Twenty-channel grating polychromator diagnostic system for electron cyclotron emission measurement in JT-60

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Abstract
A twenty‐channel grating polychromator diagnostic system has been built to measure the temporal evolution of local electron temperatures in JT‐60. A cross Czerny–Turner diffraction grating spectrometer is utilized for the measurement of second‐harmonic electron cyclotron emission with extraordinary modes in the range 85–300 GHz, in which a grating plate grooved on both faces with different grating periods is applied effectively to yield a wide coverage for the toroidal fields. The grating angle is automatically set up by control of a stepping motor according to the relation of the grating equation. The diffracted light is detected by 20 indium‐antimonide hot‐electron bolometers cooled at 4.3 K in a modified Solvay cycle cryogenic refrigerator. A typical resolving power of the instrument was measured to be λ/Δλ∼130, providing a spatial resolution of 2.3 cm at the plasma center. The transmission line over ∼38 m long is composed of oversized S‐band waveguides. The total transmissivity of this system is estimated to be ∼0.01.