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SUMMARY:Transport bifurcation &amp\; layering in tokamak plasmas - Yanick 
 Sarazin (CEA\, Cadarache)
DTSTART:20240507T103000Z
DTEND:20240507T113000Z
UID:TALK216694@talks.cam.ac.uk
DESCRIPTION:Heat and particle transport across the nested magnetic surface
 s of tokamak plasmas is mainly governed by turbulence. Several underlying 
 instabilities coexist\, but the most virulent bear similarity with the Ray
 leigh-B&eacute\;nard instability in neutral fluids. In addition\, drift wa
 ves are also present\, the plasma analogue of Rossby waves in rotating atm
 ospheric turbulence. Sheared flows efficiently contribute to their saturat
 ion.\nAs will be shown\, turbulence self-organization in magnetized plasma
 s tends to develop localized regions of shear flow layers with fronts prop
 agating in between. This results in the local storage of turbulent energy 
 into corrugations\, leading to a staircase pattern of the pressure profile
 . The segregation of these complementary structures &ndash\; fronts and la
 yers &ndash\; is believed to be key to the achieve good confinement proper
 ties.\nOne of the issues deals with their possible role in triggering bifu
 rcations towards the macroscopic transport barriers which are observed exp
 erimentally. By means of a reduced flux driven nonlinear model that captur
 es "fluctuations &ndash\; mean" interactions\, we explore some of the char
 acteristics of layering and transport. Layering dynamics turns out to depe
 nd on the turbulent regime. Flow layer nucleation will be shown to involve
  phase curvature dynamics and to require the positive feedback loop with t
 he pressure profile. Implications will be discussed.
LOCATION:Seminar Room 1\, Newton Institute
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