In-vessel Mirrors Research Articles

Design of in-vessel mirror of ITER poloidal polarimeter

Diagnostics in-vessel mirrors receive high neutron heating loads and high radiation heat loads because it directly faces the plasma. The in-vessel mirror of this study consists of three layers; tungsten mirror, copper-alloy heatsink and stainless-steel mirror mount. The copper-alloy heatsink is actively cooled by cooling-water pipe. The tungsten mirror is attached to the heatsink with contact pressure of 3.5 MPa and is cooled by thermal conduction to the heatsink. The mirror is not rigidly fixed to the heatsink but is able to slide on the surface of the heatsink because thermal stress in the dissimilar materials during baking needs to be avoided. The structural integrity was evaluated according to RCC-MRx code. The highest primary load to the mirror was the acceleration load during plasma disruption, and the highest secondary load was the baking temperature (240 °C). All design criteria specified in RCC-MRx code are satisfied. In addition, the design of the in-vessel mirrors was developed to facilitate adjustment of mirror angle with accuracy of 0.1° A prototype mirror was made to confirm the surface deformation owing to the assembly load. Surface deformation was 6.3 μm and was sufficiently small compared to the laser wavelength.

Demonstration of a two-dimensional divertor Thomson scattering system on DIII-D

Recent upgrades providing two-dimensional divertor Thomson scattering (DTS-2D) measurements of Te and ne during a DIII-D plasma shot and a thorough description of system components and their functionality are presented. This system expands the capabilities of the existing single divertor Floor measurement location by introducing seven additional laser beam path options in the poloidal plane, spanning major radii from 1.062 to 1.335 m. The system redirects ∼1 J, 50 Hz Nd:YAG laser pulses to the new beam paths within 20 ms, stepping through each path on the divertor Floor every 200 ms during a plasma shot. The laser is redirected using an ex-vessel, fast-steering mirror to one of eight in-vessel beam paths oriented underneath the vessel tiles. Up to eleven measurement positions per beam path, from −1.35 to −1.13 m below the machine midplane, are available by dynamically refocusing the ex-vessel collection fiber array using a high-speed linear stage. Current measurement positions above the divertor Shelf are retained via a hole in the fixed, in-vessel mirror, allowing laser pass through.

Design of optical transmission line of ITER poloidal polarimeter

ITER Poloidal Polarimeter is laser-aided diagnostics that measures the change of polarization of injected far-infrared laser beam in order to identify plasma current profile. PoPola consists of thirteen measurement chords, and each chord is comprised of a set of optical components. This study shows the design of optics in the ITER vacuum vessel and those in high radiation area outside the vacuum vessel. The design takes into account zero-offset configuration, the minimization of the neutron streaming, remote alignment, accessibility, manufacturability and radiation hardness. The laser power loss due to vignetting loss and absorption is evaluated, and the signal-to-noise ratio at a detector is estimated at 50,000. This study evaluates the acceptable errors of manufacturing and assembly of the optical transmission line on the condition that the demonstrated alignment method using the two steering mirrors outside the vacuum vessel is applied. The evaluation results show that the medium tolerance class of ISO 2768 is basically enough for manufacturing ITER poloidal polarimeter. Only for assembling the in-vessel mirrors to the mirror module, the mirror angles need to be carefully adjusted with accuracy of 0.1° Finally, radiation hardness of a steering mirror is presented. The steering mirror consists of an angle sensor and a piezo actuator. The angle sensors are developed by the authors and free from semiconductor and soft resin. The prototype test results show that angle sensor can measure the mirror angle in the range of ±2.3° with accuracy of 0.37%. As for the piezo actuator, the irradiation test was carried out. The test results show that the piezo actuator is able to operate after the neutron fluence of 3.5 × 1015 n/cm2 and the gamma dose of 0.99 MGy.

Calculations for the optical system for the first ITER plasma

A dedicated optical system is designed for the breakdown of the first ITER plasma, when the machine will not be equipped with the equatorial ECRH-launcher. Seven beams from the upper launcher will be transmitted via several in-vessel mirrors through the resonance layer to a beam-dump. The whole system was simulated with the PROFUSION package to obtain the beam parameters, estimate spill-over effects and determine the required polarization in the launcher in order to have X-mode polarization in the resonance.

Design validation of in-vessel mirrors and beam dump for first plasma operations in ITER

First plasma operation in ITER will start after completing the assembly of the tokamak vessel and the installation of the main sub-systems, but prior to the installation of the blanket modules and the divertor cassettes. Utilization of temporary limiters and divertor replacement structures will provide a poloidal and toroidal reference side position to the plasma edge to protect the vacuum vessel and other components already installed during operations. An additional set of mirrors is required to reflect the power injected from the upper launcher towards the plasma resonance for EC-assisted breakdown of the plasma up to a beam dump needed to trap and absorb the power of the beams after crossing the plasma in order to reduce the stray radiation escaping back into the vacuum chamber down to less than 10% of the total power. The quasi-optical system has been designed, with shape and size of the mirrors compliant with the requirements provided by ITER for their installation, realization and plasma performances, resulting in two standard focussing mirrors and one grating mirror. The beam dump consists of a box with five metal plates, the first providing a spreading of the high incident power and the others coated with absorbing material with thickness distribution studied to gradually reduce the power during the multiple reflections inside the box, avoiding damages to the coating itself. This work focuses on the validation of the quasi-optical design of the mirrors and the assessment of the dump performances, based on a multi-bounces model developed ad-hoc for this purpose. The study includes a tolerance analysis for the beam dump to include the effect of uncertainties in the thickness of the absorbing coating and misalignments of the mirrors, to verify the performances of the dump also when operating in different conditions with respect to the nominal ones.

Design optimization of the Upper Steering Mirror Assembly (USMA) for ITER ECHUL in view of disruptive events

Four Electron Cyclotron Heating Upper Launchers (ECHUL) will be used at ITER to counteract magneto-hydrodynamic plasma instabilities by targeting them with up to 20 MW of mm-wave power at 170 GHz. The millimeter waves are guided through a set of fixed mirrors (M1, M2 and M3) and the front steering mirror set (M4), aiming at the correct location in the plasma for suppression of the q = 3/2 and q = 2/1 Neoclassical Tearing Modes (NTMs). At the M4 reflecting mirror surfaces, part of the mm-wave power is converted into heat by ohmic dissipation, totaling ca. 25 kW of absorbed power and reaching a peak power density of up to 1.8 MW/m2 in each of the 4 beam center spots.The latest in-vessel mm-wave mirrors Components Load Specification (CLS) data imposes an increase of the electromagnetically induced loads relative to those anticipated in earlier designs, resulting in higher mechanical load on the Crossed Flexure Pivot (CFP) due to Vertical Displacements Events (VDEs). The present paper reports the main design optimizations as well as the finite elements analyses carried out with the objective to: 1) reduce the electromagnetic loads on the components due to induced Eddy currents, 2) dissipate the thermal loads coming from the beams themselves and the plasma following the design requirements in terms of coolant temperature rise, pressure drop and admissible corrosion rate values, 3) assure the components structural integrity enforcing the ITER Structural Design Code for the In-Vessel Components (SDC-IC).

First measurements of line-integrated electron density in an ITER-like configuration using the JET far infrared polarimeter diagnostic

Polarimetry exploits the optical activity and birefringent properties of thermonuclear plasmas to calculate some important quantities for their control like the line-integrated electron density and magnetic field distribution. The Joint European Torus (JET) far infrared polarimeter shares the same probing laser beams of the interferometer, with eight channels, four vertical and four lateral. While the vertical channels were already optimised to provide accurate measurements of the Faraday rotation angle, Cotton–Mouton (CM) phase shift and ellipticity, the lateral channels had been only optimised for Faraday rotation angle alone. By setting the initial polarisation angle of the lateral channel at zero degrees the CM effect is minimised, the ellipticity is almost zero, and the CM phase shift angle is impossible to measure. During the recent JET experimental campaign (C38 in 2019–2020), the input polarisation for the lateral channels was altered for a class of pulses to analyse the possibility to measure the CM phase shift angle and the ellipticity, and, more important, to assess if it is possible to provide information of line-integrated electron density using the lateral channels of JET polarimetry in an ITER-like configuration. As a note, ITER will have only tangential channels with reflectors buried deep inside the first wall, so this setup was truly an ITER-like configuration. The results clearly show huge improvements, which can be achieved by just changing the input polarisation. Moreover, the analysis of the measurements shows that the polarimetric measurements have a systematic error, which is probably due to the effect of refraction and to the in-vessel mirrors, which was only partially taken into account during the calibration phase before the plasma. Thus, a new calibration method was developed and the results presented on a statistical basis. It has been demonstrated that, varying the input polarisation of the polarimeter and using the new calibration method, it is possible to measure the line-integrated electron density, using the CM phase shift (or the ellipticity) of the lateral channels, with good accuracy with respect to the electron density measured by the interferometer that was considered the reference.

Estimation of the Degradation Rate of the In-Vessel Mirrors for the H-Alpha and Visible Spectroscopy Diagnostics in ITER

Mirrors for light transmittance will be used in more than 30 optical and laser diagnostics installed in the ITER tokamak reactor. Their reliable performance depends largely on the degradation rate of the first mirror directly facing the plasma. The fluxes of neutral atoms, mostly D, T, and Be, from the plasma are the most dangerous for the first mirror. The interaction with the high-energy atoms can lead to sputtering of the mirror surface; redeposition of beryllium sputtered from the walls of the vacuum chamber can lead to the formation of a film on the mirror. This paper gives a conservative estimate of the degradation rate of in-vessel mirrors in the equatorial channels of the H-alpha and Visible Spectroscopy diagnostics. Calculations were performed for stationary ITER operation using the Zemax OpticStudio program. The results show that the working area of the first mirror will be sputtered at a rate of no more than 100 nm/year, which is an acceptable erosion load for mirrors made of single-crystal molybdenum. Beryllium film formation is expected only at the edges of the mirror (outside the working area). Possible contamination of the working part of the mirror as a result of accidents or plasma events can be eliminated using the mirror cleaning system installed into the first mirror unit. For the other in-vessel mirrors, the estimate predicts no significant degradation owing to their location at a long distance from the plasma.

High power characterisation of the ECRH transmission lines and power deposition calculations in the TJ-II stellarator

Abstract In the TJ-II stellarator, the last mirror of both ECRH transmission lines is an in-vessel steerable phase correcting mirror that distorts the power distribution of the reflected beam whenever the mirror orientation angles deviate from the design position. Beam distortions cannot be measured due to space restrictions when accessing the inner part of the device, particularly if we have in mind the wide range of orientation angles needed to carry out ECRH and ECCD experiments, for which such experimental characterisation should be performed. Instead, a set of full wave electromagnetic simulations allows us to estimate the power distribution of the beam coupled to the plasma and how this distribution changes when the in-vessel mirror is steered. High power characterisation of the transmission lines is first accomplished and documented in order to determine the beam power distribution before entering the vessel. For the perpendicular injection case, the additional contribution to the beam distortion produced by its passage through the structure supporting the window is calculated. Leaving aside the power losses due to beam trimming (7% and 10% for the first and second line respectively), the added distortion produced by the window housing is of minor importance compared to the effect of the internal mirror. The impact of beam distortion on the width of the power deposition profiles for several launching directions is addressed using the ray/beam tracing code TRUBA. Results show that the beam waist in the plasma appears smaller and ahead of the position predicted by quasi optical theory, leading to beam widening in the magnetic axis where the resonant layer is located. Despite the evident distortion, power deposition profiles calculated with beam tracing, refined with a Gabor expansion of the wave field distribution in case of strongly oblique incidence, do not differ much from the predictions of the standard ray tracing calculation.

Development of Microwave Imaging Diagnostics for WEST Tokamak

WEST is an upgrade of the Tore Supra tokamak to test ITER like divertor elements over very long pulses. For the studies of MHD instabilities affected by tungsten impurities, two microwave imaging diagnostics have been developed to obtain density and temperature fluctuation maps. The core reflectometer was refurbished and now measures the core density profiles in few microseconds. Measurements of several thousand profiles in sequence with microsecond dead time (burst mode) will be available in 2019 enabling the reconstruction of density maps with unprecedented time resolution. The major issue for the implementation of an electron cyclotron emission imaging diagnostic was the heat radiation during long pulses on the large vacuum window and the first optics. Two metallic in-vessel mirrors inside the personnel access tube solved this issue. The first mirror can withstand the heat radiation without water cooling, the second mirror redirects the beam toward the personnel access port. The oblique and small view angle of this access eases also the thermomechanical constraints on the vacuum window. As the space between the port flange and the tokamak access lobby was tight, a compact optical enclosure with vertical aligned optics was designed. To ensure a good reproducibility of the mirrors and the optical enclosure positions after a maintenance period and thus avoid re-alignment, different mechanical systems are used to manipulate the mirrors and the optical enclosure and to fine tune their position and their orientation. The first channel covers the low field side and will provide 192 pixel images of temperature fluctuations with 1–2 microsecond time resolution. The installation of the in-vessel mirrors is scheduled during March 2019. Diagnostic qualification tests will be done before the restart of the experiments late May 2019.

RF plasma cleaning of water-cooled mirror equipped with notch filter based on shorted λ/4 line

Physical and engineering aspects of RF plasma cleaning developed for actively cooled first mirror serving as grounded DC-coupled electrode are under consideration. Feasibility of the water-cooling lines implemented as the coaxial notch filter for ITER in-vessel diagnostic mirrors was demonstrated for Capacitively Coupled Radio Frequency (CCRF) discharge in neon at 1–10 Pa. The impinging ions energy as a function of absorbed RF power was measured for both the DC-coupled and -decoupled schemes. Sputtering yield variation over rectangular surface of stainless steel mirror was studied experimentally. The measured sputtering yield profiles were found to be in qualitative agreement with the RF plasma numerical simulations. It was demonstrated that Al/Al2O3 used as Be/BeO proxy can be removed from the water cooled mirror surface by 100 eV neon ions with the use of the notch filter. The 14 h plasma exposure didn’t affect the mirror reflectivity.

Integration of an Electron Cyclotron Imaging diagnostic system on the WEST tokamak

An Electron Cyclotron Emission Imaging (ECEI) diagnostic system for the WEST tokamak has been developed under the UNIST-WEST collaboration. This diagnostic system is designed to overcome accessibility and thermomechanical constraints for long pulse operation. The first O-mode channel will be installed in the first trimester of 2019 to probe the low field side (LFS) of the WEST plasma. Two large metallic reflective mirrors are installed inside the duct which is being used for maintenance access. They are suspended on a rail to facilitate mirror manipulation. The ex-vessel optical system (lens, detection array, etc.) is housed in a compact optical enclosure that fits in a tight free space between the port flange and tokamak access lobby. The design emphasized reproducibility of the precise alignment between in-vessel mirrors and optical enclosure since the both elements must be removed during shutdown period for maintenance access. The overall optical system was fully tested at UNIST last year. The test results demonstrated that the imaging optics can full access at any radial position on the LFS. The 2D beam pattern measurements were consistent with the design values.

Thomson scattering measurements on DIII-D using in-vessel laser mirrors and lenses to diagnose a new divertor location.

Translatable in-vessel mirrors have enabled the DIII-D Thomson scattering system to diagnose the divertor plasma in high triangularity shaped plasmas. Previous divertor Thomson scattering measurements in DIII-D were restricted to spatial locations along a Nd:YAG laser beam that was directed through a vertical port. This only allowed measurements to be made in low triangularity shaped plasmas. The new mirrors re-route the laser underneath floor tiles to a position of smaller major radius as necessary for high triangularity plasmas. New in-vessel collection optics transmit scattered light from regions inaccessible to external lenses. Damage to mirrors and high stray light levels are challenges that were overcome to successfully make these measurements. Through the careful use of baffles and light shields, stray light leakage into polychromator detector channels was reduced to negligible levels, allowing temperature measurements below 1 eV. The system is described and the initial results presented.

Image quality method as a possible way of in situ monitoring of in-vessel mirrors in a fusion reactor.

The plasma-facing (first) mirrors in ITER will be subject to sputtering and/or contamination with rates that will depend on the precise mirror locations. The resulting influence of both these factors can reduce the mirror reflectance (R) and worsen the transmitted image quality (IQ). This implies that monitoring the mirror quality in situ is an actual desire, and the present work is an attempt towards a solution. The method we propose is able to elucidate the reason for degradation of the mirror reflectance: sputtering by charge exchange atoms or deposition of contaminated layers. In case of deposition of contaminants, the mirror can be cleaned in situ, but a rough mirror (due to sputtering) cannot be used anymore and has to be replaced. To demonstrate the feasibility of the IQ method, it was applied to mirror specimens coated with carbon film in laboratory conditions and to mirrors coated with contaminants during exposure in fusion devices (TRIAM-1M and Tore Supra), as well as to mirrors of different materials exposed to sputtering by plasma ions in the DSM-2 plasma stand (in IPP NSC KIPT).

Asymmetries in the motional Stark effect emission on the DIII-D tokamak

Spectrometer measurements and filter upgrades to a motional Stark effect polarimeter measuring the outer half-radius of the DIII-D tokamak helped to identify asymmetries in the polarization angle of Stark-split emission. The measured polarization angle of the π components differs and is not orthogonal to the σ component. These differences persist over a range of densities and with low levels of background light. It is suggested that the difference in the polarization angle between components is from a change in the ellipticity of the emitted light across the Stark components coupled with imperfect polarization preservation from an in-vessel mirror.

Поведінка металевих діагностичних дзеркал із різною структурою в умовах, що імітують умови їх роботи в термоядерному реакторі ІТЕР

In the paper by V.S. Voitsenya et al. (Plasma Phys. Rep. 20, 217 (1994)), a methodology aimed at an optimal selection of materials for in-vessel mirrors used in optical and laser methods of plasma diagnostics in the experimental fusion reactor ITER was elaborated. The corresponding systematic simulation studies concerning the behavior of mirror specimens fabricated from different metals with different structures – polycrystalline (Be, Al, SS, Cu, Ti, Mo, W, Ta), single-crystalline (SS, Ni, Mo, W), and film (i.e. the film/substrate structure, namely, Be/Cu, Cu/Cu, Rh/Cu, Rh/V, Rh/SS, Mo/SS, Mo/Mo) – as well as mirrors fabricated from amorphous alloys of the ZrTiCuNiBe type, under long-term sputtering by deuterium (in some cases, argon) plasma ions were carried out. Amorphous mirror specimens were shown to be much more resistant to the development of roughness in comparison with mirrors with any other structure, which results from the complete absence of any ordered structure on the surface on a scale exceeding a few nanometers. The most important results were confirmed experimentally on such fusion installations as the TEXTOR (J¨ulich, Germany), ASDEX-U (Garching, Germany), and Tore Supra (Cadarachе, France) tokamaks, the heliotron Large Helical Device (Toki, Japan), on the small tokamak TRIAM-1M (Kyoto, Japan), and on special stands at Lausanne University (Switzerland) and in the Institut f¨ur Plasmaphysik, Association EURATOM-FZJ, FZ-J¨ulich (Germany).

Engineering aspects of a fully mirrored endoscope

The development of optical diagnostics, like endoscopes, compatible with the ITER environment (metallic plasma facing components, neutron proof optics, etc.) is a challenge, but current tokamaks such as JET provide opportunities to test fully working concepts. This paper describes the engineering aspects of a fully mirrored endoscope that has recently been designed, procured and installed on JET. The system must operate in a very strict environment with high temperature, high magnetic fields up to B=4T and rapid field variations (∂B/∂t∼100T/s) that induce high stresses due to eddy currents in the front mirror assembly. It must be designed to withstand high mechanical loads especially during disruptions, which lead to acceleration of about 7g at 14Hz. For the JET endoscope, when the plasma thermal loading, direct and indirect, was added to the assumed disruption loads, the reserve factor, defined as a ratio of yield strength over summed up von Mises stresses, was close to 1 for the mirror components. To ensure reliable operation, several analyses were performed to evaluate the thermo-mechanical performance of the endoscope and a final validation was obtained from mechanical and thermal tests, before the system's final installation in May 2011. During the tests, stability of the field of view angle variation was kept below 1° despite the high thermal gradient on endoscope head (∂T/∂x∼500K/m). In parallel, to ensure long time operation and to prevent undesirable performance degradation, a shutter system was also implemented in order to reduce impurity deposition on in-vessel mirrors but also to allow in situ transmission calibration. One of the main specifications of the shutter system was high reliability. This was considered as achieved when the prototype was successfully tested to 3000 cycles at a temperature of 300°C.

Experimental study of contamination and cleaning of in-vessel mirrors for ITER optical diagnostics on T-10 and QSPA-T facilities

Abstract ITER-relevant hydrocarbon films deposited under controlled conditions in the SOL of T-10 tokamak during working and cleaning discharges and by using QSPA-T plasma gun at plasma shots onto W and graphite targets were investigated by XPS, XAES and ellipsometry. XPS survey, valence band and C1s, O1s core level spectra showed differences in chemical composition and electronic structure as between T-10 and QSPA-T films and between T-10 films obtained in working and cleaning discharges. Hydrogen content evaluation in the films was made independently by analysis of XPS valence band edge position and ellipsometry data. The Csp 2 carbon content was determined from XAES CKLL spectra. Classification of T-10 and QSPA-T films was undertaken using ternary phase diagram (H, Csp 2 and Csp 3 ). It was shown that all studied hydrocarbon films are positioned between diamond- and polymer-like a-C:H films. The observed difference between XPS and ellipsometry evaluations of hydrogen content may be connected with strong structural non-uniformity of the films along thickness.

Low-energy helium irradiation on in-vessel mirror materials

Many metallic mirrors will be used for optical diagnostics in ITER. This study investigates the effects of low-energy helium (He) irradiation on the reduction in optical reflectance for molybdenum (Mo) and tungsten (W). When the surface temperature of the materials was sufficiently low, (i.e., lower than 700K for W and 600K for Mo), optical reflectance did not change; however, at higher temperatures, optical reflectance decreased significantly due to the He irradiation. Surface analysis indicated that the reduction in optical reflectance was caused by the morphology change, likely due to the formation of He bubbles. Based on the experimental results, the influences of He irradiation on the diagnostic mirrors in ITER are discussed.

Alignment Principles for ITER In-Vessel Diagnostic Mirrors

Optical mirrors inside the ITER torus vacuum are required by many of the ITER port-based diagnostic systems. As an example, the core charge exchange recombination spectroscopy (core CXRS) concept is based on a mirror labyrinth of seven consecutive in-vessel (exposed to the torus vacuum) mirrors. For this system, alignment of up to three mirrors during assembly might be needed. Because of the unique and demanding environment in the ITER diagnostic port plugs, particularly in the first meter from the plasma, existing designs of adjustable mirror holders cannot be used without modification. The wish to allow the realignment of the mirrors during maintenance of the diagnostic systems, if possible by remote handling only, adds to the complexity. This paper discusses mirror alignment principles in the light of the requirements by ITER diagnostics. The core CXRS system is used as example throughout this paper. Loads and requirements for in-vessel diagnostic mirrors are looked into. Three basic groups of alignment principles are assessed for applicability in close proximity to the ITER plasma. The strength and weaknesses of the different alignment principles are discussed, and a conclusion about their general applicability in ITER is given.