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SUMMARY:Two etudes on unexpected behaviour of drift-wave turbulence near s
 tability threshold
DTSTART:20180226T103000
DTEND:20180226T113000
DTSTAMP:20260506T015800Z
UID:9c8744106b3787989a5982145287eeebe984b6d7b9a1f3dbc0e97514
CATEGORIES:Conferences - Seminars
DESCRIPTION:Prof. Alex Shekochihin\, Univ. of Oxford\, UK\nI will discuss 
 some recent results — numerical and experimental — on the nature of dr
 ift-wave turbulence in MAST\, obtained in the doctoral theses of my studen
 ts Ferdinand van Wyk [1\,4]\, Michael J. Fox [2] and Greg Colyer [3]. At i
 on scales\, in the presence of flow shear\, we find numerically a type of 
 transition to turbulence that is new (as far as we know) in the tokamaks\,
  but reminiscent of some fluid dynamical phenomena (e.g.\, pipe flows or a
 ccretion discs in astrophysics): close to threshold\, the nonlinear satura
 ted state and the associated anomalous heat transport are dominated by lon
 g-lived coherent structures\, which drift across the domain\, have finite 
 amplitudes\, but are not volume filling\; as the system is taken away from
  the threshold into the more unstable regime\, the number of these structu
 res increases until they overlap and a more conventional chaotic state eme
 rges. Such a transition has its roots in the subcritical nature of the tur
 bulence in the presence of flow shear. It can be diagnosed in terms of the
  breaking of the statistical up-down symmetry of turbulence: this manifest
 s itself in the form of tilted two-point correlation functions and skewed 
 distributions of the fluctuating density field\, found both in simulations
  and in BES-measured density fields in MAST [2]. The governing (order) par
 ameter in the system is the distance from the threshold\, rather than indi
 vidual values of equilibrium gradients\; the symmetries — and drift-wave
 /zonal-flow turbulence of conventional type — are restored away from the
  threshold. The experiment appears to lie just at the edge of this latter 
 transition rather than at the exact stability threshold. At electron scale
 s in MAST\, the conventional streamer-dominated state of ETG turbulence tu
 rns out to be a long-time transient\, during which an initially unimportan
 t zonal component continues to grow slowly\, eventually leading to a new s
 aturated state dominated by zonal modes\, rather similar to ITG turbulence
  [3]. In this regime\, the heat flux turns out to be proportional to the c
 ollision rate\, in approximate agreement with the experimentally observed 
 collisionality scaling of the energy confinement in MAST. Our explanation 
 of this effect is based on a model of ETG turbulence dominated by zonal–
 nonzonal interactions and on an analytically derived scaling of the zonal-
 mode damping rate with the electron–ion collisionality. These developmen
 ts open some intriguing possibilities both for enterprising theoreticians 
 tired of the V&V routine and for ingenious experimentalists interested in 
 making use of tokamaks to probe transitions to turbulence in nonlinear pla
 sma systems.  \n\n\n[1] F. van Wyk et al.\, J. Plasma Phys. 82\, 90582060
 9 (2016)\n[2] M. F. J. Fox et al.\, Plasma Phys. Control. Fusion 59\, 0340
 02 (2017)\n[3] G. J. Colyer et al.\, Plasma Phys. Control. Fusion 59\, 055
 002 (2017)\n[4] F. van Wyk et al.\, Plasma Phys. Control. Fusion 59\, 1140
 03 (2017)
LOCATION:PPB 019
STATUS:CONFIRMED
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