Local edge parameters on the ASDEX Upgrade tokamak are investigated at the L-mode to H-mode transition, during phases with various types of edge-localized modes (ELMs), and at the density limit. A scaling law for the boundary electron temperature,e,b B 0.8 t I 0.5 p , is found which describes the H-mode threshold for deuterium-puffed discharges with favourable ion ∇B-drift direction. The region of stable operation is bounded by type I ELMs near the ideal ballooning limit and by a minimum temperature necessary to avoid thermal instability of the plasma edge. Stationary operation with type III ELMs imposes an upper limit on the edge temperature. Within the entire range of boundary densities investigated (n e,b 8 × 10 19 m −3 ), both L-mode and H-mode are found to be accessible. During type I ELMy H-mode, a relation of global confinement with the edge pressure gradient is found which is connected with a loss of the favourable density dependence predicted by the ITER-92P and ITER-93H ELMy H-mode scalings. At high density, better confinement is achieved in H-modes with an edge pressure gradient below the ideal ballooning limit, e.g. during type III ELMy H-mode with impurity-seeded radiation.
An investigation of subnanosecond switching of 119-microm radiation achieved by irradiating high-resistivity silicon wafers with 1.7-ns, 337-nm pulses from a nitrogen laser is presented. The experimental results are compared with a one-dimensional numerical multilayer model, which accounts for the generation, recombination, and diffusion of the free carriers and the resulting change of the far-infrared optical properties of the Si wafer.
By combining the time-of-flight or LIDAR principle with a Thomson backscatter diagnostic, spatial profiles of the electron temperature and density are measured in a magnetically confined fusion plasma. This technique was realized for the first time on the JET tokamak. A ruby laser (3-J pulse energy, 300-ps pulse duration, 0.5-Hz repetition rate) together with a 700-MHz bandwidth detection and registration system yields a spatial resolution of about 12 cm. A spectrometer with six channels in the wavelength range 400–800 nm gives a dynamic range of the temperature measurements of 0.3–20 keV. The stray light problem in the backscatter geometry is overcome by spectral discrimination and gating of the photomultipliers. A ruby filter in the spectral channel containing the laser wavelength allows calibration of the vignetting along the line of sight by means of Raman scattering, enabling the measurement of density profiles. The low level of background signal due to the short integration time for a single spatial point yields low statistical errors (ΔTe /Te ≊6%, Δne /ne ≊4% at Te =6 keV, ne =3×1019 m−3 ). Goodness-of-fit tests indicate that the systematic errors are within the same limits. The system is described and examples of measurements are given.
The Thomson scattering system of the ASDEX upgrade (AUG) tokamak is described. One of the main objectives of AUG is to investigate plasma wall interaction in reactor relevant discharges with a magnetic divertor. The very successful Nd:YAG scattering system developed for its predecessor ASDEX, has been upgraded to give higher spatial and temporal resolution, reliability, and flexibility to different discharge conditions. The system consists of two independently operating devices, each using a cluster of six lasers: One measures the electron temperature and density along three possible vertical chords alternatively through the magnetic axis, or the inner or outer boundary layer; a second chord in the equatorial plane will always cover the magnetic center even in the case of considerable Shafranov shifts. An additional compact spectrometer has been designed for measurements with high radial resolution in the equatorial plane across the separatrix. A third system, using the laser beams for the vertical arrangement once again, has been designed for profile measurements in the energy deposition zone 2 cm above the outer divertor plate. Each laser is run at a repetition rate of 20 Hz and 1 J per pulse. A variety of synchronization modes are available, e.g., 20 Hz/6 J, 120 Hz/1 J etc., or repetitive bursts at 20 Hz. In this case the minimum delay between two pulses is presently limited to ≊30 μs by the existing data acquisition. This mode will be used for investigating fast phenomena such as sawteeth or disruptive instabilities. During the time intervals between the laser pulses the bremsstrahlung radiation (line integral) will be measured by the Thomson scattering detection system to calculate Zeff.
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