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Melting of Tungsten by ELM Heat Loads in the JET Divertor

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Presentation on theme: "Melting of Tungsten by ELM Heat Loads in the JET Divertor"— Presentation transcript:

1 Melting of Tungsten by ELM Heat Loads in the JET Divertor
Guy Matthews, Gilles Arnoux, Boris Basylev, Jan Coenen,…. …and JET Contributors EX/4-1, Fusion Energy Conference 2014, St. Petersburg

2 Outline Background The JET melt experiment Simulation of the results
Conclusions and future plans

3 Decision on material for ITER first divertor
Risks for tungsten melting in early ITER operation had to be reviewed by IO

4 Bulk W melting well studied in medium sized tokamaks
Bulk melting can cause disruptions = dangerous for ITER Bulk melting risk was considered low for ITER divertor due to tile shaping & protection systems ASDEX-Upgrade divertor manipulator

5 Looking down into the tungsten divertor - after
The JET tungsten divertor Tungsten lamella shaping and divertor protection systems work in JET at ITER relevant inter-ELM heat fluxes 2011 As installed 2012 After ~3500 pulses ~20Hrs plasma Stack A Stack B

6 QSPA ELM simulations looked worrying for ITER
W melting by ELMs QSPA ELM simulations looked worrying for ITER QSPA-T W target N. Klimov et al., JNM (2009) 721 Plasma flow direction q > 2.2 MJm-2, t ~0.5 ms  3.8 MJm-2 for a 1.5 ms pulse (ITER minimum tTQ) …but plasma pressure lower and B higher in ITER (and JET)  stabilises surface waves Extrapolation to ITER requires a benchmark for the MEMOS code JET is capable of achieving a similar transient heat-flux at normal incidence 6

7 Typical temperature rise due to ELMs in JET
Geometric factor for a vertical edge ~ ×20 Surface T depends mainly on pedestal pressure not ELM size! So increase heating power or Ip to raise T Existing data – normal lamellas T during ELMs Temperature rise (C) [T.Eich] ITER ~105 Pa Pedestal pressure (Pa) 7

8 Outline Background The JET melt experiment Simulation of the results
Conclusions and future plans

9 Exposing a tungsten edge in JET
JET pulse 84779 B q|| A B

10 Typical JET W melt pulse 3MA/2.9T 23MW
300kJ ELMs q|| ~3GWm-2 ~0.5GWm-2

11 Limitations of the top IR viewing

12 Melt pulses are reproducible and no disruptions
Special lamella Temperature from IR (C) Standard (Ref.) lamella Time (s)

13 q|| q|| 84686 – Before melting HFS LFS Special lamella 5.5mm

14 After 84724 Special lamella LFS HFS

15 After 84778 Special lamella LFS HFS

16 After 84779 Special lamella LFS HFS

17 After 84781 Special lamella LFS HFS

18 After 84782 Special lamella LFS HFS

19  Jthermionic After 84783 JxB B 5.5mm LFS HFS
Erosion: m per pulse, 5-10m per ELM (frequency 30Hz) Total volume moved: ~6mm3

20 Erosion centres on the ELM footprint
Tref(C) 20

21 Indirect evidence for W droplet expulsion
W droplet event during melt pulse Laser blow off with W target A few droplets reach the main plasma with diameters ~ 100m - Small perturbations only and no disruptions 21

22 Outline Background The JET melt experiment
Simulation of the results MEMOS = key tool used for ITER predictions Stefan problem in 3D geometry accounting surface evaporation, melting and re-solidification solved by implicit method - Surface power density vs time from IR is the input - Vapour shielding - Temperature dependent thermophysical data applied - Moving boundaries are attached to melt layers - All forces: Gradient of plasma pressure, gradient of surface tension, JxB, tangential friction force [Bazylev TH/P3-40] Conclusions and future plans 22

23 Power density q|| from reference lamellas
IR camera T(t) MEMOS 3D Theodor 2D inverse code qn (MWm-2) q|| = qn / sin  B Reference lamellas

24 Power density on the special lamella
PIC model says: fs >0.6 during ELMs fs~1 inter-ELM and L-mode MEMOS input = qn from IR (Theodor) fs iterated to match: evaporation rate, synthetic IR image and Planck radiation q|| from reference lamellas qs = fs q|| cos  qn= q|| sin  qs Special lamella qs = fs q|| cos  24

25 fs chosen to fit evaporation rate and Planck radiation
Temperature #84779 – IR (unresolved) and peak (MEMOS) Melting IR W evaporation rate from WI (400.88nm) MEMOS fs=1 MEMOS fs=0.4 Best fit to all data with fs=0.4 25

26 W melt evolution #84779 – MEMOS [Bazylev TH/P3-40]
fs=0.4 26

27 Hierarchy of forces – MEMOS [Bazylev TH/P3-40]
JET pulse # MEMOS Plasma pressure (6kPa) + surface tension gradients Surface: -40 to +10m HFS LFS Shadow 2.5mm 60mm m Surface: to +400m Plasma pressure (6kPa) + surface tension gradients + thermionic emission (JxB) B J JxB m Melt depth and motion match JET data

28 Conclusions W melting by ELMs in JET provided important inputs to ITER: Shallow melts with a few small droplets ejected but no disruptions JET melt results are consistent with MEMOS assuming J×B dominant Unexpectedly large power mitigation factor found for exposed W edge New lamella at 15: Fully resolved IR temperature Grazing field angle / more ITER-like Simpler geometry Future JET plans 28

29 Thermionic emission during ELM - MEMOS
JET pulse # MEMOS [Bazylev TH/P3-40] Unlike JET experiment, suppression of thermo electron emission is predicted in ITER due to grazing field angles

30 Larmor radius smoothing - PIC code [Dejarnac NF]
Equivalent to fs~0.8 - Calculated for ELMs only - insufficient to explain fs=0.4 in H-mode - No effect expected in L-mode where we find fs=0.2 30

31 MEMOS suggests significant vapour shielding
…..but we are not able to prove it experimentally due to lack of a consistent physics model for fs=0.4 in H-mode and fs=0.2 in L-mode #84779

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