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The density profiles of the light impurities in the plasma and the comparison with quasi-linear and non linear gyrokinetic simulations are shown

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INTRODUCTION

A series of experiments was carried out in JET ILW L-mode plasmas in order to study the transport of light impurities and their effects on core thermal transport. These discharges feature the presence of 3He, Be, C, N, whose profiles are all measured by active Charge Exchange, although with different degrees of accuracy. To study the effects on ion heat transport, ICRH power was deposited on- and off-axis mainly to ions in (3He)-D minority scheme, in order to have a scan of the ion heat flux versus R/LTi, and also modulated for ion heat wave propagation. The density profiles of the light impurities in the plasma and the comparison with quasi-linear and non linear gyrokinetic simulations are shown. The impact on ions and electrons heat transport of the presence of nitrogen in the plasma is studied both analysing the experimental data and with gyrokinetic simulations.

LIGHT IMPURITY TRANSPORT

C-wall, L-mode plasmas. B0~ 3.3 T, ICRH~3-4 MW on- off-axis in (3He)-D minority scheme, NBI~ 3 MW, Ip~ 1.5 MA, ne,0~ 3·1019 m-3, Te,0~ 5 keV, Ti,0~ 2.5 - 4 keV. Nitrogen puffed in discharges 86749/56/58 (N~1.2%). N~0% in discharges 86740/43/46.

N.Bonanomi1,2, P. Mantica2, C, Giroud3, C. Angioni4, J. Citrin5,6, E. Lerche7, P. Manas4, S. Menmuir3, C. Sozzi2, M. Tsalas5,3, D. Taylor3, D. Van Eester7 and JET contributors*

EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK

Light impurities in JET plasmas: transport mechanisms and effects on thermal transport

EFFECTS ON THERMAL TRANSPORT

Figure 5: Ti radial profile (left), Te profile (center) and Te/Ti profile (right). Discharge 86740 (blue circles) representative of discharges without N. Discharge 86749 (red squares) representative of discharges with N.

CONCLUSIONS

ACKNOWLEDGMENTS

The authors would like to thank Yann Camenen for precious suggestions and discussions.

This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No.DE-AC02-05CH11231. A part of this work was carried out using the HELIOS supercomputer system at Computational Simulation Centre of International Fusion Energy Research Centre (IFERC-CSC), Aomori, Japan, under the Broader Approach collaboration between Euratom and Japan, implemented by Fusion for Energy and JAEA.

REFERENCES 1 Università di Milano-Bicocca, Milano, Italy

2 CNR - Istituto di Fisica del Plasma “P. Caldirola”, Milano, Italy

3 Culham Centre for Fusion Energy, Abingdon, OX14 3DB, UK

4 IPP-Garching Garching bei Munchen, Germany

5 FOM Institute DIFFER, 5600 HH, Eindhoven, The Netherlands

6 CEA, IRFM, F-13108 Saint Paul Lez Durance, France

7 LPP-ERM/KMS, TEC partner, 1000 Brussels, Belgium

* See the Appendix of F. Romanelli et al., Proceedings of the 25th IAEA Fusion Energy Conference 2014, Saint Petersburg, Russia

Figure 2: Neoclassical/turbulent contributions to the particle transport for N (red) and C (black).

Left oriented triangles represent the convective part of the transport while right oriented triangles represent the diffusive part. The neoclassical contribution to the impurity particle transport is calculated with the NEO code [1] and results to be negligible , compared to the turbulent transport, outside ρtor ~ 0.2.

ρtor

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

n 3 He

×1018

0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2

86749

86758: 13MW NBI high rotation (0.33*real density)

ρtor

0 0.2 0.4 0.6 0.8 1

n C

×1016

2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6

j86740, C profile j86740, fitted profile

ρtor

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

n Be

×1017

1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

86749

ρtor

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

n N

×1017

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

86749

86758: 13MW NBI high rotation (1.6*real density)

ρtor

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Dneo turb , RVneo turb

10-2 10-1 100

JET 86749, D

N,C neo

i turb

JET 86749, |RV

N,C neo|/χ

i turb

JET 86740, |RV

C neo|/χ

i turb

JET 86740, |RV

C neo|/χ

i turb

7N

6C

4Be

32He

R/Limp EXP QL NL

32He r= 0.33 2.5 +/- 0.5 1.25 / r= 0.5 2.8 +/- 0.5 0.7 /

4Be r= 0.5 peaked 0.7 /

6C r=0.2 2.5 +/- 0.5 / / r = 0.5 0.3 +/ 0.5 1.3 1

7N r= 0.33 1.8 +/- 0.3 1.3 1.6 r = 0.5 0.3 +/- 0.3 1.5 1-1.3 Figure 3: Radial density profiles of 3He, Be, C and N. The density profiles of the same species are similar in all the discharges in which they are measured. The C profile is measured only for the discharge 86740 (N=0%), as the nitrogen has a big impact of the CX analysis of the C lines. The Plume effect is not considered for the 3He profile. The dashed purple lines are the profiles for a discharge with high rotation.

Ta b l e 1 : C o m p a r i s o n b e t w e e n t h e experimental peaking of the impurities density profiles, the quasi-linear gyrokinetic simulations and with nonlinear gyrokinetic simulations (with GWK [2] and GENE [3]).

The simulations underestimate the density peaking of the 3He and overestimate the peaking of 6C and 7N. The simulations indicate that important mechanisms [4] for the turbulent particle transport are the thermo-diffusion and the pure pinch, while the roto-diffusion is less important (~1/20 of the other terms).

ρtor

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

T i (eV)

500 1000 1500 2000 2500 3000 3500 4000 4500

j86740 j86749 j86740 fitted j86749 fitted

ρtor

0 0.2 0.4 0.6 0.8 1

s/q

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

86740 86749

ρtor

0 0.2 0.4 0.6 0.8 1

T e/ T i

0.2 0.4 0.6 0.8 1 1.2 1.4

86740 86749

ρtor

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Plasma resistiivity

×10-7

0 0.5 1 1.5 2 2.5

86740, Zeff ~ 1.4 86749, Zeff ~ 1.9

(Profiles are almost equal if Z

eff is fixed)

kyρ

s

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

γ (R/c s)

0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6

ρtor = 0.5

86749, 1.2% N

ITG <--> TEM/ETG dominant

86749 no N

with 86740 s/q and T

e/T

i

86749, no N

kyρ

s

0 5 10 15 20 25 30

γ (R/c s)

0 2 4 6 8 10 12 14 16

86749, no N 86749, N = 1.2%

R/LTi

0 1 2 3 4 5 6 7 8 9 10

q i, gBs

0 5 10 15 20 25 30 35 40 45

ρtor = 0.33

GENE linear threshold N ~ 0

86740,86743,86746

N ~ 1%

86749,86756

open = EXP.

full = GENE

R/LTi

0 1 2 3 4 5 6 7 8 9 10

q i, gBs

0 20 40 60 80 100 120

GENE linear thresholds

86740,86743,86746 N = 0%

86749,86756 N ~ 1,2%

86749 N = 0%

ρtor = 0.5 open = exp.

full = GENE

R/LTe

6 6.5 7 7.5 8 8.5 9

q e,gBs

0 5 10 15 20 25 30 35 40 45 50

ρtor = 0.5 empty = exp.

full = GENE

86740, no N

86749, no N 86749, N = 1%

ρtor = 0.33

Figure 6: Radial profiles of plasma resistivity (left), and s/q (right) of discharges 86740 (blue circles, no N) and 86749 (red squares, N~1%). Difference in s/q can be a secondary effect of the N injection. Higher Zeff causes higher plasma resistivity, that change the current density radial profile and so the q (and s) radial profiles.

ρtor

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

T e (eV)

0 1000 2000 3000 4000 5000 6000

86740 86749

Figure 7: Linear gyrokinetic sim. of the effects of N, s/q and Te/Ti on the plasma micro-instabilities on ion-scale (left) and electron-scale (right). The stabilization of ITG is due to main ion dilution and higher Ti/Te and s/q [8,9].

Stabilization of TEM is due to higher collisionality and higher s [5,7].

Stabilization of ETG is due to higher τ=ZeffTe/Ti.

[1] E. Belli and J. Candy, Plasma Phys. Control. Fusion 50, 2008.

[2] A.G. Peeters et al., Computer Physics Communications 180 , 2009.

[3] F. Jenko et al., Phys. Plasmas 7, 2000.

[4] C. Angioni et al., Nucl. Fusion 52, 2012 [5] F. Ryter et al., Phys. Rev. Letter 95, 2005

[6] F. Jenko et al., Phys. Plasmas 8, 2001 [7] N. Bonanomi et al., Nucl. Fusion 55, 2015 [8] F. Romanelli, Phys. Fluids B 1 5, 1989.

[9] P. Migliano et al., Plasma Phys. Control. Fusion 55, 2013

R

Limp (Γ = 0) = − RV

D =- D

D [CT R

LT,imp +CR ⋅u'+ CP]

Contribution to the particle flux assuming no sources: CT is the thermo-diffusion coefficient, CR is the roto-diffusion coefficient and CP is the pure pinch.

Figure 8: qi,gB vs R/LTi (left and central) and qe,gB vs R/LTe (right). Open symbols=experimental points, full symbols=GENE non-linear simulations. The stabilization effects of N is visible at both radii, but is stronger at outer radii due to higher differences in s/q and Te/Ti. The role of N is visible from the simulations (full purple diamond). On electrons, with N the values of R/LTe are higher with the same flux.

•  Radial density profiles of four light impurities in a JET L-mode ILW discharge are shown.

•  Known mechanisms for impurity transport in plasma are studied. The simulations underestimate the density peaking of the 3He and overestimate the peaking of 6C and 7N.

The simulations indicate that the most important mechanisms for the turbulent transport are the thermo-diffusion and the pure pinch.

•  The effect of the N on thermal transport is studied. The general effect is a stabilization of both the electron and the ion turbulent transport. The stabilization is due to different mechanisms, directly or indirectly caused by the presence of N.

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