Transcript Impurity Transport in High Density Plasmas in JET and FTU

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
P.DuMortier, A. Messiaen, J Ongena
R.Dux,
M.F.F Nave
J.Rapp, B. Unterberg
L. Gabellieri ,D. Frigione, L. Pieroni
Consorzio RFX, Padova, Italy
Ecole Royal Militaire, Brussels, Belgium
IPP-Euratom Assoziation, Garching Germany
Centro de Fusão Nuclear, 1096Lisbon , Portugal
IPF Jülich GmbH, Jülich, Germany
ENEA, Frascati, Italy
Presented by M. Valisa
9th
EU-US Transport Task Force Workshop
Cordoba, Spain - Sept. 9- 12 / 2002
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 (> 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.
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
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
Background 2: Impurity accumulation
Accumulation of impurities depends on the combination of various
processes
Transport Processes
•Anomalous transport - Typically flattens profiles
v n T
•Neoclassical transport


D
n
T
•Edge transport/ ELM’s/ screening
PWI
•Impurity production mechanisms
•Impurity net influx
The analysis method : 1 D impurity transport model (M.Mattioli’s)
Ionisation, recombination and radial transport of the ions of charge Z:
Radiative, dielectronic, charge-exchange recombination
Impurity 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.
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 :
H98=1, bN=1.8, n/nG=0.85
•J. Ongena et al., Phys .of Plas. 8 (2001) 2188
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,
W. Suttrop
- sawtooth amplitude decreases
- Ar-.accumulates
- confinement degrades
- sometimes radiative collapse is reached
et al., Phys.of Plas.9 (2002) 2103
JET Elmy H mode / After puff/ Effect of ICRH
Moderate (2-3 MW against 1012 MW of NBI) ICRH power
deposited in the center:
• Heats the plasma core (Te
peaks)-> Screens impurity
• Increases diffusion ( ne
flattens) -> Opposes impurity
peaking
•Keeps q(0) below 1 maintains sawteeth ->
Contribute to expel Ar
• Altogether sustains the
anomalous transport ->
Reduces impurity
accumulation
M.F Nave et al. To be published
JET Elmy H mode / After puff/ Effect of ICRH
Ar density profiles reconstructed by a 1-D Collisional
Radiative Transport Code (Mattioli’s)
Septum, low d
w/o ICRH
Septum, low d
with 2 MW ICRH
JET Elmy H mode / After puff/ Effect of ICRH
EHT , Continuous
D2 Puffing,
with 2 MW ICRH
Best radiation belt. Possible contribution from CX
JET Elmy H mode / After puff/
Effect of ICRH
D’s and V’s (from Mattioli’s impurity transport model)
In shots in which accumulation is avoided
•Anomalous transport increases
• Inward convection decreases and may become outward
1.0
0.80
pulse n. 53015
0.60
0.40
r
v [m/s]
No accumulation : convection may
become outward
pulse n. 53548
0.20
0.0
-0.20
Accumulation
- Strong
inward
convection
pulse n. 52146
-0.40
pulse n. 52136
-0.60
0
0.2
0.4
0.6
0.8
1
r [m]
M.E. Puiatti et al .Plas. Phys.Contr. Fus. 44(2002)1863
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.
2.0
0.80
blue -> #52146 (Ar not acc.)
red -> #52136 (Ar acc.)
0.70
blue -> #52146 (Ar not acc.)
red -> #52136 (Ar acc.)
1.0
anomalous *3
0.60
v [m/s]
0.50
anomalous
2
D [m /s]
0.0
0.40
0.30
-1.0
-2.0
neoclassical
0.20
-3.0
0.10
neoclassical
0.0
0
0.2
0.4
0.6
r [m]
0.8
-4.0
1
0
0.2
0.4
0.6
r [m]
0.8
1
JET Elmy H mode / After puff/ Effect of ICRH
#52136 (Ar accumulating)
#52146 (Ar not accumulating)
#52136 (Ar accumulating)
#52146 (Ar not accumulating)
10
5.0
8.0
m-3]
19
3.0
ne [ x10
Te [keV]
4.0
2.0
4.0
2.0
1.0
0.0
6.0
0.0
2
2.5
3
r [m]
3.5
4
2
2.5
3
r [m]
3.5
4
JET Elmy H mode / After puff/ Effect of Sawteeh
Impurity transport model results :
Sawteeth contribute to the expulsion of the impurities
from the core
1 10
4
SXR emissivity [W/m
3
]
Before ST
8 10
3
6 10
3
After ST crash
ST at 2.95s
4 10
3
2 10
3
solid line: simulation
dashed line:experimental
0
0
0.2
0.4
0.6
0.8
1
r [m]
M.Mattioli et al .EPS meeting Montreaux 2002
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
17
1.2 10
17
1.0 10
17
8.0 10
16
6.0 10
16
4.0 10
16
2.0 10
16
simulation with 3 ST and transport
of an accumulating discharge
simulation without ST
Ar density [m
-3
]
1.4 10
0.0 10
simulation with 3 ST
# 52146
0
0
0.2
0.4
0.6
r[m]
0.8
1
JET Elmy H mode / After puff/ Effect of continuous modes
Other MHD activity in the form of continuous modes m=1 n=1 and others -helps increasing the anomalous
transport .
M.Mattioli et al .EPS meeting Montreaux 2002
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)
Radiatively improved mode in FTU
D.Frigione, L. Pieroni et al . EPS Montreaux, 2002
Radiatively improved mode in FTU
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) .
Radiatively improved mode in FTU
•Impurity transport does not
change significantly (same v’s
and D’s) 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
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