Highly radiating type-III ELMy H-mode with low plasma core pollution

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Highly radiating type-III ELMy H-mode with low
plasma core pollution

• J. Rapp, M. de Baar, W. Fundamenski,
• M. Brix, R. Felton, C. Giroud, A. Huber,
• S. Jachmich, E. Joffrin, I. Nunes,
• G.J. van Rooij, M. Stamp, G. Telesca,
• R. Zagorski and JET EFDA contributors

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Motivation

• Proposed solution for acceptable steady-state and
  transient heat loads
• Strongly radiating Type-III ELMy H-mode by
  impurity seeding
• Advantage benign scenario for plasma wall
  components
• Disadvantage confinement reduction with respect
  to type-I ELMy H-mode
• Compensation could be ITER operation at 17 MA
  (q952.6)
• Achievements as of 2006
• Radiative power fractions up to 95
• Partial outer divertor detachment
• DWELM in outer divertor down to 0.002 MJ/m2 for
  Wdia 3 MJ
• Objectives in last JET campaigns
• Improve confinement to high values than
  H98(y,2)0.73
• Reduce plasma core pollution to lower Zeff
• Experiments at high current (up to 3 MA), high
  density (1.1x1020 m-3), optimized fuelling

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Confinement improvement

• frad0.75
• H98(y,2) was increased to 0.83 by lowering
  density (NGW0.85)
• bN1.9
• n lowered by factor 2.5
• Confinement is within Q10 ITER operation domain
  for 17MA operation

Q10 domain
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Plasma core pollution

• Zeff for nitrogen seeded type-III ELMy H-modes
  with frad gt 0.65
• Zeff drops strongly with increasing density, even
  more than expected
• Check impurity production and Zeff scaling

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Plasma core pollution

• Heating power (bN) scan with fixed radiative
• power fraction and fixed density
• ne 1020 m-3, Prad 10 17 MW, frad 0.7
• Nitrogen flux is increased by factor 4
• Zeff is dominated by nitrogen (background
  Zeff0.2-0.3, carbon concentration 1
• Increased nitrogen concentration does not lead to
  higher carbon concentration
• But physical and chemical sputtering by nitrogen
  should increase carbon sources

G s-1 Yphys (10 eV) GC s-1
D 8 x 1022 1.1 x 10-3 8.8 x 1019
N 2.5 x 1022 1.6 x 10-2 4 x 1020
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Carbon sources in divertor

• From CIII (465 nm)
• Strachan et al. Nuclear Fusion 2003
• Divertor carbon fluxes are reduced at lower
  powers to the divertor target
• High current discharges at high density frad0.7
  show only variation of 20

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Carbon sources, outer divertor

• Carbon sources (derived
• from CII, 515 nm)
• Carbon sources from CII are
• about a factor of 2-4 lower
• than derived from CIII

type-I ELMy H-mode
S/XB for CII (514nm) 9-6 S/XB for Da (656nm)
20-15
type-III ELMy H-mode
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Carbon sources, main chamber

• Type-I ELMy H-mode
• factor 10
• Agreement with the fact that
• fuel retention is lower in type-III
• ELMy H-mode Loarer, R3
• Estimation of Zeff from
• carbon sources (divertor)
• fuelling efficiency from nitrogen
• puffing
• Zeff (from carbon) 1.2

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Chemical erosion in outer divertor

• no very clear trend (if at all increase is
  observed)
• Some increase could be due to higher surface
  temperaturre on the target at higher power to the
  target

Chemical erosion in divertor 1.5 to fit carbon
sources derived from CII 10 to fit carbon
sources derived from CIII
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Zeff scaling

• Zeff scaling as developed for
• Mk-0, Mk-I, Mk-IIA divertors
• Matthews et al., Nuclear Fusion 1999
• Predictions for ITER (400 MW)
• 15 MA, NGW0.85 Zeff1.7
• 17 MA, NGW1 Zeff1.3
• Good news !

• BUT Zeff scaling does not fit experimental data
  in range 1.5 to 2.5 very well

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Zeff scaling

• Zeff scaling has been
• re-evaluated, including
• transport of impurity ions
• ITER prediction more pessimistic
• 17 MA, NGW1 Zeff1.9

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Zeff scaling summary

• Zeff scaling has been improved, however
  predictions for ITER seem to be very sensitive on
  scalings
• Simple scalings do not take into account impurity
  profiles (Zeff profile is typically hollow for
  JET nitrogen seeded discharges, so that central
  Zeff could be 20 lower
• Code simulations are necessary to better predict
  profile effects in Zeff
• Modelling started with COREDIV Zagorski et al.
  J. Nucl. Mater.. 2003
• Self consistent radial 1D energy and particle
  transport of plasma and impurities (energy
  confinement according to H98(y,2) scaling law
• coupled to
• 2D multifluid transport in SOL

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Integrated modelling for JET

• Benchmark to JET experiments
• 69354 and 69359 from heating
• power scan
• Plasma profiles (Te, ne) in the
• core have been matched
• Zeff, frad, H98(y,2) have been
• matched too
• Good agreement of modelling and
• experiments !

Erosion by nitrogen not included Carbon erosion
by deuterium and self sputtering Chemical erosion
according to Roth formula No main chamber erosion
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Simulations for ITER, 17MA, high ne
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Simulations for ITER, 15 MA
Demonstrated at JET

• SOBERING Reduction of Q might be inevitable in
  15 MA standard scenario for strongly radiating
  scenarios

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Summary

• Operational domain of type-III ELMy H-mode has
  been increased towards higher current, higher
  densities and lower Zeff
• Carbon sources are minimized in those high
  density discharges, nitrogen is the main impurity
• Main chamber erosion is lower in type-III ELMy
  H-mode than in type-I ELMy H-mode
• Zeff scaling has been re-evaluated, taking into
  account transport of impurities, but prediction
  for ITER is slightly pessimistic
• Type-III ELMy H-mode is compatible with ITER Q10
  operation at 17 MA
• Q is reduced to 6 in standard 15 MA ITER scenario