1 Max-Planck-Institut für Plasmaphysik 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila ELM resolved measurements of W sputtering MPI für Plasmaphysik EURATOM Association, Garching, Germany R. Dux, R. Neu, R. Pugno, T.Pütterich, ASDEX Upgrade Team

2 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Temperature/Energy Dependence of W Sputter Yield

3 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Erosion Yield per Deposited Energy (steady state) Value represents number of eroded W particles per deposited energy scan over T i and T e keeping a fixed ratio for a typical impurity mix a first plateau (~ 3e19 particles/MJ) is eV  at this (high divertor) temperatures erosion flux mainly given by deposited power (weakly depended on detailed plasma parameters)

4 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Erosion Yield per Deposited Energy (ELMs) high temperature ions reach targets only after re-adjusted T e T e fixed T i scan for a typical impurity mix ‘plateau’ extended up to T i ~ 1 keV  erosion flux mainly given by deposited power (weakly depended on detailed plasma parameters) dashed lines: sputtering by D/T only

5 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Influx Measurements in the Full W ASDEX Upgrade Quantify W-influx from various PFCs: visible spectroscopy influx density is a local quantity (often toroidal symmetry) many lines-of-sight needed to have full account of the total influx Caveat: measurements of W 0 * (W I) do not account for prompt redeposition

6 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Set-up for Spectroscopic Influx Measurements

7 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Example of Spectra Evolution Spectrometer for fast measurements in divertor (1 line-of-sight): wide entrance slit (200  m) standard repetition/exposure time =0.2ms (continuous) Balmer-  surrounded by OII multiplet WI line well isolated

8 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu W-Influx Profile at Outer SP (temporally averaged) Map flux densities to outer mid-plane Profile can be described with an exponential decay into the far SOL Interpolation and integration yields total source New in 2008

9 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Highest W Erosion Rates for Hot Divertor Conditions TeTe 19eV 12eV 8eV W-erosion decreases with increasing level of gas puff (recycling) leading to lower temperatures in divertor

10 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Effective Erosion Yield of Tungsten erosion yields can only be explained by sputtering by impurities data from: - H-mode (only between ELMs) - H-L threshold calculations (solid lines)

11 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu ELM Cycle at Low Divertor Density divertor temperature between ELMs: ≈ 20eV Considerable influx also inbetween ELMS ELMs contribute  50% to the W-influx

12 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu ELM Cycle at Higher Divertor Density divertor temperature is low after the ELM (detachment, phase with high recycling) and eventually recovers during the ELM cycle depending on - the distance to the strike point, - the ELM frequency - … Erosion yield is lower inbetween ELMs ELMs contribute > 50% to W-influx

13 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Effect of Ar puff on W erosion in divertor Ar puff at 2.5s During ELMs: power flux density decreases tungsten influx increases

14 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Effect of Ar puff on W erosion in divertor Ar puff at 2.5s Between ELMs: power flux density decreases temperature decreases tungsten influx decreases

15 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu W Influx During ELMs W-influx measurements with time resolution down to 40  s show sub-structure of ELMs (local / temporal?) Langmuir probe data can not be used during ELMs

16 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Erosion by ELMs at different outboard PFCs Fraction of erosion during ELMs increases with distance to the separatrix: higher fractions are observed at outboard limiters/horizontal plate lower fractions at strike point module close to separatrix Note: total erosion flux decreases  R!

17 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Summary largest W erosion flux usually at the divertor SP region dominated by impurity sputtering with strong ELM modulation relative contribution of ELMs to erosion -increases with distance to separatrix to values around 70 % -increases with decreasing divertor temperature (but total erosion decreases strongly in both cases) Ar puffing increases W erosion during ELMs; between ELMs the cooling leads to lower erosion (further investigations envisaged) Note: influence of transport on W content much larger than source rate!  ELM transport important to keep W concentration low

18 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Tools for operation with high-Z: Reduction of W content by increasing ELM frequency ELM frequency depends strongly on gas puffing, distance to H/L threshold and machine conditions  larger heating power or ELM pacemaking keep impurity content low Frequent ELMs necessary to clamp impurity content A. Kallenbach, PPCF 2005

19 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu S/XB for W I (400.9nm) Calculations of S/XB for W I (400.9 nm) I Beigman et al. PPCF 2007 ionisation rate ATOM code calculations (lowest configurations) excitation rate: - semi-empirical v. Regemorter formula (complicated coupling scheme + configuration mixing) - corona approximation: only excitation from ‘ground’ state TEC

20 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Erosion in the Outer Divertor Strongest W erosion found in the outboard divertor: determination of the total W source (temporally averaged - integrating over ELMs) Effect of ELMs Effective Erosion Yield Sputtering by Argon

21 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Sputter-flux / power load strong decrease with higher particle energies at similar power loads high-Z best for D-energies < 300 eV low-Z better at high D-energies Be W C Y C =1% Y C =5%  1nm s -1 / MW m -2

22 10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu Erosion Yield per Deposited Energy (ICRH) Fixed T e and T i Scan sheath voltage