1. Non-Local Effects on Pedestal Kinetic Ballooning Mode Stability
S. Saarelma

2. Non-local effects on KBMs, S. Saarelma 2/10/2015
Background
EPED model assumes pedestal gradient being limited by the kinetic ballooning modes (KBM).
In JET and low collisionality MAST pedestals the steepest pressure gradient region is locally in the second stable region.
However, in the pedestal r*/LT << 1 is not valid, local approximation may not be sufficient.
JET:
MAST (low n*):
<1: unstable
>1:stable
Non-local effects on KBMs, S. Saarelma 2/10/2015

3. Non-local effects on KBMs, S. Saarelma 2/10/2015
Outline
Linear ORB5 runs in the circular plasma
Linear ORB5 runs in MAST geometry
Comparison to local stability
Conclusions
Future plans
Non-local effects on KBMs, S. Saarelma 2/10/2015

4. Non-local effects on KBMs, S. Saarelma 2/10/2015
ORB5 analysis
Linear
Electromagnetic, kinetic electrons, no collisions
me/mp = 0.01
Without this pedestal modes can get dominated by core modes.
Local analysis shows a small effect on growth rates, no qualitative change.
Profiles shifted inwards to avoid edge effect on the result
No flows
Convergence tested for the grid and the number of particles
Non-local effects on KBMs, S. Saarelma 2/10/2015

5. Circular plasma, local stability
ne and Te profiles
3 cases:
no bootstrap current – entire pedestal n=∞ unstable
normal bootstrap current – middle of the pedestal 2nd stable
2 x bootstrap current – most of the pedestal 2nd stable
Ideal MHD n=∞
GS2 normalised g
GS2 real g
Non-local effects on KBMs, S. Saarelma 2/10/2015

6. Circular plasma, global stability
experiment
n=10 mode structure for f in ORB5
At low r*, there is no difference in growth rates between cases, as all of them are localised at the unstable region.
At high r* with modes extending to the locally 2nd stable region, there is significant difference in growth rate.
Non-local effects on KBMs, S. Saarelma 2/10/2015

7. Modified MAST case, local stability
Old MAST case (#24763) with flattened core profiles and inward shifted pedestal.
Ideal MHD n=∞
GS2 normalised g
Non-local effects on KBMs, S. Saarelma 2/10/2015

8. MAST global pedestal stability
No global instabilities when bootstrap current is taken into account in the equilibrium (even at b=0.08).
Without bootstrap current, normal KBM behaviour with increasing growth rate with b.
r* does not have a very large effect on unstable modes.
Modes balloon near marginal stability, then move to lower part of plasma at higher b.
Non-local effects on KBMs, S. Saarelma 2/10/2015

9. Non-local effects on KBMs, S. Saarelma 2/10/2015
Conclusions
The local 2nd stability access for KBMs discovered in global analysis.
2nd stability access closes as r* decreases if there is a sufficiently wide unstable region.
Non-local effects on KBMs, S. Saarelma 2/10/2015

10. Non-local effects on KBMs, S. Saarelma 2/10/2015
Future plans
Extend equilibria to “fake” scrape-off layer to allow wider margin between the edge and the pedestal hopefully allowing non-linear simulations.
Non-local effects on KBMs, S. Saarelma 2/10/2015