Development of a multi-channel mm-wave interferometer for the versatile experiment spherical torus (VEST)

The 94 GHz single-channel interferometer at the versatile experiment spherical torus has been upgraded into a multi-channel heterodyne interferometer for electron density profile measurements. Five Gaussian beam chords generated by a pair of Powell lenses and collimating lenses are vertically spaced at 75 mm intervals from the midplane to the lower part of the main chamber. The beams passing through the plasma are received at a detector array consisting of a half-wave rotator and modules consisting of an integrated lens antenna and mixer. The local oscillator beam is expanded by another pair of Powell lenses and received at the detector array as well. The signal frequencies are down-converted to 10-70 MHz intermediate frequencies, and then their phases are converted into line-integrated electron densities. The optical system was optimized using Gaussian beam approximation and the optics design tool, CODE V. For the commissioning of the complete system, a single-channel setup was first tested and verified by comparing the results with a V-band frequency sweeping interferometer. The data from five channels were then obtained and compared with the time trace obtained by 1 kHz Thomson scattering at the midplane of the plasma.