A.V. Gorshkov

Frequency laser damage of Mo mirrors

Abstract Laser tests were performed on first mirror (FM) prototypes manufactured from single crystal and polycrystal Mo and with reflective Mo coatings on Mo substrate. Data on laser damage thresholds under the influence of the frequency-operated pulsed YAG laser (1064 nm wavelength, 12.5 Hz repetition rate, 10–30 mJ per pulse energy, 12 ns pulse duration) were obtained for single laser shot and after about 2.2×10 5 laser shots. The output beam of the laser operating in the TEM oo mode has a Gaussian profile. The single shot damage thresholds were measured and were equal to 2±0.4 J/cm 2 for polycrystal Mo and Mo/Mo mirrors, 3±0.5 J/cm 2 for single crystal Mo mirror. The long-time effect of frequency-operated YAG laser on mirror reflectivity was studied. It was shown that diffusion scattering is a very sensitive instrument to check the surface condition. Different types of Mo mirrors show various values of multipulse laser damage threshold. It may be connected as with initial quality of the surface and with different structures of the reflection layer for investigated samples of Mo. The degradation of a single crystal Mo mirror under multiple pulse laser irradiation is described with good accuracy by a predictive model for multipulse laser damage of metal mirrors [Appl. Optics 30–36 (1991) 5239] up to 1.5×10 5 laser pulses.

MHD activity and formation of the electron internal transport barrier in the T-10 tokamak

The plasma stability and confinement have been investigated through control of the safety factor profile q(r) by the electron cyclotron current drive in the T-10 tokamak. The regimes with dq/dr0 and dq/dr<0 in the plasma core were obtained. Various types of MHD activity were observed: ordinary sawtooth, saturated sawtooth, humpbacks, hills etc. It was shown that when the minimal value qmin increases from qmin <1 to qmin = 2 the plasma becomes strongly unstable due to the corresponding MHD activity or passes to the steady-state improved confinement mode. The latter is realized when the electron internal transport barrier (EITB) is formed. The condition for the appearance of the EITB is dq/dr0, where q = m/n lies near a rational value for low m and n.

Second harmonic electron cyclotron current drive experiments on T-10

Results of the electron cyclotron current drive experiment at the second harmonic resonance on the T-10 tokamak are presented. High frequency (HF) power up to 1.2 MW was launched from the low field side. A maximum driven current of 35 kA and current drive efficiency ηCD = 0.05 A/W at an electron temperature Tc(O) = 4 keV and a density nc(0) = 1 × 1013 cm-3 were obtained. For low HF power, the current drive efficiency was less than predicted by the linear theory unless the effect of the elliptical polarization from non-perpendicular injection is considered, in which case the efficiency is close to the theoretical value. The experimental dependence of HF on the absorbed HF power indicated a strong increase of ηCD with power. Suppression of sawtooth oscillations and improvement of confinement during electron cyclotron heating has also been demonstrated

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.

Formation of electron transport barriers under ECR control of the q(r) profile in the T-10 tokamak

Abstract-the formation of transport barriers under electron cyclotron resonance heating and current drive in the t-10 tokamak is studied. in regimes with off-axis co-eccd and qL<4 at the limiter, a spontaneous transition to improved confinement accompanied by the formation of two electron transport barriers is observed. the improvement resembles an L-H transition. It manifests itself as density growth, a decrease in the Dα emission intensity, and an increase in the central electron and ion temperatures. Two deep wells on the potential profile (the first one at r/aL≈0.6, where aL is the limiter radius, and the second one near the edge) arise during the transition. the internal barrier is formed when dq/dr∼0 with q≈1 in the barrier region.

Formation of an internal transport barrier and magnetohydrodynamic activity in experiments with the controlled density of rational magnetic surfaces in the T-10 Tokamak

Results are presented from experiments on the formation of an internal electron transport barrier near the q = 1.5 rational surface in the T-10 tokamak. The experiments were carried out in the regime with off-axis electron cyclotron resonance (ECR) heating followed by a fast plasma current ramp-up. After suppressing sawtooth oscillations by off-axis ECR heating, an internal transport barrier began to form near the q = 1.5 rational surface. In the phase of the current ramp-up, the quality of the transport barrier improved; as a result, the plasma energy confinement time increased 2–2.5 times. The intentionally produced flattening of the profile of the safety factor q(r) insignificantly affected magnetohydrodynamic activity in the plasma column in spite of the theoretical possibility of formation of substantial m/n = 3/2 and 2/1 magnetic islands. Conditions are discussed under which the flattening of the profile of the safety factor q near low-order rational surfaces leads to the formation of either an internal transport barrier or the development of an island magnetic structure induced by tearing modes.

Tangential System of Thomson Scattering for Tokamak T-15

Two systems of Thomson scattering diagnostics, with vertical and tangential probing, are used in the D-shaped plasma cross section in tokamak T-15. The tangential system allows measuring plasma temperature and density profiles along the major radius of the tokamak. This paper presents the tangential system project. The system is based on a Nd:YAG laser with wavelength of 1064 nm, pulse energy of 3 J, pulse duration of 10 ns, and repetition rate of 100 Hz. The chosen geometry allows collecting light from ten uniformly spaced points. Optimization of the registration system has been accomplished. The collected light will be transmitted through an optical fiber bundle with diameter of 3 mm and quartz fibers (numerical aperture is 0.22). Six-channel polychromators based on high-contrast interference filters have been chosen as spectral equipment. The radiation will be registered by avalanche photodiodes. The technique of electron temperature and density measurement is described, and estimation of its accuracy is carried out. The proposed system allows measuring the electron temperature with accuracy not worse than 10% within the range of 50 eV to 10 keV on the pinch edge over the internal contour, from 20 eV to 9 keV in the plasma central region, and from 2 eV to 400 eV on the pinch edge over the outer contour. The estimation is made for electron density of not less than 2.6 × 1013 cm–3.

TANGENTIAL THOMSON SCATTERING SYSTEM FOR TOKAMAK T-15

Для D-образной формы плазмы токамака Т-15 будут использоваться две системы томсоновского рассеяния как с вертикальным, так и тангенциальным зондированием. Система с тангенциальным зондированием позволит измерять профили температуры и плотности плазмы вдоль большого радиуса токамака. В данной статье представлен проект тангенциальной системы. В основе системы будет использоваться лазер Nd:YAG с длиной волны 1064 нм, энергией импульса 3 Дж при длительности 10 нс и частотой следования импульсов 100 Гц. Выбранная геометрия позволит собирать свет с 10 равномерно расположенных пространственных «точек». Проведена оптимизация системы регистрации рассеянного света. Передача собранного света будет осуществляться с помощью кварцевых оптоволоконных жгутов (числовая апертура 0,22) диаметром 3 мм. В качестве спектральных приборов выбраны 6-канальные полихроматоры на основе высококонтрастных интерференционных фильтров. Регистрация излучения будет осуществляться лавинными фотодиодами. Описана методика и проведена оценка точности измерения температуры и концентрации электронов. Предлагаемая система позволит проводить измерения температуры электронов с точностью не хуже 10% на периферии шнура на внутреннем обводе тора в диапазоне от 50 эВ до 10 кэВ, в центральной области плазмы — в диапазоне от 20 эВ до 9 кэВ и на периферии шнура на внешнем обводе тора — в диапазоне от 2 до 400 эВ при плотности электронов не менее 2,6∙10 см.

ECCD Experiments on T‐10

Complete non‐inductive current drive from a combination of ECCD and bootstrap has been achieved in a regime with Ip∼75 kA. At higher currents, Ip∼175 kA, up to 110±15 kA was generated by ECCD.