Recent Studies of Dust in Tokamaks

Marek Rubel

Contributions from:M. Balden2, F. Brochard3, M. Cecconello1, J. Coennen4, M. Freisinger4, A. Huber4, D. Ivanova1, A. Kreter4, J. Linke4, H. Penkalla4, V. Philipps4,

H. Roche5, V. Rohde2, G. Sergienko4, E. Wessel4, A. Widdowson6, P. Wienhold4

1Alfvén Laboratory, Royal Institute of Technology, Stockholm, Sweden2Max-Planck Institute for Plasma Physics, garching, Germany

3University of Nancy, France4Forschungszentrum Jülich, Germamy

5CEA, Tore Supra Team, Cadarache, France6Culham Centre for Fusion Energy, United Kingdom

O U T L I N E

• Mechanisms of formation and categories of dust

• Impact of brittle destruction on carbon dust production

• Metal melting & splashing

• Dust as show stopper: impact on diagnostics

• Dust generation by fuel removal techniques

3

Major Issues

Amount of dust

Fuel content

Conversion of erosion to dust scaling

Categories & Morphology• Structure & Porosity• Elemental and chemical composition• Size distribution

4

Tracing and collection of dustCameras: standard ultra-fast (3D reconstruction)

Correlation of optical, spectroscopy and magnetic signals

Collection from all locations during the shut-down periods.

Dedicated dust collectors and traps in various locations

Electrostatic dust detectors

Dust mobilisation in controlled experiments

5

Dust in plasma (TEXTOR)

(b)

4.64 s

4.70 s 4.74 s

4.66 s

Time sequence showing a cloud of particles released during the discharge&

Spectroscopic signals recorded during the event of dust release.

6

Example of dust tracing in ASDEX: shot after disruption

X (Camera pixels)

Y (C

amer

a pi

xels

)

50 100 150 200 250

50

100

150

200

250

Hot spots

Dust fly-by

Shot 24002: dust after disruption

Camera view showing hot spots (red) and dust fly-by particles (yellow)

F. Brochard (Nancy) and V. Rohde (Garching)

7

Dust generation in ASDEX

F. Brochard and V. Rohde

1500 fast camera movies covering 5 last AUG campaigns were investigated with the TRACE algorithm in order to link dust production rates with discharge conditions

Plasma operation time (s)

Num

bero

f det

ecte

ddu

stpa

rtic

les

no camera available

0 2000 4000 6000 8000 10000 12000 140000

100

200

300 Autres conditions de déchargeTous types de disruptionELMs

# 23430

# 23488

# 25216

# 26264

ECCD deposition width

caméra indisponible

ITER Breakdown studies with flat-top

AUG small ELM regimes

# 24438H-mode pedetalstructure USN-DN-LSN

H-Mode conditionning

Most of dust observed in shots with disruptions or unstable plasma phases.

8

F. Brochard and V. Rohde

Attenuator-like effect of ICRH heating is always seen.

More dust is found in discharges with NBI and NBI+ECRH

Preliminary results! especially regarding the use of ICRH systems.

Num

bero

f det

ecte

ddu

stpa

rtic

les

Total Heating Power ( x 107 W)

Dust Generation in ASDEX Influence of Heating Power on the Amount of Detected Dust

9

Dust Detection in Tore Supra

Hélène ROCHE

Electrostatic detectors developed by PPPL were installed in Tore Supra.

Dust signals are closely correlated with particles observed by a visible CCD camera

82% of dust particles detected are due to disruptions

Dust signals are correlated with the severity of the disruption

Data from 481 shots have been anlaysed

0

100

200

300

400

500

600

700

0.0 0.5 1.0 1.5

0

100

200

300

400

500

600

700

0.0 0.1 0.2 0.3 0.4 0.5

Plasma current at disruption (MA) Disruption duration (s)

Num

ber o

f dus

t par

ticle

s

Num

ber o

f dus

t par

ticle

s

10

Dust survey in TEXTOR

11

Location of dust collected in TEXTOR

bottom of the Inconel liner

bottom of the Inconel liner

Inconelliner

Inconelliner

Poloidallimiter

Poloidallimiter

Deposition and erosion zones

on ALT-II

Deposition and erosion zones

on ALT-II

DED bottom shield

DED bottom shield

Inner bumper limiter

Inner bumper limiter Antenna ICRFAntenna ICRF

12

Flaking of co-deposits

Detached flakes = dust

Co-deposits on plasma facing surfaces (1)

13

50 µm

Various forms of dust: from fine to big

Larger debris were collected on the bottom of the liner.

Carbon debris

Ni‐Cr‐Fe

5 µm500 nm

Ultra-fine dust on the liner

30 nm

Splitting into fine strata

originating from the in-vessel installation works

14

Dust from various locations

Laminar and granular structure due to different PFC temperature.

Poloidal limiterALT-IIDeposition zone

Liner

T ~ 470-520 K T ~ 520-570 K T > 2000 K

15

Polymer-like deposit on graphite RF antenna protection tile

Effect of plasmo-chemical reaction ?

16

Conversion factor

materialerodedofamountdustofamountC =

Conversion factor (definition):

C ~ 0.5 %

Collected in TEXTOR:

Total amount of all loose material: below 2 grams.

Collected fine dust: ~ 200 mg.

Eroded material in the form of co-deposit on ALT-II: ~ 40 g.

17

Retention of H and D in dust and co-deposits

0.0870.079

34.831.1

42.743.8ALT-II tile

0.0156.68.3Antenna

0.00030.036-Poloidal limiter0.0010.21.28Floor

D/CD[1020at/g]

H[1020at/g]Location

0.0870.079

18

Dust as a showstopper

19

First Mirrors Test for ITER in JET

Minimum 80 First MirrorsSolid angle for particle bombardment:ΩPB = 5x10-5 – 1.4 sr

For tested mirrorsSolid angle for particle bombardment:Wall: ΩPB = 2x10-3 – 6x10-2 sr Divertor: ΩPB= 5.5x10-2 – 0.2 srAspect ratio (depth in channel / mirror width): 0 - 5

20

Assembly in JET

Mirrors in cassettes under the load bearingplate (divertor base)

Mirrors installed on the outer divertor carrier

shutter

cassette

holder

rotatablecollector

Bracket assembly for installation of mirrors and deposition monitors on the

main chamber wall.

Mirror test at JET is a part of a broad programme onerosion-deposition studies.

Other diagnostics are installed next to cassettes with mirrors.

21

Visual Inspection of Exposed Mirrors

Base Outer

0 cm 0 cmNote: Deposition DECREASES with depth in the channel.

Inner Divertor, Stainless Steel Mirrors

3 cm1.5 cm0 cm

Outer Wall 3E, Molybdenum Mirrors

Note: Deposition INCREASES with depth in the channel.0 cm 1.5 cm 4.5 cm

22

Detachment of Deposits: Dust formation

Outer Divertor, Molybdenum

Outer Divertor, Steel Divertor Base, Steel

• Growth of the second co-deposited layer is observed.• It indicates that the first layer peeled-off in JET.

This complex surface structure influences the analyses.

23

Brittle destructionand

Melting

24

Materials under high heat loads

CFC

Emmision of carbon debris under thepower load (electron beam).

J. Linke, FZJ

25

Brittle destruction by wall locking at the EXTRAP-T2 RFP device

(a)

(b)

(c)

port-holeedges

A burst of particles is accompanied by a drastic increase of allspectroscopy signals: ne Te Zeff ncarbon

Magnetic signals recordedduring a discharge when dust particles were generated.

27

Dust from Extrap-T2 RFP

(a)

(b)

(b)

a

15 μm

b

15 μm

Debris after passing throughthe plasma.

Large graphite debris (2 mm) with co-deposits.

28

Carbon Materials:Brittle Destruction in Fusion Devices: EXTRAP

Dark field image of a co-deposit withembedded crystallites

(bright spots).

A diffraction pattern proving thepresence of crystalline matter.

29

Carbon Materials:Brittle Destruction in Fusion Devices. TEXTOR

Dust from the inner bumper limiter: amorphous carbon co-deposit with embedded nano-size particles of crystalline carbon and corresponding diffraction pattern (c).

30

Melting

31

Tungsten in ASDEX

32

M. Rubel, G. Sergienko, D. Ivanova

Metal droplets on ALT-II (TEXTOR)

Original droplet: sphericalshape; temperature abovemelting point.

CarbonCarbon, , determineddetermined byby EDXEDX

90° observation

arc track

45° observation

vvωω

300 µm

Droplet rotated: rotation axistilted with respect to velocitydirection.

Micro-crystals on top surfaceform structures oriented in thedirection of rotation.

Arc track shape indicates the direction of magnetic field.

velocity

arc motion

rotation axis

300 µm

B

Carbon deposits on edgeswere formed after metal re-solidifications.

33

Metal droplets on ALT-II (TEXTOR)

J. Coennen

34

Castellated limiter exposed at TEXTOR

35

Castellated limiter exposed at TEXTOR

W-O leaves

Formation of „leaves” by condensation from the gas phase?

36

Tungsten Oxide WO2 on the Limiter (XRD studies)

Cu

Cu

WO2 WO2

WO2

WO2

WO2

CuCu

WO2

WO2

WO2

WW

2Θ

1500

500

1000

04030 50

M. Psoda, IPPLM

37

Dust Generation by Fuel Removal Techniques

Methods

Baking in vacuumBaking in gasPlasma treatmentPhotonic:

Laser-induced ablationLaser-induced desorptionFlash lamp

38

D. Ivanova et al.

500 500 µµmm

800 800 µµmm

Initial deposit

Flaking after oxidation at 570 K Flaking after annealing at 1273 K

800 800 µµmm

Deposits after oxidation or annealing:RF antenna protection screen

Co‐deposit is not removed by oxidation or annealing. It flakes and peels‐off. 

39

Fuel Removal and Dust Generation:Laser-induced desorption and ablation

Examples of catchers after exposure

PFC tile

with

co-depositedlayer

Laser beam

Catcher plates withvarious traps ofreleased species

Mass spectrometryof the gas phase

Cu net for transmissionelectron microscopy

Messages:• The irradiation generates: (i) micro dust

(ii) condensate of gaseous products.• The products still contain fuel species (limited efficiency of fuel removal).

1 μm 500 nm

Dust generated be laser-light impact

Ccrys

40

Positioning:

4 cm

laser

Formation of crystalline dust (up to 2µm)

TEM:

SEM: No large debris were observed

200 µm

Carbon film is partly destroyed

2 µm

400 nm 6 nm-1

Collector:

slots for TEM nets

opening for laser beem

holder for a SEM sticker

Laser-induced cleaning: Dust collectionla

ser

lase

r

Metal cylinder  with a removable capInterior covered with stainless steel foil

Comparison of fuel in target and collector:

7 pulses @ 0.7J

Deposition pattern:

VPS‐W

41

A. Kreter, FZJ

Experiments in laboratory devices PADOS and TOMAS with oxygen and ammonia

O2 removes D and C much more effective than NH3

NH3 delaminates the layers• Dust production• May support (mechanical) cleaning methods

Photograph of castellated sample arrangement

Maximum removal rates:Oxygen: 5x1015 C-atoms/cm2 min at 340oC,11 mbar O2

Nitrogen: 5x1014 C-atoms/cm2 min

Fuel removal from gaps: laboratory studies

Dust production associated with several techniquesconsidered for fuel/deposit removal points to the importanceof employing mechanical (hoovering techniques).