A new beam emission spectroscopy turbulence imaging system has recently been installed onto the MAST spherical tokamak. The system utilises a high-throughput, direct coupled imaging optics, and a single large interference filter for collection of the Doppler shifted Dα emission from the ∼2 MW heating beam of ∼70 keV injection energy. The collected light is imaged onto a 2D array detector with 8 × 4 avalanche photodiode sensors which is incorporated into a custom camera unit to perform simultaneous 14-bit digitization at 2 MHz of all 32 channels. The array is imaged at the beam to achieve a spatial resolution of ∼2 cm in the radial (horizontal) and poloidal (vertical) directions, which is sufficient for detection of the ion-scale plasma turbulence. At the typical photon fluxes of ∼1011 s−1 the achieved signal-to-noise ratio of ∼300 at the 0.5 MHz analogue bandwidth is sufficient for detection of relative density fluctuations at the level of a few 0.1%. The system is to be utilised for the study of the characteristics of the broadband, ion-scale turbulence, in particular its interaction with flow shear, as well as coherent fluctuations due to various types of MHD activity.
Beam emission spectroscopy turbulence imaging system for the MAST spherical tokamak
A. R. Field, D. Dunai, R. Gaffka, Y.-c. Ghim, I. Kiss, B. Mészáros, T. Krizsanóczi, S. Shibaev, S. Zoletnik; Beam emission spectroscopy turbulence imaging system for the MAST spherical tokamak. Rev. Sci. Instrum. 1 January 2012; 83 (1): 013508. https://doi.org/10.1063/1.3669756
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