A two color millimeter-wave interferometer for the measurement of line integral electron density on large helical device

In the National Institute for Fusion Science, the large helical device (LHD) (R=3.9 m, ap=0.65 m, B=3–4 T) was constructed in Toki City on January, 1998. The LHD is a torsatron/heliotron type device with a superconducting helical winding of pitch parameters of l/m=2/10 and three sets of superconducting poloidal coils. A two color millimeter-wave interferometer has been developed for the measurement of line integral electron density on the horizontal midplane of the LHD. To overcome the effects of the change in the transmission path length (∼100 m) during long pulse plasmas (10–1800 s), we have applied two color interferometry at 140 and 285 GHz.