that the ion flow at the magnetic presheath entrance must be supersonic in the direction parallel to the wall. With our kinetic treatment, we obtain the velocity distribution function of ions entering the thin non-neutral Debye sheath. Moreover, the dependence on the ratio of ion temperature to electron temperature is studied and the results of Chodura's fluid equations (valid when the ion temperature is zero) are recovered.
May 2, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Prof. Lise-Marie Imbert-Gerard, University of Maryland
Some mathematical aspects of wave propagation in the cold plasma model

After a brief introduction to the cold plasma model, I will discuss first a numerical method designed to handle cut-offs, and second a theoretical study of a resonance.
May 9, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Dr. Fatima Ebrahimi, Princeton Plasma Physics Laboratory
Onset and nonlinear relaxation of coherent current-carrying edge filaments during transient events in tokamaks

The onset and nonlinear evolution of coherent current-carrying filaments are examined using global nonlinear three-dimensional resistive MHD simulations in a spherical tokamak (ST). We show that physical current sheets/layers develop near the tokamak edge under different circumstances, in particular as a peeling component of ELMs (due to bootstrap currents), and during vertical displacement events (associated with the scrape-off layer currents). In all these cases, edge current sheets can become unstable to nonaxisymmetric 3-D current-sheet instabilities and nonlinearly form edge coherent current-carrying filaments. Time-evolving edge current sheets in ST configurations are identified in our nonlinear simulations. [F. Ebrahimi, Phys. Plasmas 23, 120705 (2016);24, 056119 (2017)] In the case of peeling-like edge localized modes, the longstanding problem of quasiperiodic ELM cycles is explained through the relaxation of the edge current source through direct numerical calculations of reconnecting local bi-directional fluctuation-induced electromotive force (emf) terms. Second, we examine the stability and formation of reconnecting edge peeling-driven filaments during induced vertical displacement events (VDEs) simulations. Similar to fast reconnection due to axisymmetric plasmoids, [F. Ebrahimi and R. Raman, Phys. Rev. Lett. 114, 205003 (2015)] we find that the growth rate of these edge filamentary structures becomes independent of Lundquist number. As well as edge reconnection physics in tokamaks, the 3-D coherent current-carrying fi lament structures and their nonlinear dynamics due to the dynamo effect presented here are also relevant to flares, which also exhibit ejection of field-aligned filamentary structures into the surrounding space.
May 16, Wednesday 3:30 PM
ERF 1207, Large Conference Room

May 23, Wednesday 3:30 PM
ERF 1207, Large Conference Room
Dr. Sophia Henneberg, Max Planck Institute for Plasma Physics
Design of a new quasi-axisymmetric stellarator equilibrium

A new quasi-axisymmetric, two-field-period stellarator configuration has been designed following a broad study using the optimization code ROSE (Rose Optimizes Stellarator Equilibria). Because of the toroidal symmetry of the magnetic field strength, quasi-axisymmetric stellarators share many neoclassical properties of tokamaks, such as a comparable bootstrap current which can be employed to simplify the coil structure, which is favorable for finding compact equilibria. The ROSE code optimizes the plasma boundary calculated with VMEC based on a set of physical and engineering criteria. Various aspect ratios, number of field periods and iota profiles are investigated. As an evaluation of the design, the bootstrap current, the ideal MHD stability, the fast-particle losses, and the existence of islands are examined. The main result of this extensive study - a compact, MHD-stable, two-field-period stellarator with small fast-particle loss fraction - will be presented.