Measurement of the density and velocity distribution of sputtered Al in EBT-S by laser-induced fluorescence

The density and the velocity distribution of neutral Al released from the vacuum wall in the EBT-S fusion device have been measured using laser-induced fluorescence. Al I was excited at the 308 nm resonance transition using a frequency-doubled dye laser. Absolute Al-densities (~ 10 8 /cm 3 ) were determined at distances between 5 cm and 14 cm from the wall and for various plasma conditions, by comparison of the intensity of induced fluorescence with that obtained from Rayleigh scattering. Al-velocity distributions perpendicular to the wall were determined by scanning the laser through the Doppler-broadened absorption line. The influence of geometry and ionization on the shape of the distributions is given special attention to. From these distributions mean velocities were derived ( ~ 2.6 × 10 5 cm/s ), and along with the measured densities Al-fluxes were calculated. The results indicate sputtering, the possible mechanisms of which are discussed.