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10-15 May 2021
Nice, France
Europe/Vienna timezone
The Conference will be held virtually from 10-15 May 2021

Improved Particle Confinement with 3D Magnetic Perturbations in DIII-D H-mode Plasmas

13 May 2021, 14:00
4h 45m
Nice, France

Nice, France

Board: EX/P6-38
Post Deadline Poster Magnetic Fusion Experiments P6 Posters 6

Speaker

Nikolas Logan (Lawrence Livermore National Laboratory, Livermore, California 94550-9234, USA)

Description

Experiments on DIII-D have identified a robust regime in which applied 3D fields increase the
particle confinement and overall performance while partially stabilizing the peeling, edgelocalized
modes (ELMs). Controlling ELMs using resonant magnetic perturbations (RMPs) is
the leading strategy for preserving reactor first wall components from destructive heat fluxes,
but the 3D fields used have hitherto been observed to degrade the particle confinement via the
density “pump-out” phenomenon. Recent DIII-D experiments show that there is a range of
counter-current rotation over which 3D fields instead increase the particle confinement and
stabilize certain edge peeling modes in H-mode plasmas (figures 1 and 2). This density “pumpin”
has the potential to minimize or even reverse the confinement degradation cost currently
expected in RMP reactor scenarios.

The application of n=2 3D fields consistently and robustly caused the density to rise in
ELMing H-mode, reverse Ip, upper single null discharges across a range of moderate counter-
Ip rotations (figure 2). The effect disappears near zero rotation and is not observed in “normal”
plasma current (Ip) plasmas (figure 1, orange). The extensive suite of edge and plasma material
interaction diagnostics of DIII-D do not detect any changes in the wall particle source
associated with the 3D fields responsible for the observed rise in density. While the ELM
frequency changes when 3D fields are applied, ELM-synchronized measurements of the
pedestal density evolution indicate that the net change in ELM induced particle flux is not
sufficient to explain the change in total density observed. The pump-in plasmas have finite codirectional
E×B and electron diamagnetic precession frequencies (ωE and ωe respectively) from the pedestal top to bottom,
shielding islands and providing no
inward resonant transport across
rational surfaces (which would
require ωE/ω
e < -1 in resistive
MHD [a]). The 3.5-4kA RMP
from DIII-D’s midplane “C-coil”
array is a mix of resonant and nonresonant
magnetic perturbations
and below any ELM suppression
threshold that may exist in these
scenarios. Thus, we conclude the
3D fields are modifying
fundamental confinement physics
in the pedestal of these plasmas
and that this physics is distinct
from the resonant island physics
thought to be responsible for ELM
suppression [b,c].
Generalized Perturbed
Equilibrium Code (GPEC) calculations show the neoclassical non-ambipolar ion transport in
the pedestal region is qualitatively consistent with the observed changes in the particle
confinement. The inward neoclassical ion flux is on the order of 3×1018 m-2s-1 for plasmas with
the largest density pump-in. Mitigated turbulent fluctuations are also observed coincident with
the applied fields, corresponding to reduced outward turbulent fluxes. These transport
mechanisms change the particle confinement in ways independent of the resonant island
physics governing ELM suppression.
While changes in ELM dynamics do not dominate the observed pump-in, it is notable that
the reduced ELM frequency and increased ELM size with applied fields is the opposite of the
pervasive RMP ELM mitigation behavior (smaller, higher frequency ELMs) observed
internationally on tokamak RMP experiments. This novel observation implies the 3D fields are
partially stabilizing certain edge peeling modes (identified as such using ELITE, which shows
no change in the axisymmetric boundary in current and pressure). Measurements from a
toroidal array of Mirnov probes show ELMs are only born at two particular toroidal angles
without applied 3D fields and only one location with the n=2 3D fields applied (figure 3). This
suggests that a peaking of multi-n intrinsic error fields destabilizes the ELMs at a specific
location and that this ELM-causing EF can be “corrected” with low n 3D fields coils.
New DIII-D experiments have found and detailed a new regime in which 3D fields like
those planned to suppress ELMs in future reactors increase the particle confinement and
stabilize certain edge peeling modes. These effects are distinct from the resonant island physics
thought to cause full suppression of the ELMs. These findings introduce a new path for
understanding the impact of 3D fields on particle confinement and minimizing the degradation
of plasma performance in the reactor scenarios using 3D fields.

This work was supported by the U.S. Department of Energy, Office of Science, Office of Fusion
Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility,
under Awards DE-AC02-09CH11466, DE-FC02-04ER54698, and DE-AC52-07NA27344.
[a] Q. Hu, et. al, Nucl. Fusion 54, 122006 (2014).
[b] R. Nazikian, et al., Phys. Rev. Lett. 114, 105002 (2015).
[c] Q. M. Hu, R. Nazikian, et al., Nucl. Fusion 60, 076001 (2020).

Affiliation Lawrence Livermore National Laboratory, Livermore, California 94550-9234, USA
Country or International Organization United States

Primary authors

Nikolas Logan (Lawrence Livermore National Laboratory, Livermore, California 94550-9234, USA) Qiming Hu (Princeton Plasma Physics Laboratory) Carlos Paz-Soldan (General Atomics) Raffi Nazikian (Princeton Plasma Physics Laboratory) Terry Rhodes (UCLA) Theresa Wilks (UsMIT) Stefano Munaretto (General Atomics)

Presentation Materials