1. 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

2. C. F. Maggi1, H. Meyer1, C. Bourdelle2, A. Chankin3, E
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 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, 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 | | Page 2

3. 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 | | Page 3

4. 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 | | Page 4

5. Values in L-mode before L-H transition
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 | | Page 5

6. 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 | | Page 6

7. 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
fff
SOL Er could be affected by changes in Prad in divertor and SOL
CF Maggi | T&C ITPA Meeting | | Page 7

8. 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 | | Page 8

9. 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| | Page 9

10. 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 | | Page 10

11. 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 | | Page 11

12. 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| | Page 12

13. 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)
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
2.0 < ne < 2.3x1019 m-3
CF Maggi | T&C ITPA Meeting| | Page 13

14. 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| | Page 14

15. EDGE2D/EIRENE: DZeff vs Prad
Be+D+N: N2 puffing from private flux region
Be+D+N: N2 puffing from outer midplane (puffing rate is 1.4x lower)
Be + D + N
Be + D + N
DZeff
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 | | Page 15

16. Effect of Prad on SOL Er SOL Er ~ -3 grad(Te,target)/e Te,OSP [eV]
Hhhhhh
Ffff j
EDGE2D/EIRENE
Be + D + N
Langmuir probe data
#83160: no N2
#87145: low N2
#87145: high N2
Prad
Distance along outer target [mm]
Be + D + N
Prad [MW]
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 | | Page 16

17. 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 | | Page 17

18. 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 | | Page 18

19. 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 | | Page 19

20. Extra slides
CF Maggi | T&C ITPA Meeting | | Page 20

21. Zeff at L-H transition cN [%] cBe [%] yN #87145 #87143 #87143 #87145
Increase in Zeff dominated by increase in N concentration cN
Zeff from visible Bremsstrahlung and core CXRS (N + Be) consistent within 10% yN
CF Maggi | T&C ITPA Meeting| | Page 21

22. Prad profiles at L-H yN High ne branch Total Prad within flux [MW]
#83160: JET-ILW
#87145: JET-ILW + low N2
#87143: JET-ILW + high N2
Total Prad within flux [MW] yN
CF Maggi | T&C ITPA Meeting| | Page 22