study of the pedestal dynamics and stability during the elm cycle a. burckhart advisor: dr. e....
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Study of the pedestal dynamics andstability during the ELM cycle
A. Burckhart
Advisor: Dr. E. WolfrumAcademic advisor: Prof. Dr. H. Zohm
MPI für Plasmaphysik, EURATOM Association
A. Burckhart, PhD network, September 30th, 2010
A. Burckhart, PhD network, September 30th, 2010 2/30
• Motivation: why study ELMs?
• Overview on peeling ballooning theory and ELMs
• Evolution of Te, ne and pe during the ELM cycle
• Stability calculations
• Conclusions
Overview
A. Burckhart, PhD network, September 30th, 2010 3/30
• Motivation: why study ELMs?
• Overview on peeling ballooning theory and ELMs
• Evolution of Te, ne and pe during the ELM cycle
• Stability calculations
• Conclusions
Overview
A. Burckhart, PhD network, September 30th, 2010 4/30
Motivation
• Highest performance is found in H-mode plasmas featuring type-I ELMs Forseen operation scenario for ITER
• Edge localized modes (ELMs) are instabilities of the plasma edge, leading to a periodic deterioration of the pedestal profiles
• They cause massive power loads on the divertor, up to 10% of the confined energy in less than 1ms
• They cause high energy and particle losses
• They “clean” the plasma by expelling impurities
•They are not yet fully understood• Investigating the dynamic behavior of pedestal profiles can lead to a better
understanding of ELMs, and maybe ultimately to their control
• Stability calculations are usually performed for one time point directly before the ELM crash, what about the temporal evolution of the stability?
A. Burckhart, PhD network, September 30th, 2010 5/30
• Motivation: why study ELMs?
• Overview on peeling ballooning theory and ELMs
• Evolution of Te, ne and pe during the ELM cycle
• Stability calculations
• Conclusions
Overview
A. Burckhart, PhD network, September 30th, 2010 6/30
Plasma stability
• We consider a plama equilibrium with the total potential energy W, and calculate the effect of an arbitrary small displacement ξ
• If δW > 0, the system is stable, with δW < 0 it is unstable
ξ
ξ
stable (oszillation) unstable
δW < 0δW > 0
A. Burckhart, PhD network, September 30th, 2010 7/30
,1||0
2
0
2
0
20
0
,1
0
2
1
2
22
1
2
1
Bb
B
Br
jp
pB
W
dW
WWW
plasma
plasma
vac
vacuum
vacuumplasma
Instability: The change of potential energy is negative
Stability code: Find that minimises W
Field line bending
Compression of magnetic field lines
Plasma compression
Pressure driven modes
Current driven modes
Stabilising:
Destabilising:
(vacuum field line bending, always stabilising)
Edge stability
[Saarelma, JET 13.09.2010]
A. Burckhart, PhD network, September 30th, 2010 8/30
• Current driven instabilities that are localized radially at the plasma edge and poloidally near the X-point.
• Usually low-n
n=7
Mode localized near the x-points.
[Saarelma, JET 13.09.2010]
Peeling modes
A. Burckhart, PhD network, September 30th, 2010 9/30
• Driven by the pressure gradient.
• localized on the low field side.
• Radially more extended than peeling modes.
• Usually high-n.
n=20
Mode localized at the bad curvature region
[Saarelma, JET 13.09.2010]
Edge ballooning modes
A. Burckhart, PhD network, September 30th, 2010 10/30
• Driven by both the current and the pressure gradient.
• Intermediate-n• Mode structure has
features from both modes.
Peeling component
Ballooning component
[Saarelma, JET 13.09.2010]
ELMs are thought to be combined peeling-ballooning modes
A. Burckhart, PhD network, September 30th, 2010 11/30
ELMs expel particles from the plasma
[T. Lunt]
• Glow: mostly H-alpha, cold and dense
• Filaments expelled, travelling along field lines
• Highest intensity lasts less than 1ms
A. Burckhart, PhD network, September 30th, 2010 12/30
Te and ne pedestals relax due to ELMs
• Ipolsola: currents in the divertor, used as ELM indicator
• Te relaxes quickly during ELM crash, then slowly recovers
• ne also relaxes, but evolves around „pivot point“: ne inside drops, ne in the SOL increases
A. Burckhart, PhD network, September 30th, 2010 13/30
• Motivation: why study ELMs?
• Overview on peeling ballooning theory and ELMs
• Evolution of Te, ne and pe during the ELM cycle
• Stability calculations
• Conclusions
Overview
A. Burckhart, PhD network, September 30th, 2010 14/30
Te recovery shows several distinct phases
during ELMTe is small
initial recoveryof Te
Te recoverystalls Te recovery
continues
Te exhibitslarge fluctuations
Max(Te) mapped relative to t=0
A. Burckhart, PhD network, September 30th, 2010 15/30
Only one short recovery phase ~ 2-3ms
ne recovers differently than Te
Overshoot at the end of the recovery
Last phase is stationary,but large scale fluctuations
Te pedestal takes around 7ms to fully recover, ne pedestal only 4ms
A. Burckhart, PhD network, September 30th, 2010 16/30
Interplay between ne and Te buildup
• In the phase where Te recovery stalls, ne rapidly recovers
• In the following phase where Te recovers rapidly, ne stays constant
• This is observed in all analyzed discharges, at all gas fueling levels
• Reminiscent of critical value of e=Ln/LT above which Te is clamped: possibly ETGs are responsible for pause in the recovery of the Te profile
A. Burckhart, PhD network, September 30th, 2010 17/30
New laser triggering method will allow for ahigher density of TS data points
• Next steps:
• Improve characterization of edge Te profile recovery using TS data, which does not have the shine through problem (no accurate ECE data outside
ρpol=0.995 ) long discharges necessary
• Thomson scattering diagnostic:• Six 20Hz Nd-Yag lasers One profile every 8.3ms• Or: burst mode, fire all lasers in a given time interval
• Next campaign:
ELM detectedby XVR
DelayA + n*x
optical signal triggers laser
Result:All data points from the TS system lie in time interval of the ELM cycle that we want to analyze
TS data point
A. Burckhart, PhD network, September 30th, 2010 18/30
Next steps concerning pedestal dynamics
• Test reproducibility of interplay between Te and ne in the recovery of the pedestal profiles on other devices, starting with JET
• Very good Thomson scattering profiles available, but only 20Hz long discharges and coherent data selection necessary
• Fast ECE data available, but low resolution in the pedestal region, and shine through effect often more pronounced than on AUG
• Reflectometry measurements, Li-beam and DCN also available but not suited for calculating gradients and/or temporal resolution not sufficient
• No method that combines the data from different diagnostics to one joint profile (c.f. integrated data analysis at AUG)
• Start with analyzing TS and ECE data of „old“ discharges
• Then analyze dependencies of the pedestal dynamics on collisionality and on fueling level
A. Burckhart, PhD network, September 30th, 2010 19/30
AUG
• Length of the last, quasi stationary phase, is dependent on fueling level
• Length of initial recovery phase barely changes with fueling
• Next Step: trying to find a solid dependence of the different phases in the ELM cycle on the fueling level (AUG and JET)
„Steady state“ phase dependent on fueling level
JET
Fueling
[E. Giovannozzi, EPS 2009]
A. Burckhart, PhD network, September 30th, 2010 20/30
Reaching the pe limit does not necessarily lead to an ELM
• Clearly, a limit to max(pe) exists, but reaching it does not automatically trigger an ELM
• Some ELMs seem to be triggered by reaching max(pe) (consistent with JETTO simulation [J. Lönnroth, PPCF 2004])
• Others can sit at the pressure limit for several ms before ELM happens (seen before on AUG [T. Kass, NF 1998] and DIII-D, [R. Groebner, NF 2009])
‚fast‘ ELM frequency ‚slow‘ ELM frequency
A. Burckhart, PhD network, September 30th, 2010 21/30
Current diffusion cannot explain delayed ELM
peeling boundary
ballo
on
ing
bo
un
dary
[J. Connor, Phys.Plasmas 1998]
• ELM thought to be driven by combination of pressure and current gradient• Pressure gradient limited by ballooning mode• Edge current gradient limited by peeling (=very edge localized kink) mode
• Bootstrap current roughly proportional to pe, but delayed by about 1ms because of current diffusion cannot explain 6-7ms observed between reaching final pe and occurrence of the next ELM
A. Burckhart, PhD network, September 30th, 2010 22/30
Pressure further inside continues to increase
• Pressure further inside might influence peeling-ballooning stability• Next step: stability calculations
Pressure further inside might have an effect on ballooning stability
Pedestal slope and pedestal top stay constant
‚fast‘ ELM frequency
‚slow‘ ELM frequency
A. Burckhart, PhD network, September 30th, 2010 23/30
• Motivation: why study ELMs?
• Overview on peeling ballooning theory and ELMs
• Evolution of Te, ne and pe during the ELM cycle
• Stability calculations
• Conclusions
Overview
A. Burckhart, PhD network, September 30th, 2010 24/30
ELM-coherent averaged profiles
and magnetic data
CLISTE equilibrium (with kinetic
profiles)
ILSA stability
calculations
Stability calculations performed in several steps
HELENA High resolution
equilibrium
A. Burckhart, PhD network, September 30th, 2010 25/30
Experimental data is ELM synchronized
ne (1019 m-3)
Te (eV)
[M. Dunne]
ELM-coherent averaged profiles
and magnetic data
CLISTE equilibrium (with kinetic
profiles)
ILSA stability
calculations
HELENA High resolution
equilibrium
A. Burckhart, PhD network, September 30th, 2010 26/30
Grad-Shafranov equation is solved using CLISTE
Inputs for CLISTE
– Pressure data (10%)– Sauter Formula (1%)– MAC currents (<1%)– Magnetics (~90%)– cBoot (1%)– [SXR]– [CES/CEZ] Current
density
bootstrap
Pressure
ELM-coherent averaged profiles
and magnetic data
CLISTE equilibrium (with kinetic
profiles)
ILSA stability
calculations
HELENA High resolution
equilibrium
Output
A. Burckhart, PhD network, September 30th, 2010 27/30
Different p - j combinations are tested for stability
[C. Maggi, C. Konz]
,1||0
2
0
2
0
20
0
,1
2
22
1
Bb
B
jp
pB
Wplasma
plasma
Destabilising:
• edge current density• pressure gradient
Reminder:
ELM-coherent averaged profiles
and magnetic data
CLISTE equilibrium (with kinetic
profiles)
ILSA stability
calculations
HELENA High resolution
equilibrium
Code changes pedestal profiles to vary p and j, while keeping β and Ip constant
A. Burckhart, PhD network, September 30th, 2010 28/30
•First calculations would suggest a pedestal far away from stability limit, but:
•CLISTE output had too low gradients•Need to run CLISTE and ILSA again, carefully comparing CLISTE results with experimental data
First stability calculations performed, but need to be repeated
[C. Konz]
A. Burckhart, PhD network, September 30th, 2010 29/30
• Motivation: why study ELMs?
• Overview on peeling ballooning theory and ELMs
• Evolution of Te, ne and pe during the ELM cycle
• Stability calculations
• Conclusions
Overview
A. Burckhart, PhD network, September 30th, 2010 30/30
Conclusions and to-do’s
Study of ELM cycle with high temporal and spatial resolution sheds new light on both transport and stability of the pedestal
• Interplay between recovery of Te and ne observed on all analyzed AUG discharges, still to be studied on JET
• Better characterization of Te recovery might be possible with TS (no ECE shine through)
• Maximum of pe and jb can be reached well before ELM onset
• While peeling ballooning model is consistent with limits to pedestal pressure, there is still a physics ingredient missing to explain the ELM trigger
- Influence from pressure further inside to be assessed
- Temporal evolution of the stability to be characterized, comparison between stability of slow and fast ELM-cycles
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