A.G. Razdobarin

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.

Physical aspects of divertor Thomson scattering implementation on ITER

This paper describes the challenges of Thomson Scattering implementation in the ITER divertor and evaluates the capability to satisfy project requirements related to the range of the measured electron temperature and density. A number of aspects of data interpretation are also discussed. Although this assessment and the proposed solutions are considered in terms of ITER compatibility, they may also be of some use in currently operating magnetic confinement devices.

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.

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.

Near-infrared plasma spectroscopy in support of divertor Thomson scattering diagnostics development for ITER

One of the main challenges of the implementation of divertor Thomson scattering system on ITER is weak laser scattering signal to be detected against intense background plasma radiation. The paper review briefly the line and continuum radiation data from present magnetic fusion devices in the spectral range of interest to TS diagnostics. The results will form the basis of design and development of the TS diagnostics for the ITER divertor.

Blistering of mechanically polished molybdenum upon its exposure to deuterium-containing plasma

Recent data on the blistering process in polished polycrystalline Mo for the case of direct-current glow discharge in deuterium and D2/2–4 mol % O2 or N2 admixtures, at an energy of incident molecular ions of 70–120 eV and a pressure of 14 Pa, are reported. Depending on the method of metal surface polishing, the initial metal grains are broken up into mutually disordered regions of smaller sizes (subgrains); the chemical composition of the damaged layer changes. In the formed structure, blistering can develop easily or be impeded. Small additions of O2 or N2 to deuterium facilitate the blistering.

STUDY OF KINETIC PARAMETERS OF GLOBUS-M PLASMA BY THOMSON SCATTERING USING ADVANCED DIGITAL POLYCHROMATOR

В исследованиях физики процессов удержания, нагрева и устойчивости плазмы на термоядерных установках с магнитным удержанием незаменимую роль играет метод измерения локальных значений температуры и концентрации электронов по томсоновскому рассеянию лазерного излучения. В данной работе представлены результаты испытания прототипа цифрового фильтрового полихроматора, предназначенного для измерения интенсивности рассеянного излучения и его спектра. Отличительной особенностью данного спектрометра является использование АЦП с аналоговой памятью, работающего при частоте дискретизации 5 ГГц при разрядности 12 бит. Низкое энерговыделение и компактное исполнение позволяют интегрировать систему регистрации и обработки сигналов в корпус спектрального прибора, получив таким образом полностью автономное устройство, гальванически развязанное от других систем и оборудования. Собранное из плазмы рассеянное лазерное излучение по оптоволоконной линии поступает на вход полихроматора, а обработанные сигналы рассеяния и рассчитанные значения температуры и концентрации электронов доступны по цифровому интерфейсу. Испытания прототипа были проведены в составе системы томсоновского рассеяния (ТР) на токамаке Глобус-М с применением двух типов лазеров Nd:YAG 1064 нм и Nd:Glass 1055 нм с энергией в импульсе до 2 Дж и длительностью импульса 4 и 40 нс соответственно. Применение сверхбыстрой оцифровки позволило провести регистрацию сигналов рассеяния в осциллографическом режиме, продемонстрировав возможность временной отстройки от паразитно-рассеянного излучения даже при длительности лазерного импульса 40 нс несмотря на относительно малый размер вакуумной камеры ~1 м. В работе представлены результаты измерения в плазменном эксперименте на токамаке Глобус-М и приведён сравнительный анализ с данными действующей системы ТР.

Formation of Films of Tungsten and its Oxides in a High-Frequency Capacitive Discharge in a D 2 -O 2 Mixture

A technique for tungsten-film deposition on different substrates in asymmetrical high-frequency (1.76 MHz) capacitive discharge in a D2−6.5 mol % O2 mixture under a total pressure of 15 Pa and at 60–130°C is considered. A circular W strip near the upper inner edge of a cylindrical hollow cathode with a radius of 4.2 cm and a height of 10 cm is the source of W particles. The smooth transition from sputtering of the inner surface to deposition occurs at a distance of about 4 cm from the upper boundary of the open part of the cathode. W, Mo, ZrO2, Si, and Cu substrates are placed in the lower closed end (bottom) and on the inner lateral cathode surface. At the upper cathode edge the sputtering yield is (4–5) × 10−2 at/ion. The mass rate of W deposition on the cathode bottom does not depend on the substrate type and is 40 μg/(cm2 h). The peculiarities of the composition, morphology, and structure of W films obtained on the lateral surface and bottom of the hollow cathode are discussed.

Thomson scattering for core plasma on DEMO

This paper describes the challenges of Thomson scattering implementation for core plasma on DEMO and evaluates the capability to measure extremely high electron temperature range 0.5-40keV. A number of solutions to be developed for ITER diagnostics are suggested in consideration of their realization for DEMO. New approaches suggested for DEMO may also be of interest to ITER and currently operating magnetic confinement devices.

LASER DAMAGE INVESTIGATIONS OF OPTICAL ELEMENTS FOR ITER DIVERTOR THOMSON SCATTERING SYSTEM

Abstract The metal mirrors for deflecting laser radiation in the divertor Thomson scattering system of ITER are found to be inapplicable due to high density of laser radiation on a mirror. This is caused by the short distance from the laser focus to the mirror surface. We report on investigations of the possibility to use dielectric mirrors and protective sapphire plates in a rotary unit. We study the laser damage thresholds of optical elements, which are supposed to be used in the laser input channel. These optical elements are the dielectric mirrors deposited on different substrates (quartz, sapphire, and single-crystal molybdenum) and the protecting sapphire plate. A number of sapphire samples were irradiated by a neutron flux up to 1019 n/cm2 (E > 100 keV) and annealed. Laser damage thresholds of the elements were measured at room temperature and at the ITER operating temperature of 150°C. A YAG:Nd laser operating with 10-Hz repetition rate was used in the experiments. The laser pulse parameters were 1064-nm wavelength, 16-ns duration, and 250 to 300 mJ of energy. The experiments have not identified any dependence of optical element damage threshold versus the number of laser pulses. No damage was observed after 105 laser pulses with energy density just 10% below the damage threshold level. The applicability of these optical elements in the divertor Thomson scattering system of ITER had been proven.