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PRODID:UW-Madison-Physics-Events
BEGIN:VEVENT
SEQUENCE:3
UID:UW-Physics-Event-8570
DTSTART:20240129T203000Z
DTEND:20240129T223000Z
DTSTAMP:20260413T205309Z
LAST-MODIFIED:20240126T183432Z
LOCATION:B343 Sterling or https://uwmadison.zoom.us/my/jitsuk?pwd=Y3dW
 SW9CVnhycDgrY3pFOHZLNGdWQT09&omn=94319350163
SUMMARY:MULTI-SCALE INTERACTIONS OF TEARING MODES WITH MICROTURBULENCE
  AND ITG SATURATION-CHANNEL SELECTION\, Thesis Defense\, Taweesak Jits
 uk\, Physics Graduate Student
DESCRIPTION:Global tearing modes (TMs) can interact among themselves o
 r with small-scale instabilities\, exerting profound influence on fusi
 on plasma performance. Experiments in reversed-field pinches (RFPs) de
 monstrate that TMs couple\, cascade\, and cause robust transport\, whi
 le partial suppression of their activity can result in enhanced confin
 ement. The presence of unstable drift waves during the TM cascade in t
 he RFP allows interactions with microinstabilities. Local gyrokinetic 
 simulations with externally imposed magnetic perturbations modeling TM
 s have demonstrated that the magnetic perturbations erode zonal flows 
 that are nonlinearly generated by the microinstabilities\, resulting i
 n higher turbulence levels. Similarly\, when resonant magnetic perturb
 ations (RMPs) are applied to mitigate edge-localized modes in tokamaks
 \, the RMPs suppress the zonal flows\, which in turn can increase the 
 heat flux. These phenomena highlight the importance of multi-scale int
 eractions between large-scale magnetic fluctuations and zonal-flow-reg
 ulated microturbulence\, an incompletely understood topic.
\n
\nFor a 
 comprehensive understanding of these interactions\, self-consistent co
 mputations simultaneously evolving TMs and small-scale fluctuations ar
 e needed. Here\, calculations are performed with the global gyrokineti
 c code GENE\, which was modified to include a background current densi
 ty. This provides the TM drive\, and it is verified that the modified 
 GENE code properly models global TMs. Working towards multi-scale simu
 lations\, a non-reversed RFP discharge is studied\; linear simulations
  show that the non-reversed equilibrium is unstable to large-scale TMs
 \, which dominate in the core region\, while small-scale density-gradi
 ent-driven-TEMs dominate near the plasma edge. Nonlinear simulations w
 ith only TMs show that large-scale TMs in the core are coupled and exc
 ite smaller-scale stable TMs. The latter resonates at rational surface
 s closer to the edge\, where density-gradient-driven-TEMs are active\,
  indicating that multi-scale interactions are possible. In nonlinear g
 lobal density-gradient-driven-TEM simulations\, zonal flows dominate t
 he saturated state\, leading to negligible transport in the absence of
  TMs\, consistent with local simulations. When TMs and TEMs are simult
 aneously included in nonlinear simulations\, TMs partially erode zonal
  flows. This erosion of zonal flows disrupts energy mediation by zonal
  flows\, leading to a significant increase in heat flux. However\, the
  zonal flows remain a dominant characteristic of fluctuations\, causin
 g the transport fluxes to remain much smaller than in experiments. To 
 quantitatively reproduce experiments in future work\, higher density g
 radients are required to weaken the zonal flows\, or a stronger TM dri
 ve is needed to intensify the magnetic perturbations. These multi-scal
 e simulations offer valuable insights for understanding RFP experiment
 s and studying potential interactions of MHD phenomena and microturbul
 ence in tokamaks.
\n
\nIn contrast to TEM behavior\, static magnetic p
 erturbations do not strongly affect the ITG-driven turbulence of RFPs.
  This is because the ITG in RFPs is characterized by a slab limit and 
 does not rely as strongly on zonal flows for saturation\; instead\, it
  depends on marginal modes. Zonal flows\, on the other hand\, play a c
 rucial role in toroidal-ITG saturation. This prompts the exploration o
 f saturation-channel selection rules that capture the preference of th
 e toroidal limit for zonal flows and of the slab limit for marginal mo
 des. Nonlinear coupling quantities are determined\, and the triplet co
 rrelation time and mode overlap results are presented. Combining these
  metrics allows for predicting the dominant saturation channel for a g
 iven physical-parameter scenario\, providing a powerful new tool that 
 will aid a deeper understanding of nonlinear interactions and may also
  be used to enhance reduced models of anomalous transport.
\n
URL:https://www.physics.wisc.edu/events/?id=8570
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