In-Vessell Dust & Tritium Management
This work is a part of IAEA CRP(2008-2012): Characterization of Size, Composition and Origins of Dust in Fusion Devices presentations located at http://www-amdis.iaea.org/CRP/Dust/Presentations2/
1European Fusion Development Agreement(EFDA)
Summary of the 2nd RCM Presentation
Activities sponsored by EFDA in the associations concerning in-vessel dust and tritium management are described. These activities are implemented by means of two Task Agreements (TA): one on tritium inventory and removal and one on dust inventory and removal. Each of the two TA is supported by EFDA to the level of about 6 professional staff per year, plus some equipment support. Each TA is composed by individual Work Packages (WP) addressing specific studies or system development. The presentation provides an overview of the results achieved under the different work packages related to dust and tritium.
It is expected that dust inventory will have to be measured via net erosion. An EFDA task, WP1.1, has been established “to perform a review of possible laser-based techniques which could be employed together with a detailed feasibility study for the possible integration on ITER.” Several methods are being evaluated under this task. Field imaging LIDAR through Fourier Transform heterodyne is one such system. Drawback is that it is a single-point measurement and is sensitive to vibration masking of the measurements. Laser-induced breakdown spectroscopy (LIBS) timing is another option. It is predicted to be capable of 1 mm resolution, but the method is in an early phase of development and there are no documented results yet on fusion experiments. A third proposed system, Speckle interferometry, relies on multi-wavelength interferometry. This is also not straightforward to apply and may require consideration of vibration information obtained by other systems (by external sensors, or by Fourier transform processing of the signals).
A second EFDA task on net erosion measurements is WP1.2, “to perform a feasibility study in order to check a potential port-plug application of the Speckle interferometry on ITER.”. Integration studies for ITER show that the method is capable to cover erosion measurements on the inner wall, the upper limiter (Be melting), and the inner and outer divertor targets. Each separate system could have a spatial coverage of about 700 mm and spatial resolution of about 1 mm. The depth resolution is in the order of few micrometer and the effect of vibrations can be removed if a dual wave length setup is used. It is planned to start tests on the Magnum-PSI divertor target simulator in 2011.
A third EFDA task on erosion measurements is WP1.3, “to review the spectroscopy diagnostics foreseen on ITER to check their adequacy for erosion measurements.” Using the measurement capability of the impurity monitor (VUV system of ITER) some individual lines are proposed to perform wall influx measurements: 400.9 nm for a WI line, 514.3 nm for a CII line, and 351.6 nm for a BeI line. The results show that the measurements are possible to perform in terms of S/N and coverage however a problem with these spectroscopic measurements, especially the neutral lines, is that they can at the best measure only gross erosion as they do not recognize prompt redeposition (requiring measurements of higher states of ionization) making the net erosion measurements unpractical. Furthermore, melt layer erosion due to arcs will not be measured, molten droplets are not visible (no line radiation and it is not possible to differentiate between radiation from hot surface and droplets), and spectroscopy will be unable to quantify melt layer erosion during ELMs. The estimated uncertainties are largest for the case of tungsten erosion and it would be very valuable to have suitable ionic W lines.
On the issue of dust removal EFDA has a task WP2.1, “to assess the possibility (including integration issues) of using a photocleaning method for removing the dusts deposited on the divertor surfaces.” Laboratory experiments are performed on dusts produced by laser ablation of graphite or W target and are collected on substrate. These laser-produced dusts have similar morphologies and nature to tokamak ones. Studies on dust displacement under direct laser radiation have been done for C, W and Al (substitute for Be) on Si substrate, looking for the dependence of the displaced fraction on laser wavelength and pulse duration. Also laser shock wave method was studied for the removal of C particles from castellation between tungsten tiles (1mm width, 10mm deep, 20mm long). The shock wave induced by the focalisation of a laser close to the dust leads to the ejection of these dust and technique appears to be very efficient to move dust from castellations towards cold zones. With respect to application to the ITER port-plugs it must be noted that direct irradiation requires local dust collectors. The shock wave could be used to mobilise dust but requires integration on 8 ports for complete divertor coverage.
For measurement of tritium inventory EFDA has the task WP3.1, “to perform a feasibility study on non-invasive methods: Port-plug based scanning laser system, preferably with parallel light collection optics (LIDS, LIAS and LIBS).” The acronyms represent laser-induced desorption, ablation, and breakdown spectroscopy. LIDS has been tested on thick a-C:H layers in TEXTOR (2.5-3 μm a-C:H layer on tungsten). It has been established for reliable in situ fuel retention detection. The lower detection limit depends on the natural Hα signal fluctuations. B2-Eirene modelling was carried out in a feasibility study for LIAS and LIBS on ITER. The code is used to predict the background Hα signal intensity in ITER shots; it suggests that the diagnostic signal is good for inner wall measurements but for some divertor locations (viewed from the top port of ITER) is marginally measurable relative to the background.
A second EFDA task for tritium inventory measurement is WP3.2, “to perform a feasibility study on VV remote handling method: Remote Handling based scanning laser system (adaptation of the IVVS* system and/or plugged in the IVVS carrier or on a dedicated carrier).” Laboratory studies were performed to define the properties of the laser and of the optical setup. LIBS spectra were recorded with the Echelle spectrometer during laser ablation of CFC in argon; the results are in excellent agreement with simulations however dependences with gas pressure and accurancy are still to be optimized. Implementation on a robotic arm was assessed and several optical arrangements were analyzed in terms of avoidance of contamination of optics and operation efficiency.
Another EFDA task for tritium inventory is WP4.4, “to perform a feasibility study envisaging a possible implementation of a photonic heating technique on the IVVS carrier or a robotic arm system.” A heating model was validated against experimental data for laser irradiation of a tungsten layer (60-80 nm) and a DLC+H layer (4 μm) on a graphite substrate. For the application of the system on a robot arm the effect of laser deposition is studied for different angles and under vibrations. The deposition of laser energy is heterogeneous (about 2J ranging) for the practical values of incidence angles.
EFDA task WP4.5 is “to proceed with a feasibility study on a potential remote handled application of a photonic ablation technique on ITER, emphasising the need to guarantee an efficient collection of the wastes.” This method can be used at atmospheric pressure and also at very low pressure (between shots) for removing layers from top of the vessel and divertor. Under vacuum ablation wastes are ejected by the laser and deposited on a collector plate. Under atmospheric pressure the ejected dust may be collected by a vacuum cleaning system. Different concepts of nozzle shape have been studied for best access and in order to maximize dust collection.
The final EFDA project reported by Dr. Malaquias concerned arc-discharge cleaning of plasma-facing Components (no WP number). This is successful for Al (substitute for Be) but required much longer times to remove W layer. Improvements on the scanning approach (XY moving arc) will allow 5-10 times faster removal times. The system works under vacuum but tests under atmospheric pressure will be started soon. The average roughness is marginaly increased (from 1.25 μm to 3.36 μm in one set of experiments) so that the method can be used to remove deposit layers from the inner wall of the vessel without damage to the surface.
EFDA plans for 2011 are focussed on the following areas: (1) End effectors for use on the Multiple Purpose Deployer arm (MPD). This includes development of effectors to work simultaneously with or to support remote vacuum cleaning (ITER base line). The technology shall be tested on other devices in the associations. It also includes to develop at ITER integration level laser-based, arc-based, or other techniques for layer cleaning with collection of wastes with capability to measure and control locally the tritium inventory on places of higher retention, in particular at the top of the vessel. (2) Port plug based systems. This includes to develop proof of principle of LIAS, LIDS and LIBS in an ITER-relevant set-up in a tokamak or other experiment, and it includes erosion monitors: integration into ITER and modelling of Speckle interferometry. (3) Coordination of all activities involving laser based techniques.