First mirrors in ITER: material choice and deposition prevention/cleaning techniques

Abstract

We present here our recent results on the development and testing of the first mirrors for the divertor Thomson scattering diagnostics in ITER. The Thomson scattering system is based on several large-scale (tens of centimetres) mirrors that will be located in an area with extremely high (3–10%) concentration of contaminants (mainly hydrocarbons) and our main concern is to prevent deposition-induced loss of mirror reflectivity in the spectral range 1000–1064 nm. The suggested design of the mirrors—a high-reflective metal layer on a Si substrate with an oxide coating—combines highly stable optical characteristics under deposition-dominated conditions with excellent mechanical properties. For the mirror layer materials we consider Ag and Al allowing the possibility of sharing the Thomson scattering mirror collecting system with a laser-induced fluorescence system operating in the visible range. Neutron tests of the mirrors of this design are presented along with numerical simulation of radiation damage and transmutation of mirror materials. To provide active protection of the large-scale mirrors we use a number of deposition-mitigating techniques simultaneously. Two main techniques among them, plasma treatment and blowing-out, are considered in detail. The plasma conditions appropriate for mirror cleaning are determined from experiments using plasma-induced erosion/deposition in a CH4/H2 gas mixture. We also report data on the numerical simulation of plasma parameters of a capacitively-coupled discharge calculated using a commercial CFD-ACE code. A comparison of these data with the results for mirror testing under deuterium ion bombardment illustrates the possibility of using the capacitively-coupled discharge for in situ non-destructive deposition mitigation/cleaning.

Laser damage thresholds of ITER mirror materials and first results on in situ laser cleaning of stainless steel mirrors

“…Plasma cleaning might be implemented in the form of a local RF or DC generated plasma to erode the deposit either chemically or physically through sputtering [6]. In principle such a method could operate continuously, thereby preventing a deposit from building up at all [7]. Also, the worldwide industrial expertise in plasma processing inspires confidence that an engineering solution could be found for implementation in ITER.…”

Diagnostics development for quasi-steady-state operation of the Wendelstein 7-X stellarator (invited)

“…24 Moreover, in the pinhole based systems the efficiency in reducing the build-up of coatings by maintaining a small hydrogen gas flow throughout the discharges will be explored as a further promising measure. 25 Installing this gas flow system in 9 identical video diagnostic systems will give us the possibility to investigate what is the minimum required flow that still has a worthwhile effect while not affecting general plasma fuelling. In case of diagnostics requiring very high optical throughput and thus large windows, like e.g.…”

Cleaning of first mirrors in ITER by means of radio frequency discharges

“…3 In order to prevent or mitigate mirror degradation, the optical quality must be preserved, i.e., by applying a sputter-resistant coating on the reflective surface. 4 Alternatively, it must be restored periodically during ITER life time by appropriate cleaning techniques. Among several technologies (like laser ablation 5 or microwave sputtering), radio frequency (RF) plasma sputtering turned out to be the most promising technology for this task.…”