BEGIN:VCALENDAR
VERSION:2.0
CALSCALE:GREGORIAN
PRODID:UW-Madison-Physics-Events
BEGIN:VEVENT
SEQUENCE:0
UID:UW-Physics-Event-3783
DTSTART:20151214T180000Z
DTEND:20151214T190000Z
DTSTAMP:20260419T110623Z
LAST-MODIFIED:20151203T161017Z
LOCATION:2241 Physics Bldg 
SUMMARY:Plasma Microinstability Saturation in Toroidal Plasmas\, Plasm
 a Physics (Physics/ECE/NE 922) Seminar\, Paul Terry\, UW Madison
DESCRIPTION:The question of how microinstabilities saturate in toroida
 l plasmas is often overlooked or<br>
\ngiven insufficient attention. T
 his is particularly true in comprehensive numerical<br>
\nsimulations\
 , like gyrokinetics\, where beyond ascertaining that there is a physic
 ally<br>
\nmeaningful saturated state\, most of the analysis is best d
 escribed as characterization of<br>
\nfluctuations and the transport t
 hey produce. As a result\, certain widely held ideas about<br>
\nsatur
 ation do not hold up under closer scrutiny. In this talk we examine ne
 w<br>
\ndevelopments in the understanding of how ion temperature gradi
 ent (ITG) turbulence<br>
\nsaturates. We present evidence from gyrokin
 etic simulations that ITG turbulence<br>
\nsaturates primarily by ener
 gy transfer to damped modes within the wavenumber range of<br>
\nthe i
 nstability\, rather than by a cascade to small scale. Zonal flows cata
 lyze this transfer\,<br>
\nproviding an efficient energy transfer chan
 nel while absorbing very little of the<br>
\ntransferred energy. Zonal
  flow shearing is not the primary saturation mechanism\, but is a<br>

 \nsecondary (weaker) effect at best. We also show that zonal-flow-cata
 lyzed transfer<br>
\nexcites stable modes with tearing parity. These g
 ive rise to a stochastic magnetic field at<br>
\nvery low beta values\
 , and electron thermal transport. While small at low beta\, this flutt
 erinduced<br>
\ntransport becomes significant at beta approaching 1%. 
 Magnetic fluctuations<br>
\ncreated by transfer to damped modes affect
  zonal flows. Above a critical beta\, they<br>
\ninduce irreversible c
 harge loss from rational surfaces\, shorting out zonal flow potentials
 <br>
\nand leading to a state of very high transport. This phenomenon\
 , called the non zonal<br>
\ntransition (NZT)\, has long been observed
  in gyrokinetic simulations\, and in experiment<br>
\ncould limit grad
 ients in stiff discharges.
URL:https://www.physics.wisc.edu/events/?id=3783
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