BEGIN:VCALENDAR
VERSION:2.0
CALSCALE:GREGORIAN
PRODID:UW-Madison-Physics-Events
BEGIN:VEVENT
SEQUENCE:1
UID:UW-Physics-Event-8819
DTSTART:20240716T140000Z
DTEND:20240716T160000Z
DTSTAMP:20240805T230741Z
LAST-MODIFIED:20240709T223210Z
LOCATION:106 Engineering Research Building\; https://uwmadison.zoom.us
/j/93001906557
SUMMARY:Suppressing Drift-Wave-Driven Turbulence with Magnetic Field S
haping\, Thesis Defense\, Joseph Duff\, Physics PhD Graduate Student
DESCRIPTION:The effect of triangularity and sign of geodesic curvature
$\\mathcal{K}^x$ on ion-temperature-gradient (ITG)-driven turbulence
was investigated\, covering both extreme positive and negative triangu
larities. Triangularity had a substantial impact on linear and nonline
ar physics\, and reversing $\\mathcal{K}^x$ significantly impacted tu
rbulence saturation. Negative triangularity reduced peak linear growth
rates and broadened the growth rate spectrum as a function of radial
wavenumber $k_x$. Positive triangularity increased peak growth rates t
hat shifted to finite $k_x$ and narrowed the growth rate spectrum. Rev
ersing the sign of $\\mathcal{K}^x$ slightly lowered linear growth rat
es\, except for $\\delta=0.85$\, where the growth rates decreased sign
ificantly. The effect of triangularity on linear instability propertie
s at low perpendicular wavenumbers can be explained through its impact
on magnetic polarization and curvature. The nonlinear heat flux was w
eakly dependent on triangularity for $-0.5\\le\\delta\\le0$\, increasi
ng significantly with extreme $\\delta$\, regardless of sign. When $\\
mathcal{K}^x$ was reversed\, the heat flux decreased\, became weakly d
ependent on triangularity for $\\abs{\\delta}\\le0.5$\, and decreased
significantly at extreme triangularity\, regardless of sign. Zonal mod
es play an important role in nonlinear saturation for the configuratio
ns studied\, and artificially suppressing zonal modes increased the no
nlinear heat flux by a factor of at least two and a half\, with negati
ve triangularities having a larger increase. When $\\mathcal{K}^x$ was
reversed\, so did the trend of heat flux ratios with triangularity. P
roxies for zonal-flow damping and drive suggest that zonal flows are e
nhanced with increasing positive $\\delta$ in both $\\mathcal{K}^x$ sc
enarios. Conventional quasilinear models did not capture the nonlinear
heat flux trends\, but\, by using a reduced three-field fluid model f
or ITGs\, the effect of unstable modes coupling to stable modes via zo
nal modes was added to the quasilinear model. This three-wave-interact
ion corrected quasilinear model was only able to capture the nonlinear
trends in triangularity when $\\mathcal{K}^x$ was reversed. The failu
re of the modified quasilinear model to estimate the nonlinear trend f
or the physical equilibria was likely due to the nonlinear heat flux s
pectra extending into scales where the fluid model is not valid.
\n
\
nOptimizations resulted in two three-dimensionally-shaped magnetic con
figurations with suppressed trapped-electron-mode (TEM)-driven turbule
nce. Initial equilibria had flux surface shapes with a helically rotat
ing negative triangularity (NT) and positive triangularity (PT). The o
ptimization targeted quasihelical symmetry and the available energy of
trapped particles. In electron-temperature-gradient-driven scenarios\
, the most unstable linear modes of the TEM-optimized configurations w
ere inconsistent with TEMs\, and the nonlinear simulations showed no s
ignificant fluctuations at ion scales. When a density gradient was pre
sent\, the most unstable modes at low $k_y$ were toroidal universal in
abilities (UIs) in the NT case and slab UIs in the PT geometry. Nonlin
ear simulations showed that UIs drove substantial heat flux in the NT
and PT configurations. Increasing the ratio of plasma pressure to magn
etic pressure to $\\beta=4\\times10^{-3}$ significantly reduced linear
instability at low $k_y$\, halved the nonlinear heat flux for the NT
case\, and almost completely suppressed the turbulence in the PT confi
guration.
\n
URL:https://www.physics.wisc.edu/events/?id=8819
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