Arc-discharge cleaning of tokamak wall

Transcription

1 Arc-discharge cleaning of tokamak wall Annual Fusion Seminar Association Euratom-Tekes May, 2013 Antti Hakola VTT Technical Research Centre of Finland Collaborators: J. Likonen: VTT J. Kolehmainen, M. Koskinen, S. Tervakangas: DIARC-Technology Inc. M. Aints, M. Kiisk, M. Laan, A. Lissovski, P. Paris: University of Tartu

2 Why arc-discharge cleaning? in ITER, the administrative limit for tritium in the tokamak vessel is 700 g a large fraction of this can be found in co-deposited layers on the walls how to get rid of the extra tritium during maintenance breaks? and without destroying the walls? our idea: use arc discharges to clean the walls from deposits (and from tritium) design criteria: cleaning device should work at any pressure regime: from high vacuum to atmospheric pressures of nitrogen process gas (e.g., argon) can be used locally close to the region to be cleaned both conductive and insulating layers should be handled cleaning rate should be high and large areas should be cleaned before moving the device in a new location

3 Feedthroughs: Ar flush Water cooling Electricity To vacuum pump Schematic illustration of the DIARC cleaner head Camera, fibers & spectrometer Dust collector = metal cylinder B Igniter Anode Plasma collector plates Water-cooled region Flexible hose Cathode = cleaned wall vacuum ( 10-2 mbar) established inside the cleaner head Ar flow ( mbar) prevents the formation of nitrides and oxides the flexible hose on the cleaner head (nozzle) is in contact with the wall, elastomer sealing provides the required tightness cleaning system will handle up to mm 3 samples, anode separated by a few mm from the surface removed material migrates to the plasma collectors, pressure peaks will move dust from the nozzle into the metal cylinder before moving the cleaner head into a new position, dust is removed from the cylinder

4 Design of the DIARC cleaning system Control unit Feedthroughs Cleaner head = nozzle XY translators Turbo pump Dust filter Movable sample stage Z translator Forevacuum pump

5 Photographs of the cleaning system (1) Control unit Vacuum system Cleaner head Movable sample stage

6 Photographs of the cleaning system (2) Window for spectrometer Cleaner head Cleaner head Sample to be cleaned Elastomer sealing Movable sample stage

7 Cleaning experiments A set of coatings (thickness ~1 m) produced on stainless steel (AISI 304, size mm 3 or mm 3 ) Pure Al (simulates Be) Al-W mixtures with 5 at.% and 25 at.% of W C-W mixtures with 10 at.% of W Al-C-W mixtures with a composition of 50:40:10 All the coatings doped with H during their deposition (1-5 at.%) Examples of possible co-deposited layers in ITER

8 How does the cleaning pattern look like? Effective region Cleaned region 10 mm effective region of the cleaner head has a complicated shape, somewhat resembling the letter D anode rod is moved back and forth along a 25-mm long crescent-like path, arcs spread some mm from the contact point of the anode tip mm 2 banana-shaped pattern on the surface

9 Examples of cleaned surfaces optimal operation pressure for Ar 2.0 mbar cleaning rate: number of pulses: 100 cleaning cycle: 20 s (complete cleaning already during the first 1-2 s) cleaned volume: 350-mm 2 1 m minimum cleaning rate m 3 /s 4.8 Hz, 100 pulses 4.8 Hz, 100 pulses Al: 2 and 4 cleaning cycles C-W: 13 cleaning cycles

10 Cleaning device in action 10

11 a simple system with a fiber (and an optional lens) is enough for collecting a sufficient part of the detected light equipment used: AvaSpec-2048USB2 spectrometer, spectral resolution 0.7 nm, wavelength range nm SRS DG535 digital delay generator for syncronizing the spectrometer with the cleaning device measurements Spectroscopic detection of the cleaning process either through the side window (window remains clean but line of sight too far away from the source) or through the top window (small aperture needed to keep the window clean)

12 In practice Pure Al sample: Removal of coating Pure Al sample: Removal of hydrogen intensity ratios from the coating gradually go down with the number of arc pulses (cleaning rate reduced here) delay between the arc pulse and the onset of recording optimized to 1.4 ms noisy curves, due to unpredictable nature of the arcs

13 Summary and outlook an arc-discharge device for cleaning tokamak walls has been designed and successfully realized the prototype device can remove 1-mm thick deposits from mm 2 areas in a few seconds an A4-size area could be cleaned in a couple of minutes a spectroscopic detection system has also been realized to determine when the surface is clean enough for the device to move on to another location next steps: SIMS analyses of the cleaned samples and new experiments with thicker coatings an F4E project for cleaning first-wall mirrors and an ITER contract related to de-tritiation issues?