V. S. Voitsenya

Chapter 7: Diagnostics

In order to support the operation of ITER and the planned experimental programme an extensive set of plasma and first wall measurements will be required. The number and type of required measurements will be similar to those made on the present-day large tokamaks while the specification of the measurements—time and spatial resolutions, etc—will in some cases be more stringent. Many of the measurements will be used in the real time control of the plasma driving a requirement for very high reliability in the systems (diagnostics) that provide the measurements. The implementation of diagnostic systems on ITER is a substantial challenge. Because of the harsh environment (high levels of neutron and gamma fluxes, neutron heating, particle bombardment) diagnostic system selection and design has to cope with a range of phenomena not previously encountered in diagnostic design. Extensive design and R&D is needed to prepare the systems. In some cases the environmental difficulties are so severe that new diagnostic techniques are required. a Author to whom any correspondence should be addressed.

Diagnostic first mirrors for burning plasma experiments (invited)

The current state of investigations of the problem of providing first mirrors (FMs) for diagnostic systems in a reactor-grade fusion device is summarized. Results obtained in simulation experiments that have been conducted during recent years in several laboratories are presented. Attention is concentrated on two processes that can have an opposite effect but both can lead to degradation of mirror optical properties, namely: sputtering by charge exchange atoms which leads to erosion, and deposition which leads to surface contamination. It is shown in the analysis that when sputtering dominates, mirrors of monocrystalline refractory metals (Mo, W) can have a sufficiently long lifetime even for FMs that have to be located close to the first wall. Similarly, films of low sputtering yield metals on high thermal conductivity substrates (e.g., Rh on Cu) can be used for FMs in locations where the charge exchange flux is reduced to about a tenth of that at the first wall. However, deposition poses a serious threa...

Progress in research and development of mirrors for ITER diagnostics

Metallic mirrors will be used as plasma-viewing elements in all optical and laser diagnostic systems in ITER. In the harsh environment of ITER, the performance of mirrors will decrease mainly because of the erosion of their surfaces and deposition of impurities. The deterioration of the optical properties of diagnostic mirrors will directly affect the entire performance of the respective ITER diagnostics, possibly leading to their shutdown. Therefore, R&D on mirrors is of crucial importance for ITER diagnostics. There is a coordinated worldwide R&D programme supervised by the Specialists Working Group on first mirrors of the International Tokamak Physics Activity, Topical Group on Diagnostics. This paper provides an overview of new results in the field of first mirrors, covering the manufacturing of ITER mirror prototypes, investigations of mitigation of deposition and mirror cleaning and the predictive modelling of the mirror performance in ITER. The current status of research on beryllium deposition—a new critical area of mirror research—is given along with an outlook for future activities.

First mirrors for diagnostic systems of ITER

The majority of optical diagnostics presently foreseen for ITER will implement in-vessel metallic mirrors as plasma-viewing components. Mirrors are used for the observation of the plasma radiation in a very wide wavelength range: from about 1 nm up to a few mm. In the hostile ITER environment, mirrors are subject to erosion, deposition, particle implantation and other adverse effects which will change their optical properties, affecting the entire performance of the respective diagnostic systems. The Specialists Working Group (SWG) on first mirrors was established under the wings of the International Tokamak Physics Activity (ITPA) Topical Group (TG) on Diagnostics to coordinate and guide the investigations on diagnostic mirrors towards the development of optimal, robust and durable solutions for ITER diagnostic systems. The results of tests of various ITER-candidate mirror materials, performed in Tore-Supra, TEXTOR, DIII-D, TCV, T-10, TRIAM-1M and LHD under various plasma conditions, as well as an overview of laboratory investigations of mirror performance and mirror cleaning techniques are presented in the paper. The current tasks in the R&D of diagnostic mirrors will be addressed.

Some problems of the material choice for the first mirrors of plasma diagnostics in a fusion reactor

We present the results of simulation experiments on the effect of the fusion reactor environment on the optical properties of the first mirrors for spectroscopy and laser diagnostics. We found the greatest effect on the degradation of mirror optical properties was due to charge exchange atoms. These atoms can affect mirror quality in two ways: by sputtering and by redeposition of material sputtered from other inner components of the vacuum chamber. The degradation rates of mirrors made of different metals and subjected to long-term bombardment by ions from hydrogen or deuterium plasmas are compared. From analysis of all data, we concluded that special experiments will have to be conducted in order to make a correct choice of a first mirror material. These tests should include the following: (1) study the role of swelling on mirror surface modification; (2) find the multimillion shot laser-induced-damage threshold of metal mirrors; and (3) develop and test methods to protect the first mirrors from redeposi...

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.

First mirrors for diagnostic systems of an experimental fusion reactor I. Simulation mirror tests under neutron and ion bombardment

Among the diagnostic systems planned for use in the International Thermonuclear Experimental Reactor to control the reactor operation, a large number of these systems have to use the mirrors to input or output the electromagnetic radiation to or from the burning plasma in different parts of the spectrum. The mirrors placed inside the vacuum vessel will be subjected to the impact of several factors, resulting in degradation in their optical characteristics. The most critical factors are erosion under the bombardment with a flux of high-energy particles, deuterons and tritons, and the deposition of the products of erosion of the in-vessel components. The first part of this review presents the results of the simulation experiments studying the effect of sputtering and deposition of contaminants on the optical properties of mirrors fabricated from different materials. In the second part of the review, the results of mirror testing on the operating large-scale fusion devices are considered.

Materials selection for the in situ mirrors of laser diagnostics in fusion devices

When mirrors for the laser scattering diagnostic for large fusion devices need to be inside the vacuum chamber, they are subjected to irradiation by multiple high-energy laser pulses and bombardment by charge exchange atoms. Both of these assaults are known to degrade and eventually damage metal laser mirrors given sufficient time and flux. Our aim in this article is to use current data on these damage mechanisms to make design selections of metal mirror materials for application in fusion device diagnostics. We identify tradeoffs between low sputtering rates and multipulse laser damage resistance in candidate metals. The data for multipulse laser damage are incomplete and extend to a maximum of only 104 shots for a few metals. However, there is a clear trend of decreasing laser-damage threshold with increasing number of shots, and damage threshold fluences can fall to 10% of the single-pulse damaging laser fluence. Further experiments up to 106 or 108 laser shots need to be conducted on the likely mirror...

Diagnostic First Mirrors for Burning Plasma Experiments

The lifetime of refractive components exposed to reactor grade plasmas will be very short and so all diagnostics which use UV (λ > 5nm), Visible and IR radiation (λ up to ∼100μm) have to view the plasma via a mirror. The diagnostic first mirrors (FM) must survive in extremely hostile conditions and maintain a good optical performance for the duration of reactor operation. In ITER-FEAT the FMs will receive intense UV and X-ray radiation, neutron and gamma fluxes, and particle fluxes (due to charge exchange atoms (CXA)). In addition, they will be subjected to the deposition of material eroded from the divertor and first wall. Of the different kinds of radiation and fluxes only CXA impact will result in direct surface modification of metallic FMs that can lead to degradation of optical properties. The fluxes of all radiation components to the mirror surface depend strongly on the mirror location. For example, the FMs of a wide-angle observation system (endoscope with open architecture) in ITER will be bombarded by CXA fluxes of about the same magnitude as the first wall. On the other hand, the FMs in the LIDAR system are located in a long duct (∼2 m in length) and will receive CXA fluxes ∼ 10−2 of the first wall flux. This corresponds to about the lowest flux received by a FM in ITER.

Simulation of environment effects on retroreflectors in ITER

Two plasma diagnostics in ITER will use cube-corner reflectors (CCR): poloidal polarimetry and toroidal interferometry/polarimetry. The multichannel poloidal polarimetry system is planned to operate at a wavelength of 118.8μm. The multichannel toroidal interferometry/polarimetry system is based on a CO2 laser operating at wavelengths of 10.6 and 9.27μm. The long term sputtering by charge exchange atoms and/or deposition of carbon-based (or beryllium-based) contaminant layers can affect the optical properties of the CCR. The role of both these potentially deleterious effects on the CCR operation is analyzed in this article, taking into account the probing beam wavelength and the CCR locations. The conclusion is that for the intended use of a CCR in the poloidal polarimetry at 118μm neither erosion nor deposition should pose a problem. On the other hand, in the toroidal interferometry/polarimetry system operating at 10μm, care must be taken to reduce the charge exchange atom flux and it is likely that depos...