10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila 1

Max-Planck-Institutfür Plasmaphysik

ELM resolved measurements of W sputtering

MPI für Plasmaphysik EURATOM Association, Garching, Germany

R. Dux, R. Neu, R. Pugno, T.Pütterich, ASDEX Upgrade Team

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 2

Temperature/Energy Dependence of W Sputter Yield

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 3

Erosion Yield per Deposited Energy (steady state)

Value represents number of eroded W particles per deposited energy

• scan over Ti and Te

• keeping a fixed ratio

• for a typical impurity mixa first plateau (~ 3e19 particles/MJ)is reached @ 20-40 eV

at this (high divertor) temperatures erosion flux mainly given by deposited power (weakly depended on detailed plasma parameters)

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 4

Erosion Yield per Deposited Energy (ELMs)

high temperature ions reach targets only after re-adjusted Te

• Te fixed

• Ti scan

• for a typical impurity mix‘plateau’ extended up to Ti ~ 1 keV

erosion flux mainly given by deposited power (weakly depended on detailed plasma parameters)

dashedlines:

sputtering by D/T only

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 5

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 W0* (W I) do not account for prompt redeposition

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 6

Set-up for Spectroscopic Influx Measurements

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 7

Example of Spectra Evolution

Spectrometer for fast measurements in divertor (1 line-of-sight):

• wide entrance slit (200m)

• standard repetition/exposure time =0.2ms (continuous)

• Balmer- surrounded by OII multiplet

• WI line well isolated

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 8

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

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 9

Highest W Erosion Rates for Hot Divertor Conditions

Te

19eV

12eV

8eV

W-erosion decreaseswith increasing level of gas puff (recycling) leading to lower temperatures in divertor

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 10

Effective Erosion Yield of Tungsten

erosion yields can only be explained by sputtering by impurities

data from:

- H-mode (only between ELMs)

- discharge @ H-L threshold

calculations (solid lines)

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 11

ELM Cycle at Low Divertor Density

• divertor temperaturebetween ELMs:≈ 20eV

• Considerable influx also inbetween ELMS

• ELMs contribute 50% to the W-influx

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 12

ELM Cycle at Higher Divertor Density

• divertor temperatureis low after the ELM(detachment, phase with high recycling) and eventually recovers during the ELM cycledepending on - the distance to the strike point, - the ELM frequency - …

• Erosion yield is lower inbetween ELMs

• ELMs contribute > 50% to W-influx

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 13

Effect of Ar puff on W erosion in divertor

Ar puff at 2.5s

During ELMs:

• power flux density decreases

• tungsten influx increases

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 14

Effect of Ar puff on W erosion in divertor

Ar puff at 2.5s

Between ELMs:

• power flux densitydecreases

• temperature decreases

• tungsten influx decreases

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 15

W Influx During ELMs

• W-influx measurements with time resolution down to 40s show sub-structure of ELMs (local / temporal?)

• Langmuir probe data can not be used during ELMs

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 16

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!

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 17

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

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 18

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

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 19

S/XB for W I (400.9nm)

Calculations of S/XB for W I

(400.9 nm)

I Beigman et al. PPCF 2007

• ionisation rateATOM code calculations

(lowest configurations)

• excitation rate:- semi-empirical v. Regemorter

formula (complicated coupling

scheme + configuration mixing)

- corona approximation: only

excitation from ‘ground’ state

TEC

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 20

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

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 21

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

Deuterium SputteringDeuterium Sputtering

101 102 103 104 105

E (eV)

1017

1018

1019

1020

1021

1022

Sp

ut.

Flu

x D

en

s. /

Po

w. L

oa

d (

m-2s-1

/ M

Wm

-2)

Be

W

CYC=1%

YC=5%

1nm s-1 / MW m-2

10th ITPA meeting on SOL/Divertor Physics, 8/1/08, Avila R.Neu 22

Erosion Yield per Deposited Energy (ICRH)

Fixed Te and Ti

Scan sheath voltage