Beam emission spectroscopy on the Alcator C-Mod Tokamak

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Sampsell, Matthew Brian

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A beam-emission spectroscopy (BES) system has been installed on the Alcator C-Mod tokamak for study of turbulence and transport. The system collects light from excited diagnostic neutral hydrogen beam (DNB) particles, the excitation due mostly to collisions with deuterium plasma and impurities (e

/D+ /Z+

  • H0 → e- /D+ /Z+
  • H0
  • → e- /D+ /Z+
  • H0
  • hνHα). Optics relay the light to low-noise photodiodes. Along with beam emission, the optics collect light from the plasma. A spectral model was developed to simulate emission, aide in design of bandpass filters, and optimize filter tuning. Fluctuations in the measured emission are proportional to plasma density (n) fluctuations, allowing calculation of relative amplitude (δn/n), wavenumber (k), frequency (ω), and phase velocities. These quantities are of interest because of their connection to energy and particle transport [1, 2]. The diagnostic is optically capable of localized measurements from the plasma edge to the core. However, beam imprinting and smearing, two beam effects that reduce radial localization, must be included in the analysis. Measurements of the quasi-coherent mode (QCM), associated with enhanced Dα (EDA) high confinement plasmas, indicate amplitudes of δn/n ~27%, a peak located ~ 1-2 mm inside the last closed flux surface (LCFS), and a radial extent of ~ 5 mm. Low-amplitudes have been detected extending outside the LCFS as far as 7 mm. These measurements support the notion that the QCM plays a prominent role in particle transport. Correlation analysis and multidiagnostic studies find kθ’s at the midplane of ~ 1-2 cm -1 and propagation along flux surfaces consistent with the requirement that k•B = 0. All measured characteristics agree with those of a boundary turbulence simulation which suggests the QCM may be driven by resistive ballooning. Studies of low frequency fluctuations in L-Mode discharges show amplitudes of δn/n ~ 15% in the edge, falling below the noise floor of ~1% inside a normalized minor radius of 0.8. When monotonically decreasing amplitudes are assumed, the results put a unique upper bound on core fluctuations. Improving this constraint will be valuable to C-Mod transport studies. Recommendations for upgrades to the system are discussed.