Transcript L-H Transition and Pedestal Studies on MAST

L-H Transition and Pedestal
Studies on MAST
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Meyer ,
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Bock ,
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Conway ,
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Freethy ,
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Gibson ,
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Hiratsuka ,
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Kirk ,
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MFM De
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SJ
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J
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CA Michael , T Morgan , R Scannell , G. Naylor , S Saarelma , AN Saveliev , VF Shevchenko ,
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W Suttrop , D Temple , RGL Vann and the MAST and NBI Teams
1EURATOM/CCFE
2Eindhoven
Fusion Association, Culham Science Centre, Oxon, OX14 3DB
3University of York, Heslington, York, YO10 5DD, UK
5Ioffe Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russia
7Imperial College of Science, Technology and Medicine, London, UK
University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands
4The University of Tokyo, Kashiwa 277-8561, Japan
5Max-Planck-Institute for Plasma Physics, Boltzmannstr. 2, Garching, Germany
V. Pedestal formation
I. Motivation
 ITER success relies on H-mode operation with high confinement and tolerable ELMs
 Limited theoretical understanding of H-mode access and pedestal width and stability.
 H-mode access in non-activation phase (hydrogen) questionable  helium operation.
 Global access conditions studied in He and for different X-point height.
 Change of vertical position in single null (SN), change in elongation in double null (DN).
 Evolution of local parameters at the low field side mid-plane studied during the
L-H transition and the ELM cycle.
 Evolution of Er, Te, ne, Er, Te and ne measured on 0.2ms time scale through L-H transition.
 Evolution of pe and j studied during ELM cycle  peeling-ballooning model seems incomplete.
 Novel measurement of Ti shows Ti depends on collisionality on MAST.
 Measured Er at 4 radial points with t = 0.2ms
 Synchronous bursts of Thomson scattering.
II. H-mode access in helium:
 Righi et.al.: PLH
Aeff-1
 t  10ns, every 0.2ms.
(Aeff=aMana/ana) [1]
Note tLH is defined slightly differently in
these graphs:
Te, ne: top of last dither (reproducibility).
Er: top of first dither.
vis. light: bottom of last dither.
 Sequence of L-H, H-L and L-H transitions
L/H
forced using the magnetic configuration [4]
t = 13µs
transition
t
 Improved statistics, jitter of a few ms.
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Prior to L-H transition:
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 No change in Er, Er, Te, Te, ne or ne.
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After L-H transition:
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 Shear layer develops at R-Rsep~ -1 cm
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 Increase of ne and ne.
 Er = Er/tEr ~ 0.6 ms, n ~ 3 ms;
 L-mode filaments vanish in less than 0.1ms.
Range shown in the 2D images of visible light
 visible light acts as proxy for ne
 Ion temperature profiles are flat at low *
 PLH on ITER in hydrogen difficult.
 New measurements at AUG: PLHHe = PLHD [2]
 High priority ITER task to improve database.
 Compare PLH in D and 4He at similar parameters.
 Ip = 0.7 MW, Spl= 24 m2, <ne>=2.41019m-3, Bt=0.5 T
 PLHHe = (2.10.2)MW compared to PLHD = (1.40.2)MW
 Aeff~3.7 (85% He), detailed TRANSP analysis since
D-NBI and fast-ion redistribution DFI~1 m2/s.
 Ploss = Pabs+P-dW/dt-Prad (PabsHe~0.8 PNBI, PabsD~PNBI)
 Similar pedestal at Ploss-PLH~0.3 MW
 Different atomic physics; 2 ion species, no molecules
 different velocity distribution.
 CX: C6+ + D*(n=2)  C5+(n=8) + D+
 localised gas puff; 10ms < t < 20ms)
 Fit suggests narrower in He (caution).
 L-H transition dynamics differ between He and D
 Ti increases with increasing *.
 D: Dithering H-mode over wide power range.
 He: Sharp transition after modest power increase.
 Measurement averages over ELMs at high *.
 Similar density, but different Ip, Bt and Te,i.
 Profiles show dimensionless * scan.
 Ti ~ Te at high collisionality.
 MAST ions are in the banana regime.
pol
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 Flat profiles consistent with i  ln Ti 1
 Conservation of entropy [5].
 Ti >> Te changes total p for stability calc..
III. Effect of X-point height on H-mode access:
 JET: PLH decreases with decreasing X-point height [3]
 Only if strike point (SP) is on horizontal plates
 With and without septum – even septum limited.
 MAST: previously SP close/far from X-point in SN [4]
 H-mode/L-mode. (2m < Lc< 4m).
 New: PLH increases by factor of 2 with Zx~ + 10 cm (SN).
 No significant change in Lc (Lc~ -1m; different direction.)
 Increased   |Zx| ~ 8 cm  better H-mode access.
 Usually pe = pi assumed  clearly wrong.
VI. Edge current density
 Edge pitch angle, m, measurement:
 MSE (t = 2ms, R = 2cm)
 2D analysis of EBE (t ~ 40 ms,
R ~ 0.2 cm depends on ne).
 MSE resolves inter ELM period, but
spatial resolution is marginal.
 Total edge current exceeds
neoclassical prediction.
 No change in edge Te or ne prior to L-H transition
 Critical Te or Te favoured by many L-H transition theories
 no evidence for this on slow time scale.
 MSE: wider profile  spatial resolution?
 EBE: narrow profile with high m.
 Evolution of p and j suggest
peeling-ballooning model incomplete.
IV. Changes in the L-mode Er
 Er measured using active Doppler spectroscopy on He.
 L-mode Er averaged over 20ms of the dithering phase (average over many dithers).
 Little change in mean Er during power scan as Ploss approaches PLH
 No change in mean Er with
increase of  from 1.8 to 1.9.
 n=3 field  Er more positive:
No H-mode
edge
VII. Conclusions:
 On MAST PLH is about (5030)% higher in dominant 4He than in D plasmas.
 The reduction of PLH observed with the X-point closer to the targets can hardly be explained by
changes in the SOL connection length, Lc.
 In SN a change of PLH by a factor of two is observed with only 8% change in Lc.
 But, increased power (80%) to regain
H-mode  no change in Er.
 Mean Er does not correlate with PLH
 EB > max: sufficient, but not
necessary for the transition  E B
This work was funded by the RCUK Energy Programme under grant EP/G003955 and the European
Communities under the contract of Association between EURATOM and CCFE. The views and
opinions expressed herein do not necessarily reflect those of the European Commission.
 Profiles unstable with flat Ti and
max( j )   j dr /  ped
 Changes of the mean (equilibrium) Er don’t correlate with changes in PLH.
 Er seems to be a sufficient condition, but not a necessary.
 No changes of Er, Er, Te, Te, ne, ne prior to the L-H transition with t < 0.2ms.
 Ti profile on MAST reflect the fact that entropy is conserved over the whole pedestal at low *.
 Edge current density measurements indicate that the peeling-ballooning model is incomplete.
[1] E. Righi et.al., Nucl. Fusion 39 (1999) 309
[2] F. Ryter et.al., Nucl. Fusion 49 (2009) 062003
[3] Y. Andrew et.al., Plasma Phys. Control. Fusion 46
(2004) A87
[4] H. Meyer et.al. Plasma Phys. Control. Fusion 50
(2008) 015005
[5] G. Kagan et.al. Plasma Phys. Control. Fusion 50
(2008) 085010
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