Twenty-channel grating polychromator for millimeter wave plasma emission measurements

A 20-channel grating instrument based on the Ebert--Fastie grating monochromator has been built to measure electron temperature profiles and electron temperature fluctuations in TFTR (tokamak fusion test reactor). Second-harmonic electron cyclotron emission for the plasma over the frequency range of 120--400 GHz is utilized; this corresponds to a central tokamak magnetic field of 2.9--5 T. System sensitivity is typically 10-eV rms for a 2.5-kHz bandpass with a 2.5-cm radial resolution and a 5-cm antenna spot size at the plasma center. The ratio of the largest to smallest plasma radius observed by the instrument can be as large as 1.65, which maximizes spatial coverage of the plasma. Emission is detected by 20 liquid-helium-cooled indium antimonide hot-electron bolometers mounted in a long hold cryostat. The 6.5-l helium reservoir must be filled every 19 days, while the liquid-nitrogen reservoir is filled automatically once a day. This instrument has about twice the number of channels and much better spatial resolution than other instruments of this type used for plasma diagnostics