Speaker
Wouter Oosterbeek
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1055.pdf
Evaluation of core beta effects on pedestal MHD stability in ITER and
consequences for energy confinement
W. Oosterbeek1, T. Weyens2, A. Loarte2, G.T.A. Huijsmans1,3. F.J. Artola2,4
1
Eindhoven University of Technology, Eindhoven, The Netherlands
2
ITER Organization, 13067 St. Paul Lez Durance, France
3
CEA, IRFM, 13108 St. Paul Lez Durance, France
4
Aix-Marseille Université, CNRS, PIIM UMR 7345, 13397 Marseille, France
High confinement mode (H-mode) in fusion plasmas is characterized by a steep pressure
gradient, or pedestal, that is limited by Peeling-Ballooning instabilities driven by pressure
gradients and edge currents. Ideal MHD studies of the pedestal stability have shown that the
maximum stable pedestal pressure increases with more peaked core pressure profiles through
the effect of the Shafranov shift. Because of stiffness of the core pressure profile this can lead to
a positive feedback between core and edge pressure but
this is found to saturate beyond given values of core beta
[1]. Such positive feedback has been found to lead to a
more favourable scaling of the plasma energy with
input power in tokamak experiments of that expected
from the ITER-H(98,y2) scaling (e.g. [2]) but the
extrapolation of this experimental results remains
uncertain.
This paper deals with the ideal MHD pedestal stability
aspects of this positive feedback for ITER, which may
differ from present experiments given the larger levels
of bootstrap current expected in ITER (due to low Figure 1. Stable (empty circles) and
plasma collisionality). Ideal MHD stability studies have unstable (mode number given by colour)
equilibria for different core (normalized
been performed for a range of ITER plasmas in which
beta) and pedestal (pedestal beta) ITER
the stability boundary for the pedestal pressure has been 7.5MA/2.65T plasmas pressure.
identified for a range of plasma betas. This is done by
self-consistently changing the pressure profile and scaling the corresponding bootstrap current
and modelling the corresponding equilibrium with HELENA and calculating the ideal MHD
stability with the MISHKA code (an example with the results of such analysis is shown in Fig.
1 for a 7.5 MA/2.65T H-mode plasma). The modelled pedestal marginal stability relation
obtained (ped = f (N)) will be used to determine the parameter global energy confinement
𝛼
scaling 𝛽𝑁 ~ 𝑃𝑖𝑛𝑝𝑢𝑡 , for a range of assumptions on core pressure profile changes with additional
𝛽𝑐𝑜𝑟𝑒 𝛾
heating power ~𝑃𝑖𝑛𝑝𝑢𝑡 , where characterizes the stiffness of the core pressure profiles.
𝛽𝑝𝑒𝑑
[1] Wolfrum, E., et al., Nuclear Materials and Energy 12 (2017) 18.
[2] Challis, C. D., et al., Nuclear Fusion 55 (2015) 053031.
