Rubidium Vapor Filter Spectrally Filtered Thomson Scattering

Abstract

Improvements have been made to a spectrally filtered Thomson scattering apparatus which consists of an external cavity diode laser injection-locked titanium:sapphire laser and an optically thick rubidium vapor filter at 780 nm. In a detailed set of measurements the spectral punty, P, of the laser, defined as the ratio of the narrow band component of the laser output to the total output, has been studied as a function of circulating seed power. Incorporation of a feed back loop to “lock” the titaniwn:sapphire laser cavity to the cw seed frequency and addition of a Stimulated Brillouin Scattering Phase Conjugate mirror results in a measured spectral punty in excess 0.99999. These improvements have enabled Thomson scattering measurements to be performed with the vapor filter temperature reduced from 320 “C to 270 “C, corresponding to a decreased filter linewidth by a factor of approximately three. The utility of the improved system for measurement of electron density and temperature by Thomson scattering was demonstrated in an argon dc constricted glow discharge. At 100 mamps current and 30 torr of argon an electron density of 6.75 x 1013 cm-3 was measured on the discharge center line with a 20 value of statistical uncertainty equal to 0.4 x lo’* cms3. The corresponding electron temperature was measured to be 0.27 _+ 0.036 eV .