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SUMMARY:Global Gyrokinetic Particle Simulations of Microturbulence in W7-X
and LHD Stellarators
DTSTART;VALUE=DATE-TIME:20210513T101000Z
DTEND;VALUE=DATE-TIME:20210513T103000Z
DTSTAMP;VALUE=DATE-TIME:20211026T024400Z
UID:indico-contribution-17485@conferences.iaea.org
DESCRIPTION:Speakers: Javier H. Nicolau (University of California)\nWith r
educed neoclassical transport\, turbulent transport becomes a critical iss
ue for plasma confinement in optimized stellarators. Therefore\, it is imp
ortant to understand properties of microturbulence in stellarators\, which
is complicated by the 3D equilibrium. Furthermore\, the interactions betw
een neoclassical and turbulent transport in stellarators and the extrapola
tion to the reactor regime have not been widely studied by simulation or t
heory. The capability of simulating 3D equilibrium in GTC [1] has previous
ly been developed and applied to simulate linear toroidal Alfvén eigenmod
es in LHD [2]\, nonlinear microturbulence in the DIII-D tokamaks with RMP
fields [3]\, and the effects of magnetic islands on bootstrap current [4]
and on microturbulence [5]. This paper reports linear and nonlinear physic
s of microturbulence in LHD and W7-X stellarators from GTC simulations.\n\
n**Global mode structures--** GTC simulations show that the electrostatic
ion temperature gradient (ITG) eigenmode structure is extended in the magn
etic field direction but narrow in the perpendicular direction\, and peaks
at bad curvature regions in both LHD and W7-X stellarators. The eigenmode
is strongly localized at the outer mid-plane in the LHD\, similar to that
in a tokamak. On the other hand\, the eigenmode in W7-X is localized to s
ome magnetic fieldlines or discrete locations in the poloidal plane\, whic
h is due to the mirror-like magnetic fields varying strongly in the toroid
al direction that induce coupling of more toroidal n harmonics to form the
linear eigenmode. The linear GTC simulation results are in reasonable agr
eement with results from EUTERPE simulations of the same ITG eigenmode in
the W7-X using identical magnetic geometry and plasma profiles [6].\n\n**E
ffects of zonal flows on ITG turbulence--** GTC nonlinear electrostatic si
mulations show that regulation by self-generated zonal flows is the domina
nt saturation mechanism for ITG instabilities in both LHD and W7-X. The ef
fects of zonal flows appear to be more prominent for the W7-X than the LHD
in reducing the radial correlation length and the thermal transport [6].
Furthermore\, in the W7-X simulation with zonal flows\, the nonlinear spec
tra are dominated by low-n harmonics (e.g.\, n=5\,10\,15)\, which can be g
enerated both by nonlinear coupling of high-n harmonics (e.g.\, n=205 and
n=210) and by linear toroidal coupling of these low-n harmonics with large
amplitude zonal flows (n=0). Note that the linear toroidal coupling of zo
nal flows with non-zonal modes is induced by the 3D magnetic fields (e.g.\
, with n=5\,10\,15 harmonics) in the stellarators\, an interesting new phy
sics that does not exist in the axisymmetric tokamaks.\n\n**Dynamics of zo
nal flows in 3D equilibrium--** Zonal flow dynamics in 3D equilibria have
been studied in GTC linear electrostatic simulations. In the LHD\, the rel
axation process of an initial zonal flow perturbation exhibits a damped GA
M oscillation and a lower frequency oscillation (LFO) before reaching a st
eady state with a residual zonal flow. On the other hand\, zonal flow damp
ing in W7-X only exhibits the LFO oscillation. The GAM oscillation is not
visible since it is strongly damped because of the small safety factor q~1
.1. Our simulations show that LFO is generated mainly due to the helical m
agnetic inhomogeneity\, consistent with existing theory that the LFO frequ
ency is a characteristic of non-axisymmetric devices due to the presence o
f helically trapped particles. When the radial wavelength of the zonal flo
ws decreases\, the zonal flow residual level increases and the damping of
GAM and LFO oscillations becomes stronger. Finally\, the 3D magnetic field
s generally enhance the GAM damping and decrease the zonal flow residual l
evel\, which is similar to that observed in GTC simulations of zonal dynam
ics in the DIII-D tokamak with resonant magnetic perturbations (RMP). GTC
simulations using VMEC equilibrium (which preserves magnetic flux surfaces
) shows that increasing the amplitude of the 3D RMP fields leads to a decr
ease in the residual level. Furthermore\, GTC simulations using M3D-C1 equ
ilibrium (which includes magnetic islands) show that the presence of RMP m
agnetic islands further enhanced GAM damping and reduce the zonal flow res
idual level in DIII-D RMP plasmas.\n\n**References:**\n[1] Z. Lin et al.\,
Science 281\, 1835 (1998).\n[2] D. A. Spong et al.\, Nuclear Fusion 57\,
086018 (2017).\n[3] I. Holod et al.\, Nuclear Fusion 57\, 016005 (2017).\n
[4] G. Dong and Z. Lin\, Nuclear Fusion 57\, 036009 (2017).\n[5] K. S. Fan
g and Z. Lin\, Phys. Plasmas 26\, 052510 (2019)\; Phys. Plasmas 26\, 08250
7 (2019).\n[6] H. Y. Wang et al.\, submitted to Nuclear Fusion (2020).\n\n
https://conferences.iaea.org/event/214/contributions/17485/
LOCATION:Virtual Event
URL:https://conferences.iaea.org/event/214/contributions/17485/
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