The effect of divertor detachment on hydrocarbon sources in JET L-mode discharges

1. The effect of divertor detachment on hydrocarbon sources in JET L-mode discharges S. Brezinsek, A.G. Meigs, J. Rapp, R. Felton, A. Huber, S. Jachmich, V. Philipps, A. Pospieszczyk, R. Pugno, O. Schmitz, G. Sergienko, M. F. Stamp, and EFDA-JET Contributors Motivation Plasma configuration and settings Applied diagnostics Detachment characterisation Hydrocarbon chemistry Conclusion Outline:

2. Introduction • Motivation: • A semi-detached outer divertor is foreseen as the operating regime for ITER. • Reduction of the hydrocarbon flux (& chemical sputtering) in AUG and DIII-D with detached outer divertor of observed. • Comparison with the hotter target (~180°C) of JET would help to close the gap to ITER. • ITPA D-SOL task Brezinsek et al. Phys. Scripta 2007 Goals: i) Controlled and steady-state outer divertor detachment operation. ii) Characterisation of the detached outer divertor. iii) Hydrocarbon chemistry under detached plasma conditions.

3. Settings and Configuration • Settings: • L-mode (I=1.7MA, B=2.7T, PNBI=1.7MW) • Deuterium plasma • Outer strike point fixed on the LBSRP – optimised for diagnostics • Strong deuterium fuelling in the PFR • Local methane injection at the OSP (only a few shots) LBSRP

4. Plasma scenario Time evolution: Partial detachment and full detachment scenario is made with feedback control after approaching the programmed density in the density ramp.

5. Diagnostics KS3 KT3 GIM14 • Diagnostics: • Divertor spectroscopy (KT3,KS3) • Langmuir probes (KY4D) • CCD camera with Da filter (KL1) • IR camera (KL7) • Divertor bolometry (KB5) • Gas injection module (GIM14) • OSP is fixed on the LP close to GIM 14 location (local injection in the SOL) • KS3 and GIM 14 used for in-situ calibration of hydrocarbons (octant 5) • KT3 (octant 8) and KY4D used for detachment characterisation

6. Detachment characterisation (Langmuir Probes) Density ramp: Ion saturation current at injection location Degree of detachment • Roll over in ion saturation current indicates “start” of detachment • Complete detachment of the outer divertor at ~62 s

7. Detachment characterisation (Langmuir Probes) Plasma parameters in the outer divertor (LBSRP): Attached plasma (close to roll over) Complete detached plasma KY4D • Plasma (too) hot for recombination? • Peak ion flux: 3.4*1022 m2s-1 • Neutral flux in the sub-divertor: 2*1022m2s-1 • High density and hot plasma • Peak ion flux: 1.7*1023 m2s-1 • Neutral flux in the sub-divertor: 0.8*1022m2s-1

8. Detachment characterisation (spectroscopy) Deuterium volume recombination in the outer divertor: Paschen recombination line analysis Spatial distribution (tile 5) full detached roll over (LP) Time [s] Time [s] semi detached Paschen continuum Paschen recombination lines SOL PFR Paschen line (8->3) Wavelength [A] Radius [a.u.] Plasma parameters from Balmer recombination line analysis: Te ~ 1-2 eV ne ~ 2-3 1020 m-3 Intensity [arb. units] Wavelength [A] • Much colder and denser than plasma parameters from Langmuir probes KT3C #70574

9. Emission pattern in the outer divertor: Dg and CD Spatial distribution: Dg CD – Gerö band roll over (LP) Time [s] Time [s] Intensity [arb. units] roll over (LP) SOL SOL PFR PFR Radius [a.u.] Radius [a.u.] Strong drop of intrinsic CD light! Intensity [arb. units] Radius [a.u.] Time [s] Radius [a.u.] Time [s] KT3B #70574

10. Hydrocarbon chemistry in the outer divertor Integrated CD and Dg intensities: density ramp KT3B #70574 • Hydrocarbon emission (and flux) dropped prior to ion flux roll over • No complete suppression of the hydrocarbon flux in complete detachment • Possible reasons: flux dependence / threshold energy for Ychem

11. Attached – Detached oscillation (with feedback) Spatial distribution during attached – detached oscillation: Spatial distribution Dg Spatial distribution CD Time [s] Time [s] attached detached Radius [a.u.] Radius [a.u.] KT3B #70578 • Density feedback set to the density at the roll over time • Switch between attached and detached regime (gain too high) • Oscillation of Dg and CD (same for C2 and D2)

12. Local methane injection through GIM14 Hydrocarbon particle flux quantification: attached detached CII CD Dg attached detached Extrinsic source is detectable! Photon efficiencies in attached and detached case are comparable! Dg Dg CD CD CII KS3A #70579 • Reduction of CD light is related to reduction of particles! (Same for C2!) • Injected amount is too high (valve operation restriction) • Absolute quantification not yet done (need of calibration)

13. Conclusion Outer divertor plasma scenario: • Detached and volume recombined outer divertor plasma established • Plasma parameters from spectroscopy and Langmuir probes differ Hydrocarbon chemistry: • Hydrocarbon “footprint” drops prior to roll over of the ion flux • Extrinsic source is detectable • Substantial reduction of the erosion yield • But absolute quantification not yet done • Effect of flux dependence (ions and atoms at low energies) • Energy threshold of chemical sputtering? Comparison with other experiments: • Similar behaviour with respect to DIII-D and AUG experiments – • but hydrocarbon flux is still detectable during detachment