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VERSION:2.0
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SUMMARY:Stability of tokamak and RFP plasmas with an exended region of low
  magnetic shear
DTSTART:20150817T103000
DTEND:20150817T113000
DTSTAMP:20260916T043944Z
UID:98e76985e7c6700146a7829a8d0d42a736b0c7a30fb00614a9e998b4
CATEGORIES:Conferences - Seminars
DESCRIPTION:Daniele Brunetti (CRPP-EPFL)\nMagnetically confined plasmas\, 
 characterised by the safety factor q with a small or slightly inverted mag
 netic shear\, have good confinement properties. Such plasmas typically hav
 e no internal transport barrier\, operate with q95~4 and are good candidat
 es for long pulse operation at high fusion yield in the reactor ITER. We p
 resent a study of the dynamics of hybrid plasmas\, with weak or almost zer
 o magnetic shear\, in tokamak and Reversed Field Pinch (RFP) configuration
 s\, when q in the central region assumes values close to one (tokamaks) or
  to a rational number (tokamaks\, RFPs)\, though the exact resonance is av
 oided.\nWe first focus our attention on tokamak and RFP equilibria with sl
 ightly reversed shear when an extremum in the safety factor is close to a 
 low order rational. These equilibria are characterised by the possible pre
 sence of internal helical cores (with a symmetric plasma edge)\, which can
  be understood as the result of the nonlinear saturation of ideal MHD mode
 s. The amplitude of large scale m=1 helical displacements is investigated 
 by means of 3D equilibrium and nonlinear stability codes. The nonlinear am
 plitude of such saturated modes obtained with the stability code is compar
 ed both with the helical core structure resulting from equilibrium numeric
 al calculations\, and with analytic predictions which extend the nonlinear
  treatment of reversed q plasmas to arbitrary toroidal mode numbers.\nThen
  we analyse (analytically and numerically) the stability of an initially a
 xisymetric tokamak configuration when the safety factor is almost flat and
  very close to a rational value over a macroscopically extended region in 
 the plasma centre. Such conditions typically occur either in hybrid scenar
 ios or following reconnection of a global instability such as a sawtooth. 
 A dispersion relation has been derived both for ideal and resistive modes 
 with additional non-MHD effects\, showing that the resistive sidebands cou
 pled to a core kink-like mode exhibit extremely fast growth\, though addit
 ional non-MHD effects tend to reduce the extreme growth rate. The existenc
 e of such modes has been confirmed numerically\, where the sensitivity of 
 the growth rate to changes in resistivity and two-fluid effects has been d
 emonstrated.
LOCATION:PPB 019
STATUS:CONFIRMED
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