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PRODID:UW-Madison-Physics-Events
BEGIN:VEVENT
SEQUENCE:0
UID:UW-Physics-Event-9595
DTSTART:20260401T153000Z
DTEND:20260401T173000Z
DTSTAMP:20260405T100737Z
LAST-MODIFIED:20260209T183437Z
LOCATION:Chamberlin 5310
SUMMARY:Some statistical results in ion temperature gradient-driven tu
 rbulence theory\, Thesis Defense\, Augustus Azelis
DESCRIPTION:Understanding the nonlinear saturation of microturbulence 
 remains a central problem in magnetically confined fusion plasmas. In 
 ion temperature gradient (ITG) driven turbulence\, saturation emerges 
 from a complex interplay between linearly unstable drift waves\, linea
 rly stable eigenmodes\, and nonlinearly generated zonal flows. This di
 ssertation develops and applies weak turbulence closure theory to inve
 stigate these processes\, with particular emphasis on eigenmode dynami
 cs\, nonlinear energy transfer\, and the emergence of intermittent sta
 tistics. Analytical predictions are systematically compared with numer
 ical simulations of a reduced fluid model for ITG turbulence.<br>\n<br
 >\nA self-consistent weak turbulence closure is constructed for the se
 cond-order correlations governing energetic dynamics in a collisionles
 s ITG fluid model. In the collisionless limit\, the governing equation
 s exhibit parity–time reversal symmetry\, resulting in centrosymmetr
 ic correlation evolution operators. The resulting saturated states are
  characterized by equipartition of energy between stable and unstable 
 eigenmodes at each wavenumber. These results highlight the central rol
 e of zonal-flow–catalyzed nonlinear energy transfer from unstable to
  stable modes and suggest a mechanism by which turbulent heat flux can
  vanish despite the presence of linear instability. Numerical simulati
 ons confirm the predictions of the closure theory.<br>\n<br>\nA novel 
 weak turbulence closure technique is then developed to calculate two-p
 oint\, two-time correlation functions and power spectra in systems of 
 instability-driven turbulence. Application of this framework to zonal 
 flows yields a complex nonlinear decorrelation frequency that governs 
 both oscillatory behavior and temporal decay of the two-time energy sp
 ectrum. The resulting power spectra demonstrate that nonlinear interac
 tions strongly modify zonal flow dynamics\, imparting wavenumber-depen
 dent oscillation frequencies absent at the linear level.<br>\n<br>\nWe
 ak turbulence closure theory is further extended to fourth-order stati
 stics to investigate the Dimits shift through the lens of intermittent
  turbulence. A new closure method is developed to compute the growth r
 ate of a fourth-order cumulant associated with energy fluctuations. Th
 e resulting evolution equation predicts the development of intermitten
 t\, non-Gaussian statistics at large spatial scales below the nonlinea
 r critical gradient\, and their decay above it. These predictions are 
 validated through comparisons with numerical simulations\, including m
 easurements of probability distribution functions\, kurtosis\, and ent
 ropy of the turbulent heat flux. The emergence and suppression of inte
 rmittency are traced to qualitative changes in nonlinear mode coupling
  between stable and unstable eigenmodes across the nonlinear threshold
 .<br>\n<br>\nOverall\, this work demonstrates that weak turbulence clo
 sure theory provides a unified framework for describing saturation\, t
 emporal correlations\, and intermittency in ITG turbulence\, and offer
 s new insight into the dynamical mechanisms underlying zonal-flow–re
 gulated transport and the Dimits regime.
URL:https://www.physics.wisc.edu/events/?id=9595
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