Abstract
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, kθ<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.
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