A SUBMILLIMETER POLYCHROMATOR FOR PLASMA SPECTROSCOPY

A 12-channel grating polychromator for the measurement of electron cyc lotron emission on the Frascati tokamak upgrade (FTU) has been built. Three gratings can be exchanged remotely, to allow the instrument to c over three different spectral ranges (between 200 and 600 GHz) which c orrespond to the second harmonic of electron cyclotron emission for to kamak magnetic fields of 4, 6, and 8 T. Three sets of low-pass filters , whose cut-off frequencies are optimized for each grating, are used t o cut the high diffraction orders. In order to exchange the filters to gether with the grating it was necessary to integrate the groups of fi lters with the polychromator itself. This has been done reducing the g rating sizes and assembling them all closely on a moveable structure. The exchange of gratings and filters can be performed remotely via com puter in few seconds without any lost of the alignment. InSb cryogenic hot-electron bolometers are used as detectors. The cryostat hold-time is more than 30 days. Spectral resolution lambda/delta lambda is abou t 40 corresponding to a spatial resolution of delta R approximate to 2 .3 cm. The minimum detectable temperature is about 10 eV at 20 kHz ban dwidth. The spatial structure and temporal evolution of several kinds of fast temperature perturbations (as the ones due to magnetohydrodyna mic activity, temperature fall during pellet injection and plasma disr uption) have been measured on FTU by means of this diagnostic system. (C) 1996 American Institute of Physics.