Speaker
Christopher Ham
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/O5.J602.pdf
Nonlinear ballooning flux tubes in tokamak geometry
C.J. Ham1, S.C. Cowley2, H.W. Wilson,1,3
1
CCFE, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK
2
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford, UK
3
York Plasma Institute, Department of Physics, University of York, Heslington, York UK
The nonlinear phase of MHD ballooning modes determines whether they are essentially
benign or disruptive. Disruptive or hard limits are produced by ballooning modes across
magnetic confinement fusion, for example; as ELMs, some tokamak disruptions [1], and
perhaps the LHD Core Density Collapse [2]. This work improves our understanding of how
these instabilities develop and might allow us to design plasma profiles such that hard limits
are avoided and so improve machine availability and performance.
A nonlinear theory for ballooning flux tubes in large aspect ratio toroidal geometry has been
developed [3] which shows that linearly ballooning stable profiles can be unstable to finite
amplitude displacements, i.e. they are metastable. We now use the generalized Archimedes'
principle developed in [3] to study the nonlinear phase of ballooning flux tubes in realistic
tokamak equilibria. We see that saturated
filamentary ballooning states are available even
when the profile is linearly stable. We qualitatively
compare the saturated amplitude of these states to
those seen experimentally, for example on KSTAR
[4]. We also investigate if this model is applicable
Figure 1: Elliptical (orange) flux tube
to type II ELMs which are thought to be purely sliding along (blue) surface S parting
surrounding (black) field lines. The tube’s
ballooning in character [5]. displacement is larger on the outboard side
This model focuses on the saturation of the ideal of the flux surfaces – the tube balloons. The
magnetic shear (s = rq′/q) causes the twist
MHD ballooning flux tubes which is likely to occur and narrowing of the tube on the inside.
on a fast time scale. However, once this occurs we may see the flux tube break off due to
magnetic reconnection and we assess the likely location of this reconnection site.
[1] E D Fredrickson et al Phys. Plasmas 3 (1996) 2620
[2] S Ohdachi et al Nuclear Fusion 57 (2017) 066042
[3] C J Ham et al Phys Rev. Lett. 116 (2016) 235001
[4] G S Yun et al Phys Rev. Lett. 107 (2012) 045004
[5] S Saarelma et al Plasma Phys. Control Fusion 51 (2009) 035001
