ContributorsEuratom Associations

1
Task Forces S1 and T/impurity transport
Impurity Transport in High Density Plasmas in JET
and FTU
Contributors Euratom Associations L.
Carraro, M. Mattioli, M.E. Puiatti, P. Scarin, B.
Zaniol Consorzio RFX, Padova,
Italy P.DuMortier, A. Messiaen, J Ongena Ecole
Royal Militaire, Brussels, Belgium R.Dux,
IPP-Euratom Assoziation, Garching
Germany M.F.F Nave Centro de Fusão Nuclear,
1096Lisbon , Portugal J.Rapp, B.
Unterberg IPF Jülich GmbH, Jülich, Germany
L. Gabellieri ,D. Frigione, L. Pieroni
ENEA, Frascati, Italy
Presented by M. Valisa
9th EU-US Transport Task Force Workshop
Cordoba, Spain - Sept. 9- 12 / 2002
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Content

• High density regimes (relative to the Greenwald
  limit) of good confinement quality can be
  obtained in several ways
• Here we concentrate the impurity transport
  analysis on the high density Radiatively Improved
  Modes experiments carried out in JET (ELMy H
  mode) and FTU (Ohmic)
• JET injection of ICRH on top of NBI heating
  changes transport in the core and avoids
  impurity accumulation in Ar seeded quasi
  stationary D discharges with high density (ne/nG
  0.9), good confinement (H98 1) and high power
  radiated fraction (gt 50 ).
• FTU Ne seeding of D plasmas avoids saturation
  of confinement with density and the radiation
  belt at the edge reduces significantly the metal
  influx, with no major modification of the
  impurity transport.

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Motivation

• Increasing interest in High Density regimes -
  around Greenwald limit - because reactor
  relevant.
• In this context impurities are an important
  issue
• - radiative effectiveness /core power
  dissipation Prad ne nimp L(Te..)
• - risk of accumulation in the core when
  confinement improves
• - beneficial effects in accessing high density
  regimes
• w/o confinement degradation (e.g. RI-modes )
• - beneficial effects as a heat exhaust channel
  
• Same impurity transport model used to analyze
  the two different experiments

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Background - 1 Radiatively Improved mode

• Integrated scenario combining
• - high confinement ( increasing with density)
• - high density
• - good heat exhaust capability (edge radiating
  belt)
• - acceptable Zeff.
• Obtained in Textor-94 ( ISX results of 1984) by
  seeding the plasma with impurities (Ne, Ar, Si)
  and then reproduced in several experiments (
  Asdex-UG, TFTR, D III-D, JT-60, FTU, JET) .
• For an overview see J. Ongena et al., Physics of
  Plasmas 8 (2001) 2188

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Background 2 Impurity accumulation

• Accumulation of impurities depends on the
  combination of various processes
• Transport Processes
• Anomalous transport - Typically flattens
  profiles
• Neoclassical transport
• Edge transport/ ELMs/ screening
• PWI
• Impurity production mechanisms
• Impurity net influx

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The analysis method 1 D impurity transport
model (M.Mattiolis)
Ionisation, recombination and radial transport
of the ions of charge Z Radiative,
dielectronic, charge-exchange recombination Impur
ity influx is given as boundary condition, its
time evolution is determined by tracking the
brightness of peripheral lines. The transport
coefficients D and v, radius and time dependent,
are chosen in such a way as to obtain the best
global simulation of the available experimental
data Emission line spectra SXR Bolometry.
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Radiatively improved modes in JET Elmy H mode

• Radiatively improved modes obtained in Jet in
  various configurations, heating schemes and
  puffing rates.
• Example Shot 53030
• Low triangularity (d 0.22)
• X-point on septum. Ar Puffing.
• ITER ref. Scenario
• H981, bN1.8, n/nG0.85

• J. Ongena et al., Phys .of Plas. 8 (2001) 2188

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JET Elmy H mode / After puff/ Ar accumulation

• The after puff phase features higher particle
  confinement time and density peaking.
• With strong Ar puffing
  -q(0)
  increases,
  -
  sawtooth amplitude decreases
  - Ar
  accumulates
  -
  confinement degrades
  - sometimes
  radiative collapse is reached

W. Suttrop et al., Phys.of Plas.9 (2002) 2103
-.
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JET Elmy H mode / After puff/ Effect of ICRH

• Moderate (2-3 MW against 10-12 MW of NBI) ICRH
  power deposited in the center
• Heats the plasma core (Te peaks)-gt Screens
  impurity
• Increases diffusion ( ne flattens) -gt Opposes
  impurity peaking
• Keeps q(0) below 1 - maintains sawteeth -gt
  Contribute to expel Ar
• Altogether sustains the anomalous transport -gt
  Reduces impurity accumulation

M.F Nave et al. To be published
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JET Elmy H mode / After puff/ Effect of ICRH
Ar density profiles reconstructed by a 1-D
Collisional Radiative Transport Code (Mattiolis)
Septum, low d w/o ICRH
Septum, low d with 2 MW ICRH
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JET Elmy H mode / After puff/ Effect of ICRH
EHT , Continuous D2 Puffing, with 2 MW
ICRH
Best radiation belt. Possible contribution from CX
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JET Elmy H mode / After puff/ Effect of
ICRH
Ds and Vs (from Mattiolis impurity transport
model)

• In shots in which accumulation is avoided
• Anomalous transport increases
• Inward convection decreases and may become
  outward

No accumulation convection may become outward
Accumulation- Strong inward convection
M.E. Puiatti et al .Plas. Phys.Contr. Fus.
44(2002)1863
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JET Elmy H mode / After puff/ Effect of ICRH
Neoclassical transport parameters In both cases
, with and without accumulation , transport is
anomalous, but in the shot with accumulation the
empirical peaking factor is closer to the
neoclassical one than in the case w/o
accumulation.
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JET Elmy H mode / After puff/ Effect of ICRH

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JET Elmy H mode / After puff/ Effect of Sawteeh
Impurity transport model results Sawteeth
contribute to the expulsion of the impurities
from the core
M.Mattioli et al .EPS meeting Montreaux 2002
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JET Elmy H mode / After puff/ Effect of Sawteeh
However their sole contribution does not justify
the absence of Ar accumulation other
mechanisms are present
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JET Elmy H mode / After puff/ Effect of
continuous modes
Other MHD activity in the form of continuous
modes - m1 n1 and others -helps increasing the
anomalous transport .
M.Mattioli et al .EPS meeting Montreaux 2002
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Radiatively improved mode in FTU

• In FTU ohmic Ne seeded plasmas RI-Mode avoids
  saturation of confinement with density .
• Typical signatures
• Ne profiles peak
• Electron and ion temperature increase (for the
  same input power)
• As a consequence, confinement improves (x1.4)

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Radiatively improved mode in FTU
D.Frigione, L. Pieroni et al . EPS Montreaux, 2002
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Radiatively improved mode in FTU
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Radiatively improved mode in FTU
FTU has TZM (Mo alloy) limiters
L.Carraro et al. EPS Montreaux 2002
In Ne seeded shots metal concentration (Fe, Ni,
Mo) decreases This appears to be due to a
reduced sputtering associated with the reduced
convected /conducted power through the edge (Grad
.85) .
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Radiatively improved mode in FTU

• Impurity transport does not change significantly
  (same vs and Ds) give satisfactory simulation
  results in both shots with and without seeding)
• Impurity transport is anomalous neoclassical
  diffusion in the core 0.02 m2s-1
• Accumulation is avoided by a reduction of the
  influx

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Conclusions

• In High density regimes impurity seeded
  discharges impurity accumulation can be avoided.
• IN JET The risk of impurity accumulation with
  Ar seeding is avoided by modifying transport.
  Adding central deposited ICRH on top of NBI
  heats the core and maintains q(0) below 1 and
  flat.
• IN FTU The radiation belt in Ne seeded D
  plasmas avoids the risk of impurity accumulation
  by reducing significantly the metal influx,
  with no major modification of the impurity
  transport.
• FUTURE WORK
• 1) Extend the analysis to other High Density
  scenarios
• 2) Investigate detailed transport mechanisms