A. G. Alekseev

Li experiments on T-11M and T-10 in support of a steady-state tokamak concept with Li closed loop circulation

This paper presents a review of the last T-11M and T-10 tokamak activity in the field of Li plasma facing component (PFC) investigation. Attention is mainly paid to the realization of the concept of closed loop lithium circulation as a solution of the PFC problem of a steady-state DT volumetric neutron source on a tokamak basis. Realization of the Li PFC concept demands the decision of three main tasks: lithium injection into the plasma, Li collection before its deposition on the vacuum vessel and the return of Li to the injection zone from the collector. This emitter?collector concept assumes that the main heat flux from a hot plasma to the PFC (limiters and divertor plates) can be dissipated on the entire vessel wall surface by non-coronal Li radiation, which will smoothen the local heat load PFC. A rail limiter on the basis of a capillary porous system manufactured from tungsten felt and provided with W wings was successfully tested in the last T-11M experiments as a prototype of steady-state Li emitter?collector. A witness-sample analysis showed that the lateral sides of the rail and ring limiters crossing the plasma scrape-off layer can collect a significant (~80%) part of Li, injected into the plasma during discharges. This can be used in the future for closing Li loop circulation. As was shown by Li pellet injection in T-10, the probability of Li penetration into the hot plasma core from its boundary is lower than that of deuterium by a factor of 5?10. This result can explain the effect of plasma cleaning (Zeff (0) ~ 1) during T-10 Li experiments. Some different schemes of future lithium circulation loops are discussed.

Influence of stray light on visible spectroscopy for the scrape-off layer in ITER

Since ITER will be a full metallic wall machine, scattered photons from the strong emission in the divertor may distort the emission from the scrape-off layer (SOL). The influence of stray light for visible spectroscopy of H? and Be?I emissions in ITER was investigated with a ray-tracing simulation. It was found that the stray light would be always more than one order of magnitude greater than the real signal intensity for H? emission from the SOL. For Be?I emission, although the situation was better than the cases of H? measurement, the stray light could be much larger than the real signal from the SOL. In ITER, it will inevitably reduce the stray light somehow for visible spectroscopy. The effect of optical dumps embedded on first walls was investigated with the ray-tracing analysis.

Progress with the ITER project activity in Russia

Due to the development of the ITER project, the requirements of the technical parameters of the ITER systems were more precisely and practically determined to be at higher levels. The essential increase of the ITER system characteristics happened recently. A number of prototypes were manufactured and tests were carried out. The results of the development and manufacture of 25 ITER systems, subject to the Russian Federations obligations in the ITER project, are described.

Divertor stray light analysis in JET-ILW and implications for the H-α diagnostic in ITER

We report on the first results for the spectrum of divertor stray light (DSL) and the signal-to-background ratio for D-α light emitted from the far SOL and divertor in JET in the recent ITER-like wall (ILW) campaign. The results support the expectation of a strong impact of DSL upon the H-alpha (and Visible Light) Spectroscopy Diagnostic in ITER.

Main challenges for ITER optical diagnostics

The review is made of the problems of ITER optical diagnostics. Most of these problems will be related to the intensive neutron radiation from hot plasma. At a high level of radiation loads the most types of materials gradually change their properties. This effect is most critical for optical diagnostics because of degradation of optical glasses and mirrors. The degradation of mirrors, that collect the light from plasma, basically will be induced by impurity deposition and (or) sputtering by charge exchange atoms. Main attention is paid to the search of glasses for vacuum windows and achromatic lens which are stable under ITER irradiation conditions. The last results of irradiation tests in nuclear reactor of candidate silica glasses KU-1, KS-4V and TF 200 are presented. An additional problem is discussed that deals with the stray light produced by multiple reflections from the first wall of the intense light emitted in the divertor plasma.

Fast measurements of the limiter surface temperature and the heat flux onto the limiter in the T-11M tokamak by using an IR radiometer

The limiter surface temperature is measured with a high-speed IR radiometer (λ⋍3–6 µm) during a T-11M tokamak discharge. The IR radiometer was absolutely calibrated under steady-state operating conditions: the limiter was heated by a special inner heater, and the limiter temperature was measured with a thermocouple. Based on these measurements, the heat fluxes from the plasma onto the limiter surface during the discharge are determined. The measurement technique is discussed, and the data on the limiter surface temperature and the heat flux to the limiter for different regimes of the T-11M operation are presented.

Synthetic H-Alpha Diagnostics for ITER: Inverse Problems and Error Estimations for Strong Non-Maxwellian Effects and Intense Divertor Stray Light

Abstract The use of an all-metal first wall in future magnetic fusion reactors equipped with a divertor may impose severe limitations on the capabilities of optical diagnostics in the main chamber because of the divertor stray light (DSL) produced by reflections of the intense light emitted in the divertor. Here, we introduce a synthetic H-alpha diagnostics to estimate the errors of solutions of the inverse problems aimed at recovering the neutral hydrogen parameters (density and isotope ratio) in the scrape-off layer (SOL) with allowance for (a) strong DSL on the observation chords in the main chamber, (b) substantial deviation of the neutral atom velocity distribution function from a Maxwellian in the SOL, and (c) the data from the direct observation of the divertor. The results of recovering the relative contributions of all three sources to the signal along an observation chord in the main chamber (namely, from the high-field-side and low-field-side SOL sections of the observation chord, and the DSL), together with the isotope ratios in the SOL, are presented for the flattop stage of Q = 10 inductive operation of ITER.

Determination of divertor stray light in high-resolution main chamber H α spectroscopy in JET-ILW

The theoretical model suggested for ITER main chamber H α spectroscopy is applied to the high-resolution spectroscopy (HRS) data of recent JET ITER-like wall (ILW) experiments. The model is aimed at reconstructing the neutral hydrogen isotope density in the SOL, as well as the isotope ratio, by solving a multi-parametric inverse problem with allowance for (i) the strong divertor stray light (DSL) on the main-chamber lines of sight (LoS), (ii) substantial deviation of the neutral atom velocity distribution function (VDF) from a Maxwellian in the SOL, and (iii) data for the direct observation of the divertor. The JET-ILW HRS data on resolving the power at the deuterium and hydrogen spectral lines of the Balmer-alpha series is analysed, with direct observation of the divertor from the top and with observation of the inner wall along the tangential and radial LoS from the equatorial ports. This data allows the spectrum of the DSL and the signal-to-background ratio for the Balmer-alpha light emitted from the far SOL and divertor in the JET-ILW to be evaluated. The results support the expectation of the strong impact of the DSL upon the ITER main chamber H α (and visible light) spectroscopy diagnostics.

Asymmetry of the Balmer-alpha line shape and recovery of the effective hydrogen temperature in the tokamak scrape-off layer

An algorithm for recovering the effective temperature of atoms of hydrogen (and its isotopes) in the tokamak scrape-off layer from the asymmetry of the Balmer-alpha line shape is proposed. The algorithm is based on the parametrization of the asymmetry of the line shape caused by the nonlocal character of neutral hydrogen flux from the wall into the tokamak plasma. The accuracy of the algorithm is tested against the results of simulations of the velocity distribution function of deuterium neutrals in the scrape-off layer by the EIRENE code with the use of the source data on the main plasma component in the quasi-stationary stage of the inductive mode of ITER operation calculated by the SOLPS4.3 (B2-EIRENE) code.

A method of interpreting the Balmer-alpha high-resolution spectroscopy for tokamak edge plasmas with account of divertor stray light

A method is suggested for interpreting the data from the Balmer-alpha high- resolution spectroscopy diagnostics of the edge plasma in the tokamak main chamber, which additionally uses the data from direct observation of the divertor. Such an extension of the diagnostics is motivated by the fact that in a tokamak-reactor with the metal first wall, like ITER tokamak, a significant role of the divertor stray light (DSL), which is emitted by the plasma in the divertor in the same spectral line and reflected from the first wall of the vacuum chamber to a spectrometer in the main chamber, is expected. The results of the first applications of the developed model to interpret the data from the JET-ILW tokamak experiments, which simulate the conditions of occurrence of the DSL in ITER, are discussed.