High-resolution edge Thomson scattering measurements on the Alcator C-Mod tokamak

A high-resolution Thomson scattering diagnostic is in operation on the Alcator C-Mod tokamak, measuring radial profiles of electron temperature and density at the plasma edge. Photons are scattered from a Nd–yttrium–aluminum–garnet laser beam pulsed at 30 Hz (1.3 J, 8 ns pulse), and are measured by a filter polychromator with four spectral channels. The polychromator measures $T_e$ in the range of 15–800 eV and $n_e$ of ${\text{0.3–3×10}}^{20}\hspace{0.17em}{\mathrm{m}}^{\mathrm{-3}}.$ Twenty scattering volumes are located about the last closed flux surface, spaced for a nominal resolution of 1.3 mm in midplane radial coordinates. High resolution is essential for measuring edge $T_e$ and $n_e$ profiles on C-Mod, since these quantities exhibit gradient scale lengths as small as 2 mm in H mode. The steep profiles at the H mode edge are fit to a parameterized pedestal function for ease of analysis. Measured profiles are compared with edge profiles from electron cyclotron emission and visible continuum diagnostics.