Impact of low-Z impurities on the L-H threshold in JET

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Presentation transcript:

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 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

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 | 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 fff SOL Er could be affected by changes in Prad in divertor and SOL 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) 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| 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: 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 | 17.03.2016 | Page 15

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 | 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 [%] 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| 17.03.2016 | Page 21

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| 17.03.2016 | Page 22