[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: Plasma pressure profiles from various tokamaks are analysed. It is shown that in the gradient zone the pressure profiles are conserved under variation of the plasma density and deposited power. Usually these profiles are close to the canonical ones. Conservation of pressure profiles means that the density and temperature profiles are consistently correlated under different external actions on the plasma. A simple transport model for the plasma density based on the self-consistency of the pressure profiles is proposed.
[Show abstract][Hide abstract] ABSTRACT: It has been observed in the T-10 tokamak that immediately after off-axis electron cyclotron resonance heating (ECRH) switch-off, the core electron temperature stays constant for some time, which can be as long as several tens of milliseconds, i.e. several energy confinement times (τE), before it starts to decrease. Whether or not the effect is observed depends critically on the local magnetic shear in the vicinity of the q = 1 rational surface, which should be close to zero. It is hypothesized that a small shear can induce the formation of an internal transport barrier. Measurements of density fluctuations in the transport barrier with a correlation reflectometer show immediately after the ECRH switch-off a clear reduction in the fluctuation level, corroborating the above results. The delayed temperature decrease has also been observed in similar discharges in the TEXTOR tokamak; however, the delay is restricted to ~ 1 × τE.
[Show abstract][Hide abstract] 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 2.2×105 laser shots. The output beam of the laser operating in the TEMoo mode has a Gaussian profile. The single shot damage thresholds were measured and were equal to 2±0.4 J/cm2 for polycrystal Mo and Mo/Mo mirrors, 3±0.5 J/cm2 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 1.5×105 laser pulses.
No preview · Article · Sep 2003 · Fusion Engineering and Design