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SUMMARY:[OV POSTER TWIN] Improving the Stellarator Through Theoretical Und
erstanding
DTSTART;VALUE=DATE-TIME:20210510T162000Z
DTEND;VALUE=DATE-TIME:20210510T164500Z
DTSTAMP;VALUE=DATE-TIME:20211019T204340Z
UID:indico-contribution-17722@conferences.iaea.org
DESCRIPTION:Speakers: C. C. Hegna (University of Wisconsin-Madison)\n**Int
roduction:** The stellarator is unique among magnetic confinement concept
s in that the plasma performance is mostly determined by externally applie
d magnetic fields. There is considerable opportunity to improve the stell
arator through increased understanding of how 3D fields impact important p
lasma physics processes\, enabling innovation in configuration design. We
review recent progress in stellarator theory in the topical areas: 1) imp
roved energetic particle confinement\, 2) affecting turbulent transport wi
th 3D shaping\, 3) novel optimization and design methods\, 4) reducing coi
l complexity and 5) MHD equilibrium tools.\n\n**Energetic particle confine
ment:** Energetic particle confinement is a key issue for the scalability
of stellarators to fusion power plants. Analytically derived proxies for
collisionless energetic particle confinement have been used for the first
time in optimization schemes to produce quasi-helically symmetric stellara
tor equilibria that eliminate all collisionless losses within the plasma m
id-radius for an ARIES-CS scale reactor. The analytic proxy accounts for
the competition of net bounce-averaged radial drifts relative to poloidal
drifts with the goal of aligning contours of the second adiabatic invarian
t J|| to magnetic surfaces. Using the coil optimization codes REGCOIL and
FOCUS\, it is possible to generate coil solutions for these configurations
with sufficient fidelity that alpha particle confinement is not degraded\
, the key feature being to place the coils far enough away from the plasma
to avoid high-order harmonic induced ripple losses. \n\n**Effect of 3D s
haping on turbulent transport:** Theoretical techniques produced stellara
tor configurations with reduced neoclassical transport as demonstrated in
the HSX\, LHD and W7-X experiments. As such\, micro-instability induced t
urbulent transport is the dominant transport channel in present day optimi
zed stellarators. A frontier research area in stellarator optimization is
to use 3D shaping of the magnetic field geometry to reduce turbulent trans
port. \n\nUsing analytic theory and gyrokinetic simulations\, a regime of
weak ITG/TEM is identified that applies to both stellarators and tokamaks
. In specific geometries\, turbulent transport can be reduced by one to t
hree orders of magnitude as seen in W7X with pellets and many tokamak inte
rnal transport barriers. Appropriately optimized stellarators can access t
his regime over most of the minor radius\, as identified in equilibria for
the quasi-axisymmetric stellarator NCSX.\n\nNonlinear gyrokinetic studies
demonstrate that mixing length estimates based on linear theory can be un
reliable predictors for turbulent transport rates for the quasi-symmetric
class of stellarators. This motivates a need to understand how 3D shaping
affects turbulent saturation physics. The important nonlinear energy tra
nsfer mechanism is a coupling of linear instabilities to damped eigenmodes
at comparable wave number through a three-wave interaction. As this mech
anism is a strong function of 3D shaping\, the geometric characteristics o
f different classes of stellarators strongly impact turbulent transport ra
tes. In particular\, the relatively short connection length of quasi-helic
ally symmetric stellarators enables a very efficient nonlinear energy tran
sfer channel to saturate turbulence at lower levels for a given instabilit
y drive. \n\nBoth analytic theory and nonlinear GENE simulations are being
developed to describe the role of finite-beta on stellarator turbulence.
Linear gyrokinetic simulations in HSX geometry show that kinetic ballooni
ng modes (KBM) can be excited at beta values far below the threshold value
predicted by ideal MHD ballooning theory at long wavelength. Nevertheless
\, significant nonlinear stabilization is observed at finite beta\, with n
onlinear simulations suggesting that coupling to marginally stable linear
Alfvenic modes is an important property of the nonlinear saturation physic
s at beta values well below critical values for KBM onset. Additionally\
, global gyrokinetic simulations of finite-beta micro-turbulence can now b
e performed with the XGC code.\n\n**Optimization methods:** Substantial p
rogress has been made in optimization and design methods for stellarators.
One instance is a new method to generate and parameterize quasi-symmetri
c and omnigenous plasma configurations using analytic expansions about the
magnetic axis. This approach is orders of magnitude faster than traditio
nal stellarator optimization\, allowing wider surveys over parameter space
\, and enabling insights into the character of the solution set. These ne
ar-axis expansions have enabled the first combined plasma-and-coil optimiz
ation for quasi-symmetry that uses analytic derivatives. \n\nAnother area
of progress is the development of adjoint methods for computing shape gra
dients. These techniques\, widely used outside of plasma physics\, allow
shape derivatives to be computed extremely efficiently\, enabling derivati
ve-based optimization and sensitivity analysis. Adjoint methods have rece
ntly been demonstrated for many quantities of interest for stellarator des
ign\, including collisional transport and coil complexity. \n\n**Stellara
tor Coils:** Recent advances in computational tools are enabling efforts
to reduce coil complexity in optimized stellarators. The FOCUS code uses
a fully 3-D representation that allows coils to move freely in space avoid
ing the need to introduce a winding surface as used in conventional coil o
ptimization codes. This freedom allows more design space to be explored. F
OCUS also employs analytically calculated derivative information for use i
n fast optimization algorithms and in direct assessment of global coil tol
erances for error fields. Recent applications include using FOCUS for the
design of new stellarator experiments and applications to innovations in
magnet technology including permanent magnets and high field high-Tc super
conductors. \n\n**MHD Equilibria Tools:** The stepped-pressure MHD equilib
rium code (SPEC) code has been developed for stellarator applications. SP
EC employs a model using a sequence of sharp boundaries for which disconti
nuities in the pressure and magnetic field are present\, and allows for re
laxation and “tearing” at rational surfaces. Recent advances and appl
ications include the development of a free-boundary capability\, linear an
d nonlinear stability calculations\, and the study of possible local relax
ation events in W7-X.\n\n**Configuration Designs:** Advances in physics
understanding can be used to generate metrics for use in the stellarator o
ptimization codes STELLOPT and ROSE. These advances are being employed t
o produce new stellarator configurations with excellent confinement proper
ties.\n\n*Research supported by U. S. Department of Energy Grant Nos. DE-F
G02-99ER54546\, DE-FG02-89ER53291\, DE-FG02-93ER54222\, DE-FG02-93ER5419\,
DE-SC0014664 and AC02-09CH11466 and the Simons Foundation Grant No. 56065
1.\n\nhttps://conferences.iaea.org/event/214/contributions/17722/
LOCATION:Virtual Event
URL:https://conferences.iaea.org/event/214/contributions/17722/
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