
January 24, Wednesday 3:30PM
ERF 1207, Large Conference Room

Open

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 7X stellarator
Stellarators provide a potentially attractive concept for
fusion power production, owing to their intrinsic steadystate
capabilities, and their lack of currentdriven
disruptions. This talk will introduce the Wendelstein 7X
(W7X) experiment, a highly optimized stellarator that went
into first operation in 201516 and was operated again in
2017. With a 30 cubic meter volume, a superconducting coil
system operating at 2.5 T, and steadystate 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 W7X 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 RomeroTalamas, UMBC Measurements of Dusty Plasma Rotation in FilamentFree 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 nonconducting
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 2230 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 gradP 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 TwoFluid Study of Tokamak Edge
Turbulent Transport
A fluxdriven global code, Global Drift Ballooning (GDB) model,
based on the driftreduced 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 selfconsistently evolved across the entire edge
region: from plasma sources in the inner core to plasma sinks
in the outermost scrapeoff layer (SOL). GDB has successfully
simulated realistic Alcator CMod L and Hlike 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 closedflux region can be
explained based on the quasisteadystate ion continuity
relation $\nabla {\cdot} n \vec{v}_i {\approx} 0$. Another
interesting phenomenon exhibits in GDB simulations is the
updown 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 updown asymmetric pattern on
electron temperature is weaker than ion temperature. As a
result, density profile is driven to be updown asymmetric due
to the total plasma pressure is updown 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 thermaldiffusion 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
nontoroidal 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

Open

March 7, Wednesday 3:30 PM
ERF 1207, Large Conference Room

Diego DelCastilloNegrete, Oak Ridge National Laboratory
Fullorbit and backward Monte Carlo simulation of runaway electrons
Highenergy 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 (6D phase space) relativistic numerical simulations in general (integrable or chaotic) 3D magnetic and electric fields, including radiation damping and collisions, using the recently developed particlebased Kinetic Orbit Runaway electron Code (KORC) and (ii) Backward MonteCarlo (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 timedependent runaway distributions, RE production rates, expected slowingdown 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 highenergy 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 DIIID 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
Lowpressure 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, laserbased 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 laserbased 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 easytobuild 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, onedimensional curves embedded in threedimensional 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 gradientbased and Hessianbased 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

Open

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

Open

April 18, Wednesday 3:30 PM
ERF 1207, Large Conference Room

Prof. Christine Hartzell, University of Maryland
DustPlasma 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, nongravitational 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. Ongoing
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 ScrapeOff 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 selfconsistent 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 nonneutral 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. LiseMarie ImbertGerard, 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 cutoffs, 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 currentcarrying edge
filaments during transient events in tokamaks
The onset and nonlinear evolution of coherent currentcarrying
filaments are examined using global nonlinear
threedimensional 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 scrapeoff layer currents). In all these cases, edge
current sheets can become unstable to nonaxisymmetric 3D
currentsheet instabilities and nonlinearly form edge coherent
currentcarrying filaments. Timeevolving 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 peelinglike 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 bidirectional fluctuationinduced electromotive force
(emf) terms. Second, we examine the stability and formation of
reconnecting edge peelingdriven 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 3D coherent
currentcarrying fi lament structures and their nonlinear
dynamics due to the dynamo effect presented here are also
relevant to flares, which also exhibit ejection of
fieldaligned filamentary structures into the surrounding
space.

May 16, Wednesday 3:30 PM
ERF 1207, Large Conference Room

Open

May 23, Wednesday 3:30 PM
ERF 1207, Large Conference Room

Dr. Sophia Henneberg, Max Planck Institute for Plasma Physics
Design of a new quasiaxisymmetric stellarator equilibrium
A new quasiaxisymmetric, twofieldperiod 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, quasiaxisymmetric 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 fastparticle losses, and the existence of islands are examined. The main result of this extensive study
 a compact, MHDstable, twofieldperiod stellarator with small fastparticle loss fraction  will be presented.


