A 19‐channel fast grating polychromator for ECE measurements in the MTX tokamak

A 19‐channel grating polychromator is developed to measure the temporal evolution of the electron cyclotron emission spectrum in the second and third harmonics in MTX tokamak with an aim to determine the electron temperature in ohmically heated thermal plasmas as well as the electron energy distribution function in gyrotron and free‐electron laser‐pumped nonthermal plasmas. The spectrometer can cover a spectral range of about 0.3 to 1.4 mm and deliver a maximum spatial resolution of about 1 cm radially. A combination of a 90° off‐axis paraboloidal mirror as the collimating element and a toroidal mirror to focus the diffracted beam yields images on the exit slits with a modest astigmatic distortion. Liquid‐helium‐cooled InSb detectors are installed in nine channels and associated electronics are optimized for a very low noise and fast response time ( ∼ 2 μs). A faster detector element, with response < 100 ns, is envisioned for the remaining ten channels.