Dust Studies in Fusion Devices

1. D.L. Rudakov • Presented at the • PFC Meeting • Boston MA • July 7-10, 2009 • Including contributions from • Litnovsky, • N. Asakura, • N. Ashikawa, • G. DeTemmerman, • S. Ratynskaia • J.Yu 10 m Dust Studies in Fusion Devices Bt

2. Dust in ITER – a Licensing Issue • Dust accumulation is a licensing issue in ITER: • The total in-vessel dust inventory in ITER will be limited to 1 tonne; a lower administrative limit of 670 kg has been proposed to take account of measurement uncertainties • The enhanced chemical activity of Be and C dust at high temperatures is more restrictive and a limit of ~10 kg for Be and C dust on hot surfaces (T > 400 C) is being considered From operational standpoint, small amounts of W dust (<< 1 g) reaching core plasma can increase W concentration to unacceptable levels

3. Proposed Dust R&D work plan under ITPA DSOL ITER IO urgent tasks • Characterize dust production rates, recover conversion factor from erosion/damage to dust production High priority • Link the quantity of collected dust to erosion/damage • Local dust production rates at different surfaces and in volume • TEXTOR, ASDEX-U, Tore Supra,JT-60U, DIII-D, LHD, MAST, NSTX, FTU, EAST… • Characterisation of ejection velocities, sizes of molten droplets and the morphology and size distributions of collected dust High priority • TRINITI, QSPA, PISCES • Study the role of T removal techniques in dust creation: subject samples of re-deposited material to transient heat fluxes, photonic and plasma, as well as oxygen cleaning. Quantify the dust createdMedium priorityTRINITI, QSPA, PISCES, U. Toronto, Pilot-PSI,…. • Cross-machine studies of dust injectionDSOL-21 High priority • Investigate of dust launch velocities and subsequent transport • Benchmarking against dust transport models • DIII-D, TEXTOR, LHD, MAST, NSTX, AUG • Dust measurements High priority • Dust collection (see task #1) • Time-resolved detection: visible and IR imaging, electrostatic detectors, • capacitive microbalance, spectroscopy, Aerogel • Dust removal Medium priority

4. Stereoscopic imaging of dust motion in MAST • Camera configuration/location easily changed on MAST (no need for periscope) • 2 synchronized IR cameras installed on the same port (slightly shifted toroidally) Stereoscopic dust imaging LWIR camera: 5mm resolution MWIR camera: 7mm resolution MWIR camera view 4 LWIR camera view 4 Contribution from G. DeTemmerman

5. Stereoscopic imaging of dust motion in MAST • Reconstructed tracks for MAST shot 19374 (2008 restart) • Particles are accelerated in the direction of the plasma flow • Slower particles seem to follow the field lines • Faster particles move outwards • Range of observed particle velocities: 10-60 m.s-1 • Faster particles observed but need more analyses Contribution from G. DeTemmerman

6. 3D reconstruction of particle trajectory in LHD D1 LHD center Dust Reflection Using camera position, virtual plane of dust and reflected images, real dust position is determined. - Reflected image must be located on the first wall. -Incident angle from dust to the wall is determined. Contribution from N. Ashikawa

7. Recent result of dust in JT-60U: Dust distribution in plasma discharge was measured with YAG laser scattering (Mie scattering) Significant numbers of event signals (scattering light from dust) were observed just after large disruption (high Ip and Wdia > 3MJ): also measured by TV camera. They stayed, particularly, at the far SOL. Number density and its size are decreased near the separatrix, suggesting that ablation becomes dominant near the separatrix. Contribution from N. Asakura

8. TEXTOR: Multi topical research program 4 6 4 2 5 1 3 Bt Shot 106265 Bt t=1660 msec. t=1580 msec. Shot 106265 Ex-situ analyses of natural dust In-situ detection of natural dust Dust inventory, fuel retention in dust and particle size distribution Dust density and energies of dust particles Fast probe equipped with aerogel catchers for detection of dust particles in the SOL plasmas of TEXTOR Studies of artificially introduced dust Dust mobilization, motion and impact on core and edge plasmas Dust sampling places: deposition (1) and erosion (2) zones on ALT tiles, bottom of the liner (3), main poloidal limiters (4), DED bottom shield (5) and DED tiles (6) DSOL 21 2009 Work made within the programs of EU TF PWI: WP09-PWI-03-01 and WP09-PWI-03-02, IEA-ITPA joint Experiments, task DSOL 21 and bilateral collaborations. Dust launch, vertical view of the limiter (no filter) Dust launch, horizontal view of the limiter (CIII filter) Contribution from A. Litnovsky

9. TEXTOR: summary of results In-situ detection of natural dust Ex-situ analyses of natural dust • The total amount of collected loose „dust” is below 2 grams; • Co-deposits peel-off when exposed to air; • Long-term (3 days) baking of co-deposits at 350oC releases only 8-10% of deuterium; • Efficient fuel removal requires baking to 800oC – 1000oC. • Most of dust was collected during a flat- top phase of a discharge; • Size of collected particles: from submicron up to hundreds of micrometers; • Dust density assessment up to ~ 140 dust particles per sq.cm2. . Studies of artificially introduced dust • No effect on the core performance; • Carbon concentration in the edge rose from ~3% to ~6%, implying that around 0.01% of launched dust carbon entered the edge plasmas; • Dust primarily deposited locally on the nearby located plasma facing components. Contribution from A. Litnovsky

10. New Dust Collection Technique: Aerogel Example of EDX of the aerogel with C particle in it • Highly porous, very low density material • Used in space programs to collect dust • Allows capture of dust particles without destroying them • From the penetration depth particle velocity can be derived • First tests of aerogel performed in HT-7 and TEXTOR Contribution from A. Litnovsky and S. Ratynskaia

11. New ITPA Joint Experiment DSOL-21 • Title: Introduction of pre-characterized dust for dust transport studies in the divertor and SOL • Goals: • Characterization of core penetration efficiency and impact of dust of varying size and chemical composition on the core plasma performance in different conditions and geometries • Benchmarking of DustT and DTOKS modeling of dust transport and dynamics • Machines: DIII-D, TEXTOR, MAST, NSTX, LHD, AUG • Recent experiments: DIII-D, MAST, TEXTOR

12. Motivation for Dust Injection and Technique Used • The aims of the dust injection: • Calibrate dust diagnostics • Benchmark modeling of dust dynamics • Different types of dust are used: • Graphite flakes • Graphite spheres • Diamond • Suspension of ~30-40 mg of dust in ethanol loaded in a graphite holder and allowed to dry • Holder with dust inserted in the lower • divertor of DIII-D using Divertor Material Evaluation System (DiMES) manipulator 5 m 10 m 10 m

13. Newest Results from DIII-D – Injection of Spherical Dust • Spherical graphite dust manufactured by Tokai Carbon Co (Japan), provided by Naoko Ashikawa (NIFS) • Spherical shape, narrow size distribution – good to benchmark modeling! • Suspension of ~30 mg of dust in ethanol loaded in a graphite holder and allowed to dry • ~10 mg of loose dust sprinkled on top 10 m Diameter (m) Dried dust “crust” Loose dust

14. Dust from DiMES kills the discharge Shot number 136002 DiMES Full light, 2000 f/s, total duration ~ 90 ms

15. DiMES out, discharge runs Shot number 136003 DiMES Full light, 2000 f/s, total duration ~ 500 ms

16. DiMES in, discharge dies again Shot number 136004 DiMES Full light, 2000 f/s, total duration ~ 450 ms

17. Observations From Spherical Dust Injection • Dust becomes visible 13 ms into the discharge • From the fast camera data, dust velocities are low, ≤10 m/s • Dust could not travel a from DiMES into camera view in 13 ms • Thomson scattering diagnostic observed high level of scattered signal starting 300 ms before the discharge (when it was turned on) • Dust must have become mobile and spred around the vacuum vessel prior to the discharge • The physical mechanism that mobilized and spred the dust is presently unclear. Best guess: dust charged up and got mobilized when the E-coil was turned on ~400 ms before the discharge • Can this happen in ITER? • Tritiated dust can charge up and levitate in electric field • [C. Skinner et al., Fus. Sci. Technol. 45 (2004) 11] • If 10 mg of dust can prevent DIII-D discharge from running, • ~1 g may do that in ITER

18. Dust injection experiment on MAST • Injection of known shape/size particles in the divertor plasma to study transport Tungsten dust 50 m D. Rudakov (UCSD) Provided by Buffalo tungsten (USA) Design of the dust injection head Minimize the amount of particles introduced at once to maximize the chances of observation Observation with 2 IR cameras + 1 filtered fast camera (CII, WI) 18 Contribution from G. DeTemmerman

19. Dust injection campaign on TEXTOR Fraction (a.u.) 10 m Diameter (µm) 0.1 1 10 100 II. Carbon spherical “killer” dust I. Carbon flake-like dust Photo and analysis by Phil Sharpe 10 m 5 m Diameter m) Manufactured by Tokai Carbon Co (Japan), supplied by Naoko Ashikawa (NIFS) Manufactured by Toyo Tanso Co (Japan), supplied by Dmitry Rudakov IV. Tungsten dust III. Diamond dust TEXTOR experiment was with 4-8 micron dust, photo on the left is of 2-4 micron dust Diamond dust by DiamondTech: http://www.diamondtech.com Supplied by Dmitry Rudakov Tungsten dust manufactured by Buffalo tungsten (USA) Supplied by Gregory De Temmerman • +V. Mixed (W+C) dust – a combination of I and IV Contribution from A.Litnovsky

20. Slow and fast motion of dust 1. Relatively fast (v1>100 m/s) motion of individual dust particles along B field; # 110258 Bt # 110271 Bt Bt Bt It seems, that two independent types of motion co-exist: and 2. Really slow (v2~1-5 m/s) motion of the entire mass of dust across B field Spherical carbon “killer” dust Diamond dust Valid for both conductive and dielectric dust # 110274 Launch of W+C dust Recorded from fast camera Contribution from A.Litnovsky