Nanoparticles and hypervelocity dust grains in fusion devices

1. Nanoparticles and hypervelocity dust grains in fusion devices C. Castaldo Euratom ENEA Association, Frascati, Italy

2. DUE TO UNCERTAINTIES IN THE DUST PARAMETERS DIAGNOSTICS SHOULD COVER MAXIMUM PARAMETER RANGE • Visible imaging 500 m/s for a bright dust grain of few mm • Impact ionization phenomenon Evidence of velocities of ~few km/s in FTU for a mm particle • Light scattering Detectable size (% of l-few l), laserl~1 mm • Capture of dust (without destroying) Aerogels-light porous materials

3. VISIBLE IMAGING • Single cameraview – lower bound estimates Dust velocities projected on a plane perp. to line of view • Multiple cameras with intersecting views Unfold 3D trajectory. Set-up on NSTX [A.L. Roquemore et. al., J. Nucl. Mater. 363-365, 222 (2007).] • Individual particle observed with velocities up to 500 m/s • Estimates of size from thermal radiation can be masked by radiation from the ablation cloud • Problems with small (less than few mm) and fast dust small = high sensitivity, also fast=high contrast w.r.t. background • Calibration by injections of pre-characterized dust DIII-D results: 4 mm is smallest resolved by fast cameras [J.H. Yu et al., ” submitted to J. Nucl. Matter.]

4. LASER LIGHT SCATTERING • Use of existing Thomson scattering diagnostics detection channels at the laser lare used for dust detection based on elastic scattering • Particle size can be estimated from intensity of scattered light-with assumptions on geometrical and optical properties of grain • Averaged dust number density can be calculated as total number of scattering events divided by product of scattering volume and total number of laser pulses • First measurements: JIPPT-IIU after disruption, radius 0.4-1 mm [K. Narihara, et al. NF, 37, 1177 (1997)] DIII-D SOL during discharge, 6 103 cm-3, 80-90 nm [W. P. West et al, PPCF, 48, 1661 (2006)] FTU after disruptions, 107 cm-3, 50 nm [E. Giovannozzi et al., AIP Conf. Proc. Vol. 988, pg. 148 (2007)]

5. MIE SCATTERING ; ABLATION CLOUDS • For larger particles Mie scattering theory should be used • Laser power used is enough to vaporize dust < than few mm Scattering and absorption from ablation cloud (vapour+plasma) • Thermal evaporation + Mie theory for spherical particles DII-D results: averaged size twice larger than RLS estimates Power law r-g with g = 2.8 [R. D. Smirnov et al., PoP 14, 112507 (2007).] • Similar analysis on FTU data suggests RLS underestimates size by factor 2-5 ----------------------------------------------------------------------- • Uncertainties in refractive index • Geometrical parameters • Non-linear laser-dust interaction • Lack of statistics for scattering events by micron size dust

6. DUST IMPACT IONIZATION-FOR RARE FAST DUST • For most materials the hypervelocity regime (when the impact speed > the speed of the compression waves both in the target and projectile) has been reached when the impact speed exceeds 2-3 km/s • The resulting pressure can reach 1 TPa and the temperature can be sufficient to cause vaporization and ionization of the materials. • Diagnostics based on the phenomenon: (i) charge released (ii) craters on the target surface • Laboratory studies of impacts [M. J. Burchell et al, Meas. Sci. Technol. 10, 41 (1999).] -charge 1011-1013 e upon impact of ~1 mm Fe particle on Mo surface with velocity of few km/s -(with t=10-100 ms) ~10 mA current –feasible to measure in SOL • Probe measurements in FTU near the wall, equatorial plane [ C. Castaldo, S. Ratynskaia, V. Pericoli et al., NF 47 L5-L9 (2007).] [S. Ratynskaia, C. Castaldo, K. Rypdal et. al., NF 48, 015006 (2008).]

7. Signal detected by electrostatic probes near the FTU wall A threshold of 6 rms can identify spikes uncorrelated detected by adjacent probe tips (6 mm distance in poloidal direction) Lower amplitude spikes are correlated in poloidal direction.

8. Dimensions of the craters are function of the projectile parameters – empirical results available Craters smooth, no rough rims from ejected molten metal typical for the unipolar arcs Cracks observed - not typical for the arcs where surface damage is due to heating by the arc current Arc hops and leaves scratches on mm scale- none were found [ C. Castaldo et al., NF 47 (2007)] [S. Ratynskaia, et. al., NF 48(2008)] DUST IMPACTS -CRATERS ON THE TARGET SURFACE 10mm 20mm

9. Electro-optical probe Electrostatic probes Optical fibers Toroidal view Poloidal view

10. Plasma&neutral gasdensities after the impact vd = 10 km/s 1022 1022 ne(m-3) nv(m-3) ad = 4 mm ad = 4 mm ad = 2 mm ad = 2 mm t (ms) t (ms)

11. Photons at the detector > background W probes not necessary Yph(s-1) Yph(s-1) 1.1 mm Ø 1.5 cm distance from the source Mo Fe t (ms) t (ms)

12. Integrated signal convolution with unit pulse of 1.0 ms duration) Counts Counts Fe Mo t (ms) t (ms)

13. Highly porous, very low density material allows capture of even hypervelocity particles without destroying them Silica (SiO2)aerogels composed of clusters of 2 -5 nm solid silica spheres with up to 95 % empty space, an average pore size is 2-50 nm and mass density 0.1 g/cm3. Silica aerogels are made by high temperature and pressure-critical-point drying of a gel composed of colloidal silica structural units filled with solvents. Samples of ~cm3 are sufficient for tokamak applications AEROGELS –DUST CAPTURE WITHOUT DESTRUCTION http://stardust.jpl.nasa.gov/tech/aerogel.html

14. SILICA AEROGELS • Studies of compatibility with tokamak enviroments: -Outgasses H2O with traces of N2,CO,O2,CO2 -Withstands days of heating to 300-500 C -Easily pumped; few cm3 in 70 l chamber with 60 l/s vaccuum of 10-6 mB in 6 h and 10-7 mb in less than 24 h • Erosion is not serious problem for SOL conditions -sputtering by D charge-exchange neutrals with 1012 cm-3 and 30 eV, we find flux of SiO2 of3 1016 cm-2 s-1 • For extraction and analysis of captured dust see Burchell et al. Annu. Rev. Earth Planet. Sci. 34, 385-418, (2006). • First experiments performed: HT-7 tokamak Hefei, China (130 kA, 1.8 T and 1019 m-3) 800 traces from 10 mm to 0.5mm dimensions, length up to few mm. Now sample at SEM analysis • Currently aerogels are being installed in FTU and RP experiment “EXTRAP T2R” and in CUSP CNR-IFP • Preliminar analysis show that aereogel can be also used to capture nanoparticles (10 nm), which can be detected by SEM e.g. carbon nanodust