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OS2010| Institute of Energy Research–Plasma Physics | Association EURATOM – FZJ

Spectroscopy on laser released particles from plasma facing materials in a tokamak

B. Schweer, A. Huber, V. Philipps, M. Zlobinski, N. Gierse and U. Samm

Institut für Energieforschung – Plasmaphysik, Forschungszentrum Jülich,

Association EURATOM-FZJ, Trilateral Euregio Cluster, D-52425 Jülich, Germany

OPEN SYSTEMS 2010| Institute of Energy Research – Plasma Physics | Association EURATOM – FZJJuly 6th, 2010 No 2

• Introduction

• Wall characterisation with laser based methods

• Application in tokamaks (ITER)

outline


Introduction

1. Retention of hydrogen isotopes in bulk material (accumulation effects?)

2. Development of mixed layers fromeroded wall materials with - co-deposition of hydrogen isotopes- Formation of dust, flakes?

ITER operation

Proposed wall materials: main chamber: Beryllium Baffles: Tungsten Divertor: CFC (H-H

operation)Physical and chemical erosion of carbon

Tungsten lamellae (D-T operation)?

Physical erosion


• measurement of tritium retention• measurement of erosion and depositionLimit for allowed operation is 700 g T inventory (1017 Bq) (technical regulation) T retention from global fuel gas balance (measurement)

Strong inhomogeneous distribution of H isotopes in re-deposited material (plasma facing components, remote areas, dust, flakes)

Local characterisation of ITER wall (pre-tritium phase)Identification of major deposition areas and material composition

Results should support the decision to start tritium removal techniques, e. g.

• change of plasma configuration- strike points • wall heating (surfaces)• chemical reaction-oxidation, cleaning discharges• remote access-local heating of surface layer (flashlamp, laser)• exchange of components

Motivation: Characterisation (monitoring) of wall conditions


In-situ diagnostics preferred:

diagnostic developments:

• Laser induced desorption spectroscopy (LIDS)in-situ with main plasma pre-tritium and tritium phase

• Laser induced ablation spectroscopy (LIAS)in-situ with main plasma pre-tritium and tritium phase

• Laser induced breakdown spectroscopy (LIBS)in-situ without main plasma pre-tritium and tritium phase

• Investigation of physics requirements and principle feasibility• Technical design and construction for ITER

- remote operation - reliability- radiation constrains- large structures

Laser based diagnostics

Single laser shot analysis!


Knowledge of molecules decay process: • (dissociation, excitation, ionisation)

D2 D* + D* 2D++ 2e

h• (ionisation, dissociation, excitation) D2D2

++ e D++ D* + e 2D++ 2e

hMeasurement of absolute H intensitiesKnowledge of plasma parameter and conversion factors (S/XB)

Laser induced desorption spectroscopy LIDS

First wall bulk

Mixed layer (a:C-H)

Main plasma

H2, (HnCm)

Laser In-situ diagnostic:1. Fast heating and desorption

release of hydrogen isotopes(H2, hydrocarbon)

H

H

H

detector

2. Excitation by plasma(ionisation, dissociation, excitation) observation of line radiation


Heating sourceNd:YAG laser: L = 1064 nm, Ppulse 20 kW, (3 Hz)tpulse 20ms, EL 60 J, Pave 300 W (5 Hz)

Aspot=0.29 cm2 (d=6 mm)

Laser induced desorption spectroscopy

LIDSnecessary conditions for desorption: laser power density (single pulse): PA 70 kW/cm2 95% of hydrogenpulse duration: tp 5 ms, Aspot Alaser

necessary surface temperature Ts 1800 K (for graphite)

Penetration depth z 100 µm T 900 K

Sensitivity (inventory) N 1017/cm2

observation systemAbsolute calibrated detector:

CCD camera or PhotodiodeOptical transmissionInterference filter parameter

Edge plasma parameter (Te, ne)Conversion factors S/XB

High resolution spectrometer Discrimination H isotopes


system

Laser outsideconcrete shieldingFibre length 35 mCore diameter 400 µm

Fast camera: 7000 frames/sStandard:500 frames/shigh resolution spectrom.H/D

Experimental arrangement at TEXTOR


LIDS on graphite (roof limiter)

Camera frame frequency 1500 Hz


First wall bulk

Mixed layer

Main plasma

Laser induced ablation spectroscopy LIAS

High power laser

H,W, Be, C Laser plasma

In-situ diagnostic with plasma:1. Fast heating and ablation

destruction of surface (plasma)fast recombination(W, Be, cluster, H isotopes)

Knowledge of spectroscopic and atomic data of released species in plasmaKnowledge of plasma parameter

• Qualitative measurement• Quantitative measurement

2. Excitation by plasma(ionisation, dissociation,excitation)simultaneous observationof (absolute) line radiationH

detectors

Dichroicmirrors

W I

Be I

W I Be I

H


Heating source: (e.g.)

Nd:YAG laser (Q-switch): Laser = 1064 nm, (512nm, 353 nm)Ppulse 300 MW, (Q switch)tpulse = 8 ns, ELaser 2.5 J, Pav 25 W (10 Hz)

Aspot=3 cm2 (d=10 mm)(w/o fibre)

Laser induced ablation spectroscopy LIAS

necessary conditions for ablation: laser power density (single pulse): PA 100 MW/cm2

pulse duration: tp 10 ns, Laser ind. plasma temperature Tp 1-10 eV

Penetration depth z 1 µm

observation systemAbsolute calibrated detector:

CCD camera or diodeOptical transmission (UV to IR)Interference filter parameter

Edge plasma parameter (Te, ne)Conversion factors S/XB

Wide range high resolution spectrometer


Laser induced ablation

323.05 656.44 989.83

-22

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

Graphite Substrate

After 30 shots @ ~ 1J ~ 17um

Cra

ter

De

pth

(um

)

Lateral distance (um)

Original Surface

0.4 µm / laser shot


0,00E+00

2,00E-05

4,00E-05

6,00E-05

8,00E-05

1,00E-04

0 2 4 6 8 10 12 14 16 180 4 8 12 16

Laser energy fluence/(J cm )-2

0

20

40

60

80

100

Ab

late

d m

ass

/(µ

g c

m)

-2Material: carbon EK98

Ruby laser =694,3 nmAverage 20 laser shots


580 600 620 640 660 6800

100

200

300

400

500

600

CII(678.4nm)

CII(658nm)H(656nm)CII(589nm)

1st laser pulse 0.25J/cm2, spot No2, #96692

2nd laser pulse 0.25J/cm2, spot No2, #96693

3d laser pulse 0.25J/cm2, spot No2, #96694

inte

nsity

/ a.

u.

wavelength / nm

Laser-induced ablation spectroscopyon TEXTOR

Tungsten test limiter with 140 nm a-C:D coating

Ruby lasertlaser=10 nsP=25 MW/cm2

E=0.25 J/cm2

Threshold for ablation of layers is much lower


Investigation of ablation source on carbon bulk material

10 5 0

0

50

100

150

200

inte

ns

ity

distance mm

Blue channel

10 5 0

0

50

100

150

200

inte

ns

ity

distance mm

Blue channel

Laboratory experiment:

Vacuum

Pumps

vacuum

pumps

Ruby laser, 1J, 20ns

movable

sample holder

Vacuum

Pumps

vacuum

pumps


movable

sample holder

Vacuum

Pumps

vacuum

pumps


Quadrupole

(Balzers QMA311)

movable

sample holder

Fibre to cross dispersion spectrometer=363 nm -715 nm / =2000

Collectionoptics

Gate valve

Observationwindow

Focussing lensDeflection plates

aperture

Las

ersp

ot


Time of flight measurement on fine grain carbon

25 50 75 100Time of flight /µs

d=0.5 m

12 5 10 20 Energy/eV


First wall bulk

Mixed layer

Laser induced breakdown spectroscopy LIBS

Knowledge of spectroscopic and atomic data of released species in plasmaKnowledge of laser plasma parameter

• Qualitative measurement• Quantitative measurement

High power laser

H,W, Be, C Laser plasma

In-situ diagnostic w/o plasma:1. Fast heating and ablation

destruction of surface (formation of plasma)(W, Be, cluster, H isotopes)

H

detectors

Dichroicmirrors

W I

Be I

2. Laser induced plasma(fast recombination, excitation)no background radiationsimultaneous observation of(absolute) line radiation

W I

H

Be I


8

6

4

2

0Int

ensi

ty /

10co

unts

4

350 400 450 500 550 600 650 700

LIBS spectra on graphite

Reproducibility?Calibration?


Carbon

Divertor

First wall

Accessible range for

LIDS, LIAS and LIBS

Tungsten

Observation

Focussing mirror + pinhole

Laser

Observation

Laser

a) b)

Wallelement

Studies on technical implementation of LIDS, LIAS and LIBS in ITER


optical arrangement for port plug based diagnostic

Laser beam (ns, ms)

Observation endoscopeFor LIDS, LIAS and LIBS

Focussing mirror with hole

Rotation axis?

First wallHole in first wall


Sensitivity study for LIDS at ITER

Assumptions:Laser spot size: A =1cm2

H density: nH =31015/cm2nmLayer thickness: l =100nmPulse duration: t =1msMaxwellian source: T =0.2 eVFlux spot=31020/s

Equat

oria

l vie

w


Modelling studies on LIDS and LIAS during running ITER shots

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0Distance from strike point along separatrix / m

1017

1018

1019

1020

Equatorial plane

ITER: equatorial view low density caseLIDS: N=10 17 D2

background

Line integrated H signal

Lin

e i

nte

gra

l H

/ph

oto

ns/

(ms

r s)

2


ITER like optical system at TEXTOR

Laser beam: Focal length: 2.5 mObservation: Focal length: 0.2 mScanning area: 10 cm x 10 cm

Las

er b

eam

Lin

e ra

diat

ion

X-Y tilting

Tangential view

LIDSLIASLIBS


Summary

Laser based diagnostic for ITER:

LIDS: in-situ method with plasma for Tritium retention in the divertor

and main chamber (poloidal distribution in equatorial view)

Promising results at TEXTOR

LIAS: in-situ method with plasma for first wall characterisation in the divertor and main chamber (equatorial view)

Needs more investigation of the source (bulk / layer)

dependence on pulse duration and exposure time

LIBS: in-situ method without plasma for first wall characterisation in the divertor and main chamber

Needs more investigation of the source (bulk / layer)

dependence on pulse duration and exposure time

parameter of laser plasma must be known (dynamic)

ITER: coaxial injection and observation feasible

modelling of line integrated signals in edge plasma requested



0.0 0.4 0.80

1000

2000

Radius r / mm

calculatedradial temperatureafter1 ms x 80 kW/cm

SpotRadius

2

Tem

pera

ture

T/o C t = pulse length

D = diffusion coefficient K = heat conductivity = specific densityc = specific heat

cK

tDT

2

1-dim heat conduction sufficient for spot centre temperature calculation:

Microscopic image of a laser-desorbed spot on an a-C:D layer on graphite


Surface temperature (graphite) Laser power density

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