Review of experimental observations of plasma detachment and of the effects of divertor geometry on divertor performance

Transcription

1 Review of experimental observations of plasma detachment and of the effects of divertor geometry on divertor performance Alberto Loarte European Fusion Development Agreement Close Support Unit - Garching With contributions from : ALCATOR C-mod, ASDEX Upgrade, DIII-D, JET, JT-60U, TCV,. Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

2 Outline of the Talk 1. Introduction : basic effects of divertor geometry 2. Divertor geometry effects on neutral dynamics and recycling 3. Divertor geometry effects on detachment 4. Divertor geometry effects on divertor radiation and SOL flows 5. Conclusions Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

3 Basic effects of divertor geometry (I) various divertor geometries have been explored in most divertor tokamaks in the last decade Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

4 Basic effects of divertor geometry (II) B θ 0 near X-point + Divertor Target Geometry Γ div determine & Γ div JET Experiments : Ohmic and ELM-free H-mode Loarte-NF 1992 Basic ingredient for power handling design of all divertor targets Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

5 Basic effects of divertor geometry (III) Diffusion in Private Flux Region (PFR) is a unique Divertor Process DIII-D Experiments : ELMy H-mode Γ div Increases with X-point height Decreasing Flux expansion and Increasing angle of incidence No Diffusion in PFR Diffusion in PFR accounts for ~2 reduction in when going from AUG-Div I AUG-Div II div,sep P Lasnier - NF 1998 Loarte CPP 1992 Schneider JNM 1999 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

6 Basic effects of divertor geometry (IV) divertor geometry neutral recycling, closure, detachment, etc. Divertor geometry can be used to affect neutral re-emission λ ion /λ divertor divertor closure Taroni, CPP 1994 Divertor geometry can be used to affect local neutral ionisation pattern and fraction, pumped neutral flux, access to low T div sep conditions for given n e, etc. Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

7 Basic effects of divertor geometry (V) Optimising Divertor Geometry reduces Neutral Escape through Divertor Plasma Increased Divertor Closure Larger Divertor Pumping Divertor Closure = S ion outside-divertor /S ion total Common Result to all Divertor Experiments (AUG, C-mod, DIII-D, JET, JTF-2M, JT-60U) JET Team-NF 1999 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

8 Basic effects of divertor geometry (VI) Divertor geometry affects more weakly Total Ionisation balance (P rad does!) Γ divertor ~ n d c s T d ~ n s P SOL 2/7 c s (mbar) Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

9 Divertor geometry effects on neutrals and recycling (I) Divertor geometry is effective for pumping control Pumping Transport of neutrals from separatrix to pumping plenum Ballistic Transport Strong Dependence of Pumping on Divertor Plasma Position Diffusive Transport (Multiple Wall Collisions and C-X) Weak Dependence of Pumping on Divertor Plasma Position DIII-D JET Cryo-Pump Loarte, EPS 1997 Maingi, NF 1999 Loarte, PPCF 2001 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

10 Divertor geometry effects on neutrals and recycling (II) Similar dependence on strike point position seen in JT-60U and DIII-D well reproduced by Monte-Carlo modelling DIII-D Takenaga, NF 2001 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

11 Divertor geometry effects on neutrals and recycling (III) Noble Gases Transport has large Ballistic Component Noble Gas Enrichment in Pumping Plenum dominated by Geometry and Ionisation mean free Path Div I Ne reaches Pump He escapes Divertor Div II Ne cannot reach Pump He reaches Pump Divertor Geometry can be optimised for He pumping versus Ne/Ar used for P rad Control Schneider, IAEA 1998 Bosch, JNM 1999 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

12 Divertor geometry effects on neutrals and recycling (IV) Differences between Deuterium and Noble Gas Neutral Transport Enrichment Ne reaches Pump He escapes Divertor Groth, NF 2002 Ne cannot reach Pump He reaches Pump Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

13 ASDEX Upgrade PFR baffle optimised for improved pumping (less divertor leakage through private flux region) Divertor geometry effects on neutrals and recycling (V) Divertor leakage through private flux region JET no difference found for one-to-one L-mode discharges (10 30% expected) Schneider, JNM 1997 Rapp EPS 2003 Tsitrone PET 2003 DIII-D results? Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

14 Divertor geometry effects on neutrals and recycling (VI) Neutral Pressure compression and Divertor Leakage (+ Main Chamber Recycling) C-mod : by pass open lower P div & same P main P main independent of P div or (f leak x P div )~ constant? Pitcher, PoP 2000 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

15 Divertor geometry effects on neutrals and recycling (VII) JET : Smaller divertor leakage Decrease of Neutral Pressure in main Chamber but minimum n 0 in main Chamber (Mk IIa/p & Mk IIGB) set by anomalous Transport (ELMs?) S pump eff ~ 115 m 3 /s S leak ~ 305 m 3 /s Divertor Main Chamber S pump eff ~ 120 m 3 /s S leak ~ 114 m 3 /s Mark IIa/p P div Mk IIa:P div Mk IIa/p:P div Mk IIGB (L-mode) (H-mode) P main Mk IIa:P main Mk IIa/p:P main Mk IIGB (L-mode) (H-mode) Altmann, SOFE 1997 Horton, NF 1999 Maggi, EPS 1999 Loarte, PPCF 2001 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

16 Divertor geometry effects on neutrals and recycling (VIII) JET : Smaller divertor leakage Decrease of Neutral Pressure in main Chamber but Increased compression in JET : 1 st increase by-pass closure (Mk IIA Mk minimum IIA/p) n in nd main increase Chamber divertor (Mk closure IIa/p & (MkII Mk IIGB) A/p set MkII by anomalous GB (constant Transport P 0 (ELMs?) main ) S eff pump ~ 115 m 3 /s Divertor Main Chamber S leak ~ 305 m 3 /s S pump eff ~ 120 m 3 /s S leak ~ 114 m 3 /s Mark IIa/p P div Mk IIa:P div Mk IIa/p:P div Mk IIGB (L-mode) (H-mode) P main Mk IIa:P main Mk IIa/p:P main Mk IIGB (L-mode) (H-mode) Altmann, SOFE 1997 Horton, NF 1999 Maggi, EPS 1999 Loarte, PPCF 2001 level on neutral flux in main chamber f leak *P div = constant anomalous transport Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

17 Divertor geometry effects on detachment (I) divertor can be used to force higher ionisation near separatrix easier access to (partial) detachment C-mod Lipschultz IAEA 1996 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

18 Divertor geometry effects on detachment (II) more subtle divertor geometry effects on partial detachment Degree of detachment D.o.D > 1 detached plasma Mk IIA Vertical divertor Separatrix detachment Horizontal divertor Off-Separatrix detachment Loarte PPCF 2001 Monk, EPS 1997 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

19 Divertor geometry effects on detachment (IIb) more subtle divertor divertor corner geometry effective effects in neutral on partial baffling detachment Degree of detachment D.o.D > 1 detached plasma Mk IIA Vertical divertor Separatrix detachment Horizontal divertor Off-Separatrix detachment Loarte PPCF 2001 Monk, EPS 1997 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

20 Divertor geometry effects on detachment (III) Weaker effect of geometry on global detachment and density limit Loarte PPCF 2001 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

21 Divertor geometry effects on detachment (IV) Higher neutral baffling in TCV with larger flux expansion leads to larger D.o.D for the same density Pitts, JNM 2001 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

22 Divertor geometry effects on detachment (V) divertor geometry (+ fuelling) can be used to affect in/out detachment approach But only over a very restricted configuration range & Inner Outer Main less clear effects in ELMy H-mode Inner Outer Main Maggi, EPS 1999 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

23 Divertor geometry effects on detachment (Vb) divertor geometry (+ fuelling) can be used to affect in/out detachment approach configuration used in experiments Inner Outer Main But only over a very restricted configuration range & less clear effects in ELMy H-mode Corner configuration Vertical configuration Maggi, EPS 1999 Inner Outer Main Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

24 Divertor geometry effects on detachment (VI) recent septum/no-septum assessment for L-mode vertical plasmas no effect of septum Rapp, EPS 2003 inner Septum effects : n 0 &detachment outer Require careful septum-separatrix control to be seen in L-mode Unclear in ELMy H-modes Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

25 Divertor geometry effects on divertor radiation and SOL flows (I) large increase of radiated power (and decrease of q div ) in ASDEX Upgrade with Divertor geometry change Kallenbach NF 1999 effect associated with low T e,div sep in large range of conditions Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

26 Divertor geometry effects on divertor radiation and SOL flows (II) Change of divertor geometry in JET has caused small changes on radiation level for similar conditions 1.0 P rad /P inp Mk I Mk IIA Mk IIGB Mk IIGB-SRP P inp (MW) P rad from old definition P rad /P inp uniformly underestimated for all configurations Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

27 Divertor geometry effects on divertor radiation and SOL flows (III) difference in radiated power level due to geometry or in/out plasma asymmetries and upstream n e,t e? Kallenbach PPCF 1999 Monk EPS 1997 outer divertor in JET attached in Type I ELMy H- modes Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

28 Divertor geometry effects on divertor radiation and SOL flows (IV) Z eff changes with changing of divertor geometries but other things are also changed in devices is there any correlation between divertor geometry and Z eff? Mark IIa/p Guo NF 2000 Rapp EPS 2003 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

29 Divertor geometry effects on divertor radiation and SOL flows (V) more closed divertor allow higher particle throughput higher impurity removal rate But forced SOL flow has small effect on impurity removal and Z eff (JET, JT-60U, ASDEX Upgrade and DIII-D (?)) Asakura NF 2004 Guo NF 2000 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

30 Divertor geometry effects on divertor radiation and SOL flows (VI) divertor geometry can enhance n e range for flow reversal close to separatrix in divertor vicinity but not clear effects in main SOL (large II flows) Gafert JNM 1999 divertor sources & flows & n e profile effects n e divertor effects on SOL profiles not seen main SOL dominated by strong II flows (if M II small then divertor effects may appear) Davies EPS 1995 n e Reversed B φ M II ~ 0 Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

31 Conclusions Basic effects of divertor geometry on target power load confirmed and used in divertor design Divertor geometry can be used effectively to optimise D/T and recycling impurity pumping Neutral compression can be affected by divertor closure but maximum determined by anomalous transport Spatial variation of plasma detachment along divertor target can be influenced by divertor geometry. Global divertor detachment and density limit are not much affected Effect of septum possible seen in JET but over very small range of separatrix-septum distances Effects on divertor geometry on radiation seen in ASDEX Upgrade but not at JET (Te,div P inp?) Effects of divertor geometry on flows, SOL width and impurity shielding very minor if any (beyond increased pumping of recycling impurities) Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal

32 Divertor geometry effects on neutrals and recycling (VIII) Improved density control in DIII-D with closed divertor Alberto Loarte 5 th ITPA Divertor and SOL Physics Group CFN IST Lisboa - Portugal