Thomson-scattering diagnostic on the Frascati tokamak upgrade

Abstract

The Frascati tokamak upgrade Thomson-scattering system is used for the measurement of electron-temperature and electron-density spatial profiles along the vertical diameter of the tokamak at 19 spatial points up to 10 times in a single plasma discharge, with a spatial resolution that ranges from 2 cm in the central region to 4 cm in the plasma edge. The radiation source is a Nd:YLF laser that operates at 1053 nm, with a divergence of 0.4 mrad full angle, and is capable of delivering a burst of 10 pulses with energies of 4.5 J/pulse; the interpulse time can be regulated from 20 to 100 ms. The scattered radiation is collected by two objectives: the first looks at the plasma center, and the second at the plasma edge. Bundles of optical fibers in the focal plane of the objectives carry the scattered light from the tokamak hall to a set of 19 interference-filter polychromators, whose transmission is 70%, and the rejection of the stray light at the laser wavelength is 1/10(7). The detectors are avalanche photodiodes ith a noise-equivalent power of the order of 10(-13) W/(Hz)(½) at 1053 nm. The spectral calibration of the polychromators is presented. The absolute calibration of the scattering system for the electron-density measurement has been carried out by the use of Raman scattering on hydrogen and deuterium. Examples of the results of the temporal evolution of T(e) and n(e) spatial profiles are presented for ohmic plasma heating, lower-hybrid current drive, and a pellet-injection experiment. The electron-temperature and electron-density profiles measured through Thomson scattering are compared with the temperatures measured through the use of electron-cyclotron emission and the density profiles obtained from the interferometer data.

Recent progress in lower hybrid current drive theory and experiments

“…The effective charge Z eff , obtained from the visible bremsstrahlung measurements, stayed almost constant, near unity. The increase in the central electron temperature T e0 , measured by Thomson Scattering [22], was about 0.5 keV. The T e0 decrease, before the end of the LH pulse, was due to a sawtooth crash occurring just before the laser shot.…”

RF and microwave diagnostics for compact plasma traps and possible perspectives for fusion devices

“…Encouraged by first promising numerical results obtained on compact plasma systems, we began an upgrade of the technique to investigate possible application to large fusion reactors. Many different diagnostic techniques are routinely used in fusion machines to measure the density profile such as reflectometry [22] and Thomson scattering [23,24]. In particular, diagnostics based on Thomson scattering are widely used and have proved to be particularly efficient to measure the steep density gradient in the pedestal region of tokamaks.…”