Impact of low-Z impurities on the
L-H threshold in JET
CF Maggi, H Meyer, C Bourdelle, A Chankin, E Delabie, I Carvalho,
M Brix, P Drewelow, C Giroud, C Guillemaut, J Flanagan, L Meneses,
F Rimini and JET Contributors
C.F. Maggi1, H. Meyer1, C. Bourdelle2, A. Chankin3, E. Delabie4, M Brix1,
I.S. Carvalho5, P. Drewelow6, J Flanagan1, C Giroud1, C Guillemaut5,
L Meneses5, F. Rimini1 and JET Contributors*
EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK
1CCFE,
Culham Science Centre, Abingdon OX14 3DB, UK
2CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France
3MPI für Plasmaphysik, Garching, Germany
4Oak Ridge National Laboratory, Oak Ridge, Tennessee, US
5Instituto de Plasmas e Fusão Nuclear, IST, Universidade de Lisboa, Av. Rovisco Pais,
1049-001 Lisboa, Portugal
6MPI für Plasmaphysik, Greifswald, Germany
*See the Appendix of F. Romanelli et al., Proceedings of the 25th IAEA Fusion Energy
Conference 2014, Saint Petersburg, Russia
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 2
Outline
• Motivation: lower PL-H and Zeff in JET-ILW vs JET-C (high ne branch)
• Two possible mechanisms:
• Effect of low-Z impurities on L-mode edge turbulence  Zeff
• Changes in SOL Er  possible impact of Prad in div/SOL
• Effect of N2 injection on JET-ILW L-H transitions
• Outlook and conclusions
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 3
Lower PL-H in JET-ILW in high ne branch
JET MkII-HD divertor geometry
JET-ILW:
Be in main chamber
W divertor
• Lower PL-H in JET-ILW than in JET-C
in high ne branch
• Not a core radiation effect
[Maggi, Nucl. Fusion 2014]
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 4
Lower Zeff in JET-ILW
• Be: Lower Z & no chemical sputtering
Values in L-mode before L-H transition
Low d
High d
JET-C
JET-ILW
[Maggi, Nucl. Fusion 2014]
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 5
Low-Z impurities affect L-mode edge turbulence
Linear GK analysis (GENE) of JET edge plasmas before L-H transition
[Bourdelle, Nucl. Fusion Lett. 2014]
Experimental ExB shearing rates
•
Unstable modes are RBMs  stabilized by increased T (/power)
•
Growth rates are stabilized for T ~ experimental TL-H values
•
Larger growth rates for higher Zeff  in qualitative agreement with experiment
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 6
Higher SOL Er in HOR vs VT configuration
• Higher SOL Er  higher outer ErxB shear in horizontal than vertical target
• Proposed as mechanism to explain divertor configuration effect on PL-H
[Delabie, EPS 2015, O3.113]
SOL Er ~ -3 grad(Te,target)/e
ggg
Ggg SOL Er could be affected by changes
Ggg in P in divertor and SOL
rad
Ggg
fff
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 7
N injection into JET-ILW L-modes prior to L-H
‘Reversed experiment’:
•
Recover in JET-ILW edge radiation and Zeff
close to JET-C
•
C cannot be reintroduced in JET-ILW
•
 Use N as proxy for C, due to similarities
in Z and in edge radiating properties
Nitrogen injection into
private flux region
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 8
At high N flow rates Psep increases to JET-C levels
Externally controlled increase in Zeff at ~ constant ne
•
Threshold effect on Zeff/cN: for DZeff ≥1, increase in Psep to JET-C levels
•
Effect only observed in high ne branch and near ne,min (due to N fuelling at low
L-mode target densities and low Pin)
CF Maggi | T&C ITPA Meeting| 17.03.2016 | Page 9
Experiments at 3.0T/2.5MA, high d
•
Increase in PL-H with N injection
is independent of d and IP/BT
variations
•
Impact of N injection on PL-H
could not be tested in low ne
branch, due to N fuelling effect
in L-mode phase before L-H
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 10
Experiments at 3.0T/2.5MA, high d
•
Psep in JET-ILW + N2 approaches JET-C values when Zeff is similarly high
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 11
Local parameters at L-H transition
•
For given edge density, edge Te is lower in JET-ILW than in JET-C and
increases to JET-C levels in JET-ILW + high N injection (DZeff ≥1)
•
Note: Te,edge = Ti,edge in JET-ILW L-H transitions
[Maggi, Nucl. Fusion 2014]
[Delabie, EPS 2015]
CF Maggi | T&C ITPA Meeting| 17.03.2016 | Page 12
Stronger edge Er at high N concentrations
•
Edge Er ~ Er,dia = grad pi / (e ni) at low vtor (see Sauter NF2012 for AUG; holds
also for JET-ILW, see Maggi NF2014)
2.0 < ne < 2.3x1019 m-3
•
w/o N injection: min Er ~ constant
with ne
•
N concentration scan at ne ~ ne,min:
|min Er| increases at high N2 rates
(DZeff ≥1)
•
Mirrors increase in Psep with Zeff
CF Maggi | T&C ITPA Meeting| 17.03.2016 | Page 13
Prad fractions at L-H
Factor 2-3 higher div+SOL radiation in JET-ILW+N2 than in JET-C to achieve similar
Zeff ~2  points to Zeff being the relevant parameter (rather than Prad)
•
JET-ILW + N2: N2 injection from
divertor private flux region  high
N retention in divertor?
•
JET-C: C sources from both div and
main chamber  higher Zeff at
similar Prad(DivSOL)?
CF Maggi | T&C ITPA Meeting| 17.03.2016 | Page 14
EDGE2D/EIRENE: DZeff vs Prad
•
Be+D+N: N2 puffing from
private flux region
•
Be + D + N
DZeff
Be + D + N
Be+D+N: N2 puffing from
outer midplane (puffing rate
is 1.4x lower)
C+D
Prad [MW]
• Prad scans with EDGE2D/EIRENE deliver similar DZeff at given Prad in JET-C and
JET-ILW + N2, in contrast to experimental findings  so far result not
conclusive (on-going)
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 15
Effect of Prad on SOL Er
SOL Er ~ -3 grad(Te,target)/e
#83160: no N2
Te,OSP [eV]
Prad
Be + D + N
Be + D + N
#87145: low N2
#87145: high N2
EDGE2D/EIRENE
Prad [MW]
Langmuir probe data
Distance along outer target [mm]
• Te,OSP decreases with Prad, in qualitative agreement with experiment
• Deeper Er needed at high Prad (/N2 rates) to reach same ErxB shear keeps
SOL Er mechanism still open as explanation for N2 injection effect on PL-H
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 16
Comparison with ITPA 2008 scaling for PL-H
JET MkII-HD L-H data:
•
JET-C vs JET-ILW [Maggi NF2014]
•
Effect of divertor configuration in JET-ILW
[Maggi NF2014], [Meyer EPS2014],
[Delabie IAEA2014], [Delabie EPS2015]
•
Density scans at BT: 1.8  3.0T
•
Ip variations at constant BT
•
N2 injection in JET-ILW
[Martin, JPCS 2008]
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 17
PL-H and Psep vs ITPA scalings
JET MkII-HD L-H data
[Martin, JPCS 2008]
[Takizuka, PPCF 2004]
• Neither scaling captures all the variations in the experiment, but the scaling
which includes Zeff gives a tighter fit to the JET data…
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 18
Outlook and Conclusions
• PL-H is affected by low Z-impurities in JET:
• Lower PL-H in JET-ILW than in JET-C (high ne branch)
• Increase in PL-H with N2 injection in JET-ILW, recovering JET-C values
when Zeff is increased to similar levels as in JET-C
• Two possible mechanisms at play:
•
Effect of low-Z impurities on L-mode edge turbulence  Zeff
•
Changes in SOL Er induced by changes in Prad(div/SOL)
• Analysis of edge turbulence in L-H transitions with N2 injection is in progress:
linear GENE runs and comparison with analytical model [Bourdelle, NF2015]
• The experiments in JET-ILW + N2 injection point to the reduction in PL-H being
correlated with a reduction in Zeff rather than with changes in Prad
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 19
Extra slides
CF Maggi | T&C ITPA Meeting | 17.03.2016 | Page 20
Zeff at L-H transition
cN [%]
#87143
#87145
cBe [%]
• Increase in Zeff dominated by increase in
#87143
N concentration cN
#87145
• Zeff from visible Bremsstrahlung and core
CXRS (N + Be) consistent within 10%
yN
CF Maggi | T&C ITPA Meeting| 17.03.2016 | Page 21
Prad profiles at L-H
Total Prad within flux [MW]
High ne branch
#83160: JET-ILW
#87145: JET-ILW + low N2
#87143: JET-ILW + high N2
yN
CF Maggi | T&C ITPA Meeting| 17.03.2016 | Page 22