V.V. Semenov

Progress in the development of deposition prevention and cleaning techniques of in-vessel optics in ITER

The lifetime of front optical components unprotected from reactor grade plasmas may be very short due to intensive contamination with carbon and beryllium-based materials eroded by the plasma from beryllium walls and carbon tiles. Deposits result in a significant reduction and spectral alterations of optical transmission. In addition, even rather thin and transparent deposits can dramatically change the shape of reflectance spectra, especially for mirrors with rather low reflectivity, such as W or Mo. The distortion of data obtained with various optical diagnostics may affect the safe operation of ITER. Therefore, the development of optics-cleaning and deposition-mitigating techniques is a key factor in the construction and operation of optical diagnostics in ITER. The problem is of particular concern for optical elements positioned in the divertor region. The latest achievements in protection of in-vessel optics are presented using the example of deposition prevention/cleaning techniques for in-machine components of the Thomson scattering system in the divertor. Careful consideration of well-known and novel protection approaches shows that neither of them alone provides guaranteed survivability of the first in-vessel optics in the divertor. Only a set of complementary prevention/cleaning techniques, which include special materials for mirrors and inhibition additives for plasma, is able to manage the challenging task. The essential issue, which needs to be addressed in the immediate future, is an extensive development of techniques tested under experimental conditions (exposure time and contamination fluxes) similar to those expected in ITER.

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

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.

The ITER divertor Thomson scattering system: engineering and advanced hardware solutions

A divertor Thomson scattering (TS) system being developed for ITER has incorporated proven solutions from currently available TS systems. On the other hand any ITER diagnostic has to operate in a hostile environment and very restricted access geometry. Therefore the operation in an environment of intensive stray light, plasma background radiation, the necessity meet the requirement using only a 20 mm gap between divertor cassettes for plasma diagnosis as well as to measure plasma temperatures as low as 1 eV severely constrain the divertor TS diagnostic design. The challenging solutions of this novel diagnostic system which has to ensure its steady performance and also the operability and maintenance are the focus of this report. One of the most demanding parts of the in-vessel diagnostic equipment development is the design assessment using different engineering analyses. The task definition and first results of thermal, e/m and seismic analyses are provided. The process of further improving of the design involves identification of susceptible areas and multiple iterations of the design, as needed. One of the key points for all Thomson scattering diagnostics are the laser capabilities. A high-performance and high-power laser system using a steady-state and high-repetitive mode Nd:YAG laser (2J, 50–100Hz, 3ns) has been developed. The reduced laser pulse duration matched with high-speed low-noise APD detector can be very important under high background light level. For diagnostics such as Thomson scattering and Raman spectroscopy, a high-degree of discrimination against stray light at the laser wavelength is required for successful detection of wavelength-shifted light from the laser-plasma interaction region. For this case of high stray light level, a triple grating polychromator characterized by high rejection and high transmission has been designed and developed. The novel polychromator design minimizes stray light while still maintaining a relatively high transmission.

Detecting Dust on Plasma-Facing Components in a Next-Step Tokamak Using a Laser-Induced Breakdown Spectroscopy Technique

Abstract A technique based on laser-induced breakdown spectroscopy is proposed for detecting in situ dust on the plasma-exposed surfaces and in the grooves of plasma-facing components in the next generation of fusion devices (e.g., ITER). It is based on laser-induced ablation of wall material and spectral analysis of the laser spark flash-light collected by imaging optics and transmitted to the detection system. This could give space- and time-resolved information on the presence of dust or loosely bound films, their characteristic deposition patterns, elemental composition, and possibly their hydrogen content, without the necessity of breaking the machine vacuum. We have performed some simple proof-of-principle experiments to demonstrate the suitability of this technique, which might provide an effective nonintrusive in situ surface analysis method for surveying in-vessel dust accumulation in future fusion devices. The preliminary results are discussed, and some of the inherent advantages and difficulties of this method are highlighted. The usefulness of this technique to provide reliable information on the quantity of dust at the probed location still depends on the resolution of several aspects, which are the subject of ongoing experimental investigation. Areas of further research and development are identified, and some of the design issues to integrate this system in a next-step fusion device such as ITER are briefly discussed.

RF discharge for in situ mirror surface recovery in ITER

Almost all optical diagnostic systems in ITER will require the implementation of mirror recovery and protection systems. Plasma cleaning is considered to be the most promising technique for the removal of metal deposits from optical surfaces. The engineering and physical aspects of RF discharge application for continuous or periodic plasma treatment are discussed with a focus on implementation under ITER conditions. The ion flux parameters obtained in capacitively coupled (CC) RF discharge were measured in the mock-up of a plasma cleaning system. The uniformity of sputtering in CC RF discharge with and without a magnetic field was studied experimentally for the cylindrical discharge reactor geometry and compared with numerical simulations. The sharp increase in the sputtering rate resulting from the non-uniform radial distribution of the ion flux was observed near the electrode edges. The longitudinal magnetic field improves sputtering uniformity. It was demonstrated that Al/Al2O3 deposits can be removed in the Ne and D2 plasma of CC RF discharge but long-term exposition results in the degradation of the polycrystalline molybdenum mirror surface. The efficiency of Al sputtering in the atmosphere containing O2 and N2 fractions was studied in the D2/O2 and D2/N2 plasma of glow discharge. The addition of 2% of oxygen or nitrogen increases the sputtering yield by 3–4 times as compared with that in a nominally pure D2 discharge. The impact of metal deposits on the performance of diagnostic mirrors is discussed. It was shown that an ultrathin metallic film with a thickness as low as a few nm may cause a significant degradation of diagnostic mirrors with a transparent coating.

Perspectives of Use of Diagnostic Mirrors with Transparent Protection Layer in Burning Plasma Experiments

We evaluate using of metal mirrors over‐coated with transparent protection layer for the in‐vessel diagnostic systems in reactor‐grade fusion devices. Ideally, these should satisfy the contradictory demands of high reflectivity and small rate degradation when being bombarded by CX atoms. The serious threat to the performance of diagnostic mirrors is surface contamination with carbon‐based material eroded from carbon tiles. Via coupling the protective layer to a bulk mirror we can mitigate the deposit infiuence on the reflectance spectra. The regards are given to survivability in plasma environment of protected coated metallic mirrors.

Measurement of neutral density profile in a tokamak plasma using the principle of laser induced ionization

An application is discussed of a new method based on laser induced ionization that is an alternative to the laser induced fluorescence diagnostics. Here ionization of the hydrogen upper atomic states gives the local hydrogen densities in a plasma. On the basis of a rate equation analysis for a diffusion dominated plasma a probe saturation flux density of a few MW/cm2 was introduced and appropriate values were used for the plasma diagnostics. A multipulse neodymium laser was designed for plasma probing on the FT-I tokamak to measure the neutral density profiles along the plasma diameter. Local neutral densities as low as 1*108 cm-3, which is close to the sensitivity limit of the employed device, have been detected in a single tokamak discharge. The experimental results obtained on the FT-1 tokamak are presented and discussed

Corrosion resistance of V-4Ti-4Cr alloy in convection flow of eutectic Na-K alloy

The corrosion resistance of V-Ti-Cr alloys in contact with a eutectic Na-K melt have been evaluated from experiments on the compatibility of V-4Ti-4Cr alloy specimens with a convection flow of the Na-K eutectic. In the test system made of the same vanadium-based alloy, the maximum temperature was 650°C, the temperature drop was 150°C, and the duration of the tests was 500 h. The experiments have shown that, under the conditions of the tests carried out in the Na-K eutectic with an initial concentration of oxygen of 0.04 wt %, the V-4Ti-4Cr alloy undergoes intense mass transfer. The solid metal absorbs the oxygen impurity from the liquid melt. The observed effects agree with the previously drawn conclusions on the effect of the oxygen impurity on the solubility of vanadium in the Na-K eutectic. Provided that the intensity of the source of the oxygen impurity in the corrosion system is limited, one might expect acceptable corrosion compatibility of the V-4Ti-4Cr alloy with the Na-K eutectic.

Thomson scattering diagnostics for ITER divertor

The ITER design has highlighted the fundamental need to monitor the machine operation in more detail. The mission of the Thomson scattering diagnostics in the ITER divertor research/operation is discussed with due attention paid to challenges and capabilities of the existing diagnostic design.

Research on mirror cleaning in inductively and capacitively driven radio-frequency discharges

Bench tests are used to compare cleaning performance of inductively and capacitively driven radio-frequency (RF) discharges as a potential tool for in-situ maintenance of in-vessel diagnostic mirrors in fusion devices. The effect of erosion of hydrogenated carbon coating is studied in different processing conditions. Stainless steel (SS) mirrors have been exposed to CH4‒Ar and H2‒Ar plasmas in an RF discharge at a pressure of 10−2 Torr with an input power of 0.5 kW at 13.6 MHz. The samples, which exhibit a slow rate of chemical erosion, become essentially erosive in both inductively and capacitively driven RF discharges. The cleaning ability of a capacitively driven RF discharge is studied in dedicated experiments with SS samples retrieved from the tokamaks T-10 and Globus-M after long-term exposure to the working and wall conditioning discharges.