
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 7X
The role of the radial electric field in high performance ionroot plasmas on
Wendelstein 7X (W7X) 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 ionroot
conditions (negative Er in the plasma core) have been achieved in W7X 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
W7X 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 highdensity ion root conditions, and enables initial studies on the role of neoclassical transport in
the optimized highdensity regime of W7X.
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 highdensity ionroot 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
guidefield 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 frozenin condition at the xpoint. In
addition the electron meanderingorbit 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 twodimensional,
fullparticle 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 StOnge, Princeton Plasma Physics Laboratory
Turbulent Dynamo in a Collisionless Magnetized
Plasma
The Universe is magnetized. While
magneticfield 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 magneticfield
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 crosssection
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
opoints. 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 xpoint 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
Kadomtsevlike 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 shearflow
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 wellunderstood, in many
cases their effect on the resulting turbulence is much
less clear. I will discuss ongoing efforts towards
understanding how the KelvinHelmholtz (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
fusionrelevant 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 largescale energy sink for
turbulent fluctuations, and can change the evolution
of the mean flow. Additionally, I will discuss
gyrokinetic simulations of a driven, KHunstable flow,
where observations of unstable and stable mode
amplitudes in the fullydeveloped 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 nonuniform pressure may not exist but, mathematically, there exist 3D MHD equilibria with nonuniform, 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 nontrivial 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 zerowidth 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. Fixedboundary stability is determined by a generealization of the Newcomb crossing criterion. Freeboundary 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.


