A. R. Nemets

Charge-exchange recombination spectroscopy of the plasma ion temperature at the T-10 tokamak

Charge-exchange recombination spectroscopy (CXRS) based on a diagnostic neutral beam has been developed at the T-10 tokamak. The diagnostics allows one to measure the ion temperature profile in the cross section of the plasma column. In T-10 experiments, the measurement technique was adjusted and the elements of the CXRS diagnostics for ITER were tested. The used spectroscopic equipment makes it possible to reliably determine the ion temperature from the Doppler broadening of impurity lines (helium, carbon), as well as of the spectral lines of the working gas. The profiles of the plasma ion temperature in deuterium and helium discharges were measured at different plasma currents and densities, including with the use of active Doppler measurements of lines of different elements. The validity and reliability of ion temperature measurements performed by means of the developed CXRS diagnostics are analyzed.

Charge exchange recombination spectroscopy on the T-10 tokamak.

The charge exchange recombination spectroscopy (CXRS) diagnostics on the T-10 tokamak is described. The system is based on a diagnostic neutral beam and includes three high etendue spectrometers designed for the ITER edge CXRS system. A combined two-channel spectrometer is developed for simultaneous measurements of two beam-induced spectral lines using the same lines of sight. A basic element of the combined spectrometer is a transmitting holographic grating designed for the narrow spectral region 5291 ± 100 Å. The whole CXRS system provides simultaneous measurements of two CXRS impurity spectra and Hα beam line. Ion temperature measurements are routinely provided using the C(6+) CXRS spectral line 5291 Å. Simultaneous measurements of carbon densities and one more impurity (oxygen, helium, lithium etc.) are carried out. Two light collecting systems with 9 lines of sight in each system are used in the diagnostics. Spatial resolution is up to 2.5 cm and temporal resolution of 1 ms is defined by the diagnostic neutral beam diameter and pulse duration, respectively. Experimental results are shown to demonstrate a wide range of the CXRS diagnostic capabilities on T-10 for investigation of impurity transport processes in tokamak plasma. Developed diagnostics provides necessary experimental data for studying of plasma electric fields, heat and particle transport processes, and for investigation of geodesic acoustic modes.

Radial scanning diagnostics of bremsstrahlung and line emission in T-10 plasma

The paper describes the scanning spectroscopic diagnostics designed for measurement of line integrated plasma radiation in two visible spectral ranges. This diagnostic system is aimed at measuring the bremsstrahlung absolute values and profile with high spatial resolution. The bremsstrahlung absolute values are used to determine the value and radial distribution of effective plasma ion charge Zeff(r) in T-10 discharges. The importance of Zeff measurement is due to its strong influence on plasma heating, confinement, and stability. The spatial distribution of emission for one of the chosen spectral lines is measured simultaneously with bremsstrahlung. The spatial resolution of measurements is ~1 cm, and the temporal resolution is up to 10 ms. The spectral equipment and methods for its calibration are described. Examples of line integrated brightness distribution in a “continuum window” of 5236 ± 6 Å and brightness of the lines C5+ (5291 Å), He1+ (4686 Å), and Dβ (4861 Å) are given. Flattening of the bremsstrahlung brightness profile in the central region of the plasma column in some discharges with sawtooth oscillations in the T-10 is observed. The measured effective ion charge profiles in ohmic discharges with high plasma density and low discharge currents demonstrate accumulation of light impurities at the column axis; as a consequence, quenching of sawtooth oscillations in some discharges is observed. The developed diagnostics provides necessary data for investigation of heat, particle, and current transport in the plasma of the T-10. Successful application of the obtained data on Zeff(r) for investigation of geodesic acoustic modes of plasma oscillations in the T-10 should be specially noted.

Spatially resolved spectroscopic ion temperature measurements at plasma edge of the T-10 tokamak

Spectroscopic diagnostics of the edge ion temperature were developed on the T-10 tokamak. Spatially resolved measurements of C5+ and other ionization states of carbon spectral line shapes are provided. Spectra were measured with high spectral resolution using 14 lines of sight in one poloidal section of the tokamak. Each measured line-integrated spectrum contains a combination of multiple local spectra with corresponding values of ion temperature. Modeling of spatial distribution of line emissivity and spectral line shapes along the lines of sight allows the reconstruction of the ion temperature profile on the basis of the closest match of measured and modeled spectra. The fine structure of spectral line, Zeeman effect, and apparatus function are taken into account during data processing. Obtained ion temperature profiles, Ti(r), at the plasma edge are in good agreement with ion temperature profiles measured by Charge eXchange Recombination Spectroscopy (CXRS) diagnostics of T-10. Use of the CXRS equipment for measurements of passive spectra can provide additional information on the temporal evolution of the edge ion temperature. Developed diagnostics provide necessary data for the research of geodesic acoustic modes, which are strongly dependent on plasma edge ion temperature.

STUDY OF PLASMA IMPURITIES COMPOSITION IN THE EXPERIMENTS WITH CARBON, TUNGSTEN AND LITHIUM LIMITERS ON Т-10

Работа посвящена исследованиям примесного состава плазмы Т-10 во время экспериментальных кампаний 2014—2016 гг., в течение которых углеродный лимитер установки был заменён на вольфрамовый и был установлен подвижный литиевый лимитер. В данных экспериментах величина эффективного заряда плазмы определялась из интенсивности тормозного континуума и CXRS-измерений. Суммарная концентрация ионов вольфрама в шнуре оценивалась из интенсивности излучения, регистрируемого с помощью AXUV. Поступление в плазму лёгких примесей (C, N, O, Li), а также атомов вольфрама оценивалось из абсолютных измерений профилей линейчатого излучения. Эксперимент показал, что применение лития в качестве материала лимитера позволяет значительно снизить поступление лёгких примесей в разряд, а также уменьшить распыление вольфрамовой диафрагмы. При этом концентрация ядер лития в шнуре не достигает величин, измеримых CXRS-диагностикой Т-10 (0,3—0,5% от плотности плазмы).

MODELLING OF TUNGSTEN BEHAVIOR IN PLASMA OF T-10 TOKAMAK

В работе представлены результаты численного моделирования поведения вольфрама в разрядах с омическим нагревом (ОН) при Zэф ≈ 1 на установке Т-10 с основным вольфрамовым лимитером и вакуумной камерой, покрытой слоем лития. Показано, что в ОН-разрядах максимумы относительных концентраций ионных состояний вольфрама располагаются в шнуре на радиусах своего коронального равновесия. Выполнение условий коронального равновесия на токамаке Т-10 делает возможным решение задачи определения суммарной интенсивности излучения по полной концентрации ионов вольфрама. Показано, что моделирование профиля Prad(r) тяжёлой примеси вольфрама не позволяет получить полное совпадение с экспериментальными данными при использовании известных представлений о величине аномального переноса лёгких и средних примесей в плазме Т-10. Представлена зависимость профилей излучения вольфрама от различных данных по скоростным коэффициентам возбуждения, ионизации, рекомбинации и от величины аномального транспорта. Сделан вывод о величине и распределении в шнуре коэффициента аномальной диффузии тяжёлой примеси вольфрама в OH-разрядах Т-10.

Improvement of plasma energy confinement in tokamak under radiative cooling of the edge plasma

Improvement of plasma energy confinement in the T-10 tokamak by injection of impurity gases was studied experimentally. Injection of Ne and He in the ohmic and ECR heating regimes allows one to separate the dependences of energy confinement on the plasma density and on the edge plasma cooling rate. It is shown that the well-known dependence of the energy confinement time on the plasma density is, in fact, the dependence on the radiative loss power. This phenomenon can be explained by plasma self-organization. The experiments are described by a thermodynamic model for self-organized plasma in which the transport coefficient depends on the difference between the actual and self-consistent pressure profiles. The reduction in the heat flux at the plasma edge due to radiative cooling leads to a decrease in the transport coefficient in this region and, accordingly, improves energy confinement. Results of approximate model calculations for experiments with Ne injection are presented.

CAPABILITIES OF SPECTROSCOPIC DIAGNOSTICS CXRS IN T-10 TOKAMAK

ВОЗМОЖНОСТИ СПЕКТРОСКОПИЧЕСКОЙ ДИАГНОСТИКИ CXRS В ТОКАМАКЕ Т-10 Л.А. Ключников, В.А. Крупин, К.В. Коробов, М.Р. Нургалиев, А.Р. Немец, А.Ю. Днестровский, Н.Н. Науменко, С.Н. Тугаринов, С.В. Серов, Д.С. Деньщиков Отделение токамаков, НИЦ «Курчатовский институт», Москва, Россия Институт физики Национальной академии наук Беларуси, Минск, Республика Беларусь Частное учреждение «ИТЭР-Центр», Москва, Россия

Modernized active spectroscopic diagnostics (CXRS) of the T-10 tokamak

This work presents the results of modernization of the CXRS (charge exchange recombination spectroscopy) diagnostics [1] at the T-10 tokamak. The relevance of this work is due to the importance of measurements of the ion temperature and nuclei density of the working gas and impurities for analysis of transport processes in the plasma ion component. Measurements of radial profiles of the ion temperature are extremely important for investigating the geodesic acoustic mode behavior which is conducted at the T-10 [2]. The modernized scheme of CXRS measurements, as well as the design and operational features of the spectrometer created for the new diagnostics, is described. Principles of data recording and further processing are considered in detail; attention is given to the problem of calibration of the whole complex of equipment. The performed changes in diagnostics allow the measurements to be taken simultaneously in three spectral intervals: in the region of the beam line Hα, the CXRS line of carbon ion C5+, and the CXRS line of one of the hydrogen-like ions: He1+, Li2+, N6+, O7+ or Ne9+. This makes it possible to measure the density profiles of two plasma impurities simultaneously, as well as the ion temperature from CXRS lines of different elements. The modernized diagnostics significantly broadens the possibilities of studying the physics of transport processes and quasi-coherent modes of plasma oscillations at the T-10.