Optical measurements of core density fluctuations in TEXT-U using a helium metastable beam

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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−1, kθ−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.