Time resolved measurements of deposition and dust in NSTX

1. Office of Science Supported by Time resolved measurements of deposition and dust in NSTX College W&M Colorado Sch Mines Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Maryland U Rochester U Washington U Wisconsin C.H. Skinner, H. Kugel, L. Roquemore, E. Biewer, W. Davis, R. Maingi, N. Nishino, C. Parker, and C. Voinier NSTX Results Review December, 12-13th, 2005 PPPL. Culham Sci Ctr U St. Andrews York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAERI Hebrew U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec

2. Motivation: ITER • ITPA DSOL priority topic: “Improve understanding of SOL plasma interaction with the main chamber” Concerns on ITER Be wall: • heat load, erosion lifetime, repair difficulty • tritium migration, • coating diagnostic mirrors… • ITPA Diagnostics priority topic:“…assessment of techniques for measurement of dust and erosion.” Concerns: high dust levels are expected in ITER from long plasma duration and more intense plasma surface interactions. • How to assure dust levels are below safety limit ? • Will dust transport impurities to plasma core reducing fusion reactivity ? • Limits for C-and Be-dust are related to an explosion (e.g., H produced by Be reactivity with steam). • The limit for W-dust is related to the containment function of the ITER building (is more flexible). Open ST geometry facilitates diagnostic access and cost effective progress on these topics.

3. Dust particle trajectories in NSTX plasma: Fast Camera divertor view, 800 kA, 3 MW NBI discharge, 13,500 frames/sec. Particles seen in minority of discharges Note: Particle breakup @160 ms curved trajectory @ 340 ms Apparent velocities up to 200 m/s. Multiple camera views planned for 3D comparison to dust transport theory (S. Pigarov).

4. Microscope image of Bay B lower viewport exposed 11 March - 5 August 2004 1,659 plasmas 793 s total duration 10,372 particles / mm2 (excl. those < 1 micron) Dust est. surface area 1.2 mm2 / mm2 -exceeds area of viewport Count Median Diameter = 2.07 µm Graphite (sp2 elemental carbon) identified by Raman analysis Dust particle trajectories in NSTX plasma: Surface Dust in NSTX 100 µm (dia. human hair)

5. Dust detector installed in NSTX: Circuit grid Circuit grid Grid Spacing : 25 µm 50 µm 75 µm 100 µm 125 µm Dust collection slide for calibration Lab results: • A fine grid of interlocking traces (spacing down to 25 µm) is biased with 30-50 v DC. • Impinging dust produces a short circuit and resulting current pulse vaporises the dust. • Extensive lab data in air and vacuum ->> • Higher sensitivity needed for NSTX dust levels (~ 5 ng/cm2/discharge).- should not be a problem in ITER Bay C Bay B See poster GP1.00025 “Controlling surface dust in a tokamak” Colin Parker.

6. Deposition in NSTX Quartz crystal microbalances located at Bay H top & bottom, 7 cm ‘behind’ 7 cm wide gap in tiles + Bay I midplane 10 cm ‘behind’ limiter. Quartz Crystal Thermocouple • Quartz crystal oscillates at ~ 5.9 MHz, exact frequency depends on mass and on temperature. • Temperature effect largely subtracted using thermocouple data. Density 1.6 g/cm3 assumed. • Deposition inferred from change in frequency (measured to ~0.1 Å, ~0.1 Hz) • Data accumulated continuously 24/7.

7. Boronization not spatially uniform. Boronization Boronization Rate at midplane and divertor Bay H top • Deposition of boron highest near to glow discharge electrodes 40 cm below midplane • Recently installed movable glow probe should improve boronization of divertor. Bay I mid Bay H bottom

8. Deposition/erosion depends on plasma shape Initial conclusions: • Discharge time marked by transients from pickup and temperature differences between thermocouple and crystal. • First shot of day always experiences deposition. • Subsequent discharges can show deposition or erosion (unlike 2004 location 80 cm from LCFS which mostly accumulated deposition). • Interaction at upper divertor increases in DN discharges. • Interaction increases with plasma stored energy and pulse duration. qmb

9. Conclusions: • Incandescent particles with complex trajectories observed with fast camera in some NSTX plasmas. • Carbon dust identified on surfaces after campaign. • Electrostatic dust detector developed for time resolved measurements, however more sensitivity needed for NSTX dust levels (should not be a problem in ITER). • Quartz microbalance (qmb) show boronization is non uniform, movable glow probe installed to address this. • Quartz microbalance shows erosion and deposition on wall from plasmas. • Deposition dominates on first discharge of day • Erosion depends on plasma shape. • Analysis continuing… More in poster GP1.00025 “Controlling surface dust in a tokamak” Colin Parker.

10. Extras….

11. Deposition thickness at midnight during campaign • words

12. BBQ calculations of quiescent cross-field transport of impurities generated at the divertor strike points find little mid-plane deposition, using conventional models. ‘Bursty’ low-field side, far-SOL transport and/or ELMs should give higher deposition rates Modelling: John Hogan ORNL BBQ details: Collision model Similar to LIM, WBC, ERO codes, detailed magnetic geometry (EFIT) Background parameters assumed (ne, ,ln, Te, lT) [ local D+ flux amplification, sheath electrostatic (es) field, ESOL ] MZ dVZ/dt = -Ffriction -Fes + random // and ^ diffusion FZ = -MZ (VSOL-VZ) / ts ; Fes = Ze ESOL F// = Random // diffusion, D//=(8EZ/3pMi) t // F^ = Random ^ anomalous diffusion ( D ^ ) Particle energy (WZ) dWZ/dt = (kTi -WZ)/tT +Rfriction +Res Molecular processes: Erhardt-Langer, Janev-Reiter hydrocarbon break-up rates Atomic processes (ionization, recombination, D0 charge exchange) for carbon Birth gyro-collisions with surface Poloidal localization around strike points

13. Deposition/erosion depends on stored energy