ITER & dust Dust = major issue in term of ITER safety In vessel dust quantities: extrapolation from current experiments In vessel dust diagnostics & removal.

ITER & dust

Dust = major issue in term of ITER safety

• In vessel dust quantities: extrapolation from current experiments

• In vessel dust diagnostics & removal tools

• Dust reactivity

In vessel dust quantity: ITER prediction

Dust production :

Material erosion = redeposition dust

Cd• Normal operation• Disruption• ELMs• Conditioning• Unipolar arcs (?)

Safety people:Erosion measurements with Speckle interferometry or plasma impurities calculation and Cd=1

Operation people:To Reduce Cd value in order to constraint the intervention (removal) period Need of reliable dust diagnostics

In vessel dust quantity: ITER prediction

Cd Tore Supra evaluation:• dust recovery after a known operation period. Assume homogeneity = 31g

( 5 months, 1438 shots)• erosion processes:

• normal operation: • incident plasma flux + C sputtering yield = quantity of C eroded = upper value of C redeposited = 27gr (however, much lower quantity since 50-80% redeposited locally!)

• conditioning (HeGD mainly):• 1day of HeGD = 3g of C sputtered.• for 5 months of operation: 50gr of C eroded and then redeposited.

• by the way, no dust formation from C Arnas evaluation.• disruption:

• every disruption = 300kJ (mean value)• from plasma gun results, eroded quantity/disruption could be approximated simply by the following expression:Mc (g) = [(Ethermal*0.1) /Esublimation] * 12 (g) = 0.6 g/disruption• 80-120 gr eroded by disruption in Tore Supra (15% of frequency)

• Balance and Cd estimation Cd=30/200=15% (in JT60U = 7%)

In vessel dust quantity: ITER prediction• Cd Best estimate ~ 10%,

TO BE CONFIRMED by a comprehensive study (AUG, JET ILW and Alacotor C)

• For present ITER material choice and CD=10 % ,

dust production = 0.5 g carbon and 0.6 g for Be per 400 s, full power discharge. Negligible for W erosion is small.

• For all material, dominating processes will be:• melting and droplet formation during disruption and ELMs, • unipolar arcing (no estimation in current tokamak), • He and H embrittlement and blistering (inducing surface inhomogeneity and arcing?)

• More work needed: especially on modelisation and crosscheck with experiments on plasma gun.

In vessel dust diagnostics & removal tools• EFDA task TW6-TSS-SEA5.1 Dust measurements and removal techniques (Chair by S Rosanvallon)

dust diagnostics for ITER:• limit = 6x3 kg of dust on hot surfaces (divertor)

• due to H2 production and explosion in case of 02(Air) ingress• possible mitigation:

• injection of neutral gas: not working• limit the Air possible leaks (DCR under study) (could remove this safety limit)

• possible consequence of explosion: dust in suspension and explosion• limit of dust explosion not known yet (under study)

• almost impossible to measure (to be studied)• limit ~ 400kg in vessel quantity:

• 100kg (Be)/ 200kg (C) / 100kg (W) • 160µm / 4000µm / 100µm (homogeneous erosion, if not higher erosion)• due to release activated material to the environment

• size of dust to measure:• between 100nm to 100µm

• complex structure (fractal, fibers,…) and composition

In vessel dust diagnostics & removal tools• EFDA task TW6-TSS-SEA5.1 Dust measurements and removal techniques (Chair by S Rosanvallon)

• diagnostics for knowledge CCD & transport, laser extinction, embarked laser diffractometry, collection of dust (removal)

• diagnostics for safety: monitor the dust quantities in the VVUpper limit (>100kgs): erosion • speckle interferometry (10-40µm), • laser metrology (>100µm)• erosion assessment from plasma measurements

Lower limit (18kg in divertor): IR measurements (layers and dust identical) + LIBS evaluation(very risky, things could change if oxygen control)

link to operation issues:conversion factor evaluation: from feedback experiments, from surface diagnostics, …

• removal: use of existing or under development techniquesset of system must be used (as usual)integration very difficult(ask for support from out of fusion people, meeting in Nice end of this year)

In vessel dust diagnostics & removal tools

• Diagnostics & Removal R&D:

– Technical feasibility considering divertor design, optical accesses, reference surfaces, etc

In vessel dust diagnostics & removal tools

• Diagnostics & Removal R&D:

– Dust collection under the divertor cassette?

(washing with gas?)

Evacuation

In vessel dust diagnostics & removal tools

Removal strategy:

• Removal of dust, flakes and deposited films from the vessel is a three stage process:– Material mobilization: material must be detached from the surface on

which it is attached including gaps/castellations– Collection of the mobilized materials (Vacuum cleaner, electrostatic

transport)– Transport within/outside the vessel

• Main issues– Lack of solution for transport outside the VV (design impact),– Need of robotised arm

In vessel dust diagnostics & removal toolsDiagnostics & Removal open question:

• Quantity to be measured (ITER input, ITER TF created)• Link between local measurements and global inventory?• link between dust in suspension measurements and global inventory?(inject gases and measure the mobilised dusts) • What are the set of systems to be used for dust removal?• What are the studies needed before Tokamak demonstration? • ……………

Need to have a dedicated mock up• For dust diagnotics & removal• For dust mobilization and explosion studies• For detritiation studies (test of accessibility)

Mock up = divertor sector + vacuum vessel and port(s), with possibilities to work under vacuum or inert gas

• ITER Task Force launched 20 days ago. Goal : “Draft the effort and relevant plan of ITER Designers to provide studies and design activity necessary to bring the DCR to the TCM approval”

Dust sampling and reactivity studiesDust sampling in current machine:

Goal: correlation of dust and operation process, maintenance etc…

Dust sampling and reactivity studiesDust sampling in current machine:

Goal: correlation of dust and operation process, maintenance etc…

Surface morphology of melt zone: W Droplets or Dust

Melt zone

Dust sampling and reactivity studiesDust sampling in current machine:

Goal: correlation of dust and operation process, maintenance etc…

Dust sampling and reactivity studiesDust sampling in current machine:

Goal: correlation of dust and operation process, maintenance etc…

BET measurements done on plasma layers recovered in TS and in TEXTOR:

On going comparison between TEXTOR and TS samples

Studies to be done in order to link SSA and chemical reactivity with Oxygen, steam,….