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SEQUENCE:1
UID:UW-Physics-Event-6595
DTSTART:20210917T140000Z
DURATION:PT1H0M0S
DTSTAMP:20210927T100905Z
LAST-MODIFIED:20210907T164639Z
LOCATION:zoom link below
SUMMARY: ITG Turbulence Saturation and Near-Resonant Heat Flux Reducti
on in Gyrokinetic Dimits-Shift Analysis\, Thesis Defense\, Ping-Yu Li
\, Physics PhD Graduate Student
DESCRIPTION:Microturbulence is caused by gyroradius-scale instabilitie
s such as the Ion-Temperature-Gradient-driven (ITG) instability\, Trap
ped Electron Mode (TEM)\, Kinetic Ballooning Mode (KBM)\, etc. Underst
anding how these instabilities saturate and form turbulence is importa
nt for the optimization of magnetic confinement fusion devices in the
quest for sustained fusion energy. The objective of this thesis is to
understand the important factors and mechanisms that saturate ITG turb
ulence and to utilize said understanding to build reduced models that
capture key physical behavior as described by full-physics approach.
\
n
\nZonal-flow-catalyzed interactions that involve large-scale stable
and unstable modes are crucial for the saturation of curvature-driven
ITG turbulence. A corresponding saturation theory is built based on a
fluid model and implemented and tested numerically. The crudest satura
tion theory drops the non-zonal interactions and also the nonlinear co
rrections to frequencies\, it also truncates the wavenumber space to o
btain scalings for the saturation level with the triplet correlation t
imes with linear frequencies and coupling coefficients. It is then di
scovered that nonlinear interactions can cause nonnegligible modificat
ions on the mode oscillations for systems with higher turbulence level
. Furthermore\, the kx direction in wavenumber space needs to be resol
ved in order to break the symmetry between modes and build up the zona
l flow\, which is shown in both time-dependent and time-independent re
search. Constructing a two-predator-prey model with no free parameter
inputted base on the saturation theory is also demonstrated. This prov
ides an idea how to build a predator-prey model from the first princip
le\, which has the potential to help understanding the limit-cycle osc
illations observed in L-H transition.
\n
\nThe importance of large-sc
ale stable modes and the triplet correlation time derived from the sat
uration theory are tested in gyrokinetics. Numerical results show that
the resonance between the stable and unstable modes through the coupl
ing of zonal flow corresponds to long nonlinear interaction life times
\, or large triplet correlation times\, which increases nonlinear ener
gy transfer and leads to strong turbulence suppression beyond any pure
ly linear estimates.
\n
\nThe triplet correlation time is further used
to improve a highly reduced model for fast heat-flux prediction in gy
rokinetics\, which shows significant improvement in several cases that
demonstrate heat-flux onset upshift from the linear critical gradient
for gradient scans. The role of the coupling coefficient in gyrokinet
ics is still under investigation.
\n
\nJoin Zoom Meeting
\nhttps://uw
madison.zoom.us/j/3259617260?pwd=UU9UenJ2dmFHMzRmdlhORkFkRHRadz09
\n
\
nMeeting ID: 325 961 7260
\nPasscode: 61T1p8
\n
URL:https://www.physics.wisc.edu/events/?id=6595
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