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VERSION:2.0
PRODID:-//Memento EPFL//
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SUMMARY:Theory of nonlinear ballooning modes
DTSTART:20171109T140000
DTEND:20171109T150000
DTSTAMP:20260929T044144Z
UID:608d6e4935bed0d0496d57a0f904d7df3dd299e81ad43c384e8e0caa
CATEGORIES:Conferences - Seminars
DESCRIPTION:Dr. C.J. Ham\, Culham Center for Fusion Energy\, Abingdon\, UK
 \nIt is important to develop and control plasma scenarios which maximize t
 he confined pressure in tokamak plasmas as these reduce the size and cost 
 of a potential power plant. Instabilities limit the pressure in tokamak pl
 asmas. This limit can be a hard\, disruptive\, limit or a soft limit which
  may result in a critical pressure profile. It is well known that transpor
 t barriers allow tokamaks to achieve much higher fusion performance but th
 ey have the disadvantage of hard\, nonlinear ballooning\, instabilities: s
 uch as the ELM for the edge transport barrier\, and high plasma beta relat
 ed disruptions for internal transport barriers\, as seen experimentally in
  TFTR or numerically in nonlinear MHD calculations. An improved understand
 ing of nonlinear ballooning stability will help us design configurations t
 hat have a soft limit\, for example by staying below the critical pressure
  gradient for nonlinear stability. It may also explain experimental observ
 ations of ELMs and high plasma beta related disruptions on TFTR.\nWe revie
 w nonlinear ballooning theory and the MHD modelling of nonlinear balloonin
 g modes. We then describe recent analytic work on the nonlinear stability 
 of a large aspect ratio tokamak plasma to finite ballooning displacements 
 of thin elliptical magnetic flux tubes in the presence of a large pressure
  gradient region i.e. a transport barrier. We use a generalized form of Ar
 chimedes’ principle to derive a differential equation which models the d
 ynamics of such an ideal MHD flux tube. We solve this equation to find the
  equilibrium states of these flux tubes and calculate the energy of these 
 equilibria. Above a critical pressure the energy stored in a tokamak plasm
 a may be lowered by finite displacements of such tubes but not by infinite
 simal displacements – i.e. they are metastable. Above a higher critical 
 pressure such tubes become unstable to linear and nonlinear displacements.
  The distance the flux tubes are displaced in these states can be as large
  as the pressure gradient scale length. Triggering eruptions into these lo
 wer energy states leads to explosive dynamics\, as seen in ELMs. We discus
 s how plasma transport is enhanced by displaced flux tubes and how this re
 sults in rapid loss of confinement. We describe a scan of pressure gradien
 t and magnetic shear profiles looking at nonlinear stability. Finally\, we
  describe how this work is extended to experimental tokamak geometries.
LOCATION:ppb 019
STATUS:CONFIRMED
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