OPTICAL MEASUREMENTS OF CORE DENSITY-FLUCTUATIONS IN TEXT-U USING A HELIUM METASTABLE BEAM

A compact, portable, eight-channel, filter spectrometer has been designed for beam emission spectroscopy measurements of long-wavelength density fluctuations on TEXT-U and Phaedrus-T. The system uses radially elongated image volumes which maximize the étendue of the system while still resolving the dominant fluctuations (kr<2 cm-1, k θ<6 cm-1). Because of the vertical injection geometry of the TEXT-U diagnostic neutral beam, there is little Doppler shift of the beam emissions and thus a nonhydrogenic species must be used to distinguish between the beam and edge emissions. A He0 metastable beam has been found to provide good beam penetration with little contamination of the detected beam fluorescence by edge emission. A multistep excitation model is used to quantitatively interpret the fluctuations in the intensity of the He0 triplet (3P0-3D, 587.6 nm) as plasma density fluctuations. © 1995 American Institute of Physics.