Microinstabilities in the pedestal region• Start by identifying linear microinstabilities. David...

Microinstabilities in the pedestal

region

D. Dickinson

York Plasma Institute

Acknowledgements

David Dickinson | 7th IAEA Technical Meeting on Theory of Plasma Instabilities | Frascati | 4th March 2015 | Page 2

D. Dickinson*, B. Dudson, C. M. Roach2 and H. R. Wilson

*[email protected]

York Plasma Institute, Department of Physcs, University of York, YO5 10DQ, UK 2 EUROfusion CCFE, Culham Science Centre, Abingdon, Oxon OX14 3DB, UK The author would also like to thank F. Casson, A. Kirk, R. Scannell and S. Saarelma.


This work was part funded by the RCUK Energy programme, EURATOM and by a

EUROFusion fusion researcher fellowship (WP14-FRF-CCFE/Dickinson) and was carried

out using: HELIOS at IFERC, Aomori, Japan ; ARCHER, through EPSRC Grant No.

EP/L000237/1 ; HECToR, through EPSRC Grant No. EP/H002081/1.

Outline

• Pedestal predictions.

• Inter-ELM pedestal evolution:

• Observations from MAST

• Linear gyrokinetic simulations

• Microtearing modes at the pedestal top:

• Linear characteristics

• Non-linear transport

• Towards a pedestal transport model:

• Optimising GK codes

• Gyrofluid modelling

• Conclusions


Pedestal modelling

• EPED [1] developed to predict pressure pedestal

character at ELM crash, uses two constraints:

1. Peeling-ballooning ELM model.

2. KBM limits P’ in pedestal.

• Typically agrees within

20% of expt.

• Only predicts limit, not

evolution of pedestal.

• Only predicts pressure

pedestal, not n and T.

• Stiff transport → We need n & T pedestal character

(height, width) to predict core performance.

• But is evolution important?


[1] P. B. Snyder et al., PoP 16, (2009) 056118

Inter ELM pedestal evolution

• Composite ne and Te evolution

• MAST: shots 24452, 24459, 24763

• Density pedestal widens, ne’~constant.

• Temperature pedestal minimal change

• Understanding this evolution is important.

• Controlled by transport.

• Start by identifying linear microinstabilities.


[2] D. Dickinson et al,

PPCF 53 (2011)

115010

Linear gyrokinetics around pedestal

• Find MTMs in low P’ and KBMs in steep P’ region.

• ETGs are seen in narrow radial region at small scale.

• As pedestal evolves KBM region expands with pedestal and

MTMs become more unstable further into the core.

• Consistent with experimental observations.


MTM

KBM

ETG



MTM

KBM

ETG

[3] P. Manz et al, PPCF 56 (2014) 035010 [4] J. Hillesheim Proceedings IAEA 2014

[5] A. Diallo et al, PRL 112 (2014) 115001

• Experimental observations across devices find

characteristics of these instabilities, e.g.

• Velocimetry on AUG has seen MTM signatures from pedestal top [3].

• Doppler backscattering from MAST consistent with ETG at pedestal

top [4].

• KBM type fluctuations have been seen on C-Mod [5].


• Investigate character at pedestal top/gradient transition.

• Increasing β’ via T’ destabilises MTM.

• Increasing β’ via n’ stabilises MTM.

• KBM destabilised at critical β’ (either T’ or n’)


Suggests KBM limits β’ but MTM influences dynamics.

[6] D. Dickinson et al, PRL 108 (2012) 135002

Microtearing modes near the pedestal

• Evidence for MTMs at

pedestal top across range

of machines [2,3,7].

• MTMs may influence

pedestal evolution →

Important to understand

drive mechanism.


• Detailed study of linear mode in simplified system [8]

suggests mode driven by collisionless mechanism.

• May find modes in low collisionality tokamaks, e.g ITER.

• Study underway to improve understanding.

[7] S. Saarelma et al, NF 53 (2013) 1230123 [8] D. Dickinson et al, PPCF 55 (2013) 074006

Profile evolution → nonlinear simulations important.

Microtearing driven transport

• Nonlinear MTM simulations are challenging [9,10]: • Wide range of scales (large

box + high resolution)

• Electron dynamics important (small timestep).

• Use GKW to investigate heat flux in simplified system.


[9] H. Doerk et al, PRL 106 (2011) 155003 [10] W. Guttenfelder et al, PRL 106 (2011) 15504

• Reduce β by 5x to improve feasibility.

• Find experimentally relevant heat flux (~0.1-1 MWm-2).

• Collisions enhance transport (shift to larger scales).

• Strongly electromagnetic (EM flux >> ES flux).

Preliminary simulations suggest MTM transport significant.

Microtearing driven transport


• Further work is required to tackle more realistic cases.

• Progress has been made to improve feasibility of

simulations, e.g. by optimising initial conditions and

improving code performance.

• Using multiple codes to

improve confidence.

Despite improvements GK simulations remain very expensive

GS2 Scaling [11]

[11] http://www.idc.com/getdoc.jsp?containerId=prUS24953814

Gyrofluid modelling

• Kinetic effects important in pedestal → adopt gyrofluid model.

• Using 6 field EM GEM model [12] with BOUT++ [13], evolve:

• Energy conserving, EM model with FLR effects.

• Reduction over velocity space reduces dimensionality of

problem relative to GK simulations.

• Preliminary data suggests significantly reduced computational

cost (~102 times speed up for sample ES problem).


Whilst feasibility of GK simulations for pedestal top are improving they currently remain impractical for certain studies and long timescale simulations, such as to probe pedestal evolution.

→ Need a simpler model.

|||||||| ,,,,,,, AQQTTUN

[12] B. Scott, PoP 17 (2010) 102306 [13] B. Dudson et al, Comp. Phys. Comm. 180 (2009) 1467

Gyrofluid benchmark


• Currently benchmarking model implementation in BOUT++

• Start with widely used linear ES cyclone base case.

• Fairly good agreement with gyrokinetic code GS2.

Conclusions

• Inter-ELM pedestal not static → Modelling evolution

important for performance predictions.

• Linear GK simulations suggest KBMs set gradient limit but

MTMs can influence dynamics.

• Experimental observations broadly support simulations.

• MTMs near pedestal driven by collisionless mechanism.

• Preliminary GK simulations suggest MTMs can cause

significant transport.

• NL GK simulations expensive, whilst progress is being made

seeking simpler model to allow more extensive study.

• GEM gyrofluid model implemented with BOUT++, preliminary

benchmarks look promising, though further to go.

• Offers cheaper studies whilst retaining important effects.


Ongoing work

• Aim to build pedestal evolution model.

• EM and nonlinear benchmark first step.

• Need to move towards pedestal conditions.

• Once fluxes calculated can couple to simple transport

model to evolve pedestal profiles.

• Intending to integrate neutral model to investigate impact.


Recalculate magnetic equilibrium on

transport timescale PB unstable? Stop

No

Yes Calc sources

Calculate fluxes + profile evolution

Any questions?



This work was part funded by the RCUK Energy programme, EURATOM and by a

EUROFusion fusion researcher fellowship (WP14-FRF-CCFE/Dickinson) and was carried

out using: HELIOS at IFERC, Aomori, Japan ; ARCHER, through EPSRC Grant No.

EP/L000237/1 ; HECToR, through EPSRC Grant No. EP/H002081/1.

D. Dickinson*, B. Dudson, C. M. Roach2 and H. R. Wilson

*[email protected]

York Plasma Institute, Department of Physcs, University of York, YO5 10DQ, UK 2 EUROfusion CCFE, Culham Science Centre, Abingdon, Oxon OX14 3DB, UK The author would also like to thank F. Casson, A. Kirk, R. Scannell and S. Saarelma.

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