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, a(p) = 0.65 m, B = 3-4T) was constructed in Toki City on Janu ary, 1998. The LHD is a torsatron/ heliotron type device with a superconduc ting helical winding of pitch parameters of l/m = 2/10 and three sets of su perconducting poloidal coils. A two color millimeter-wave interferometer ha s been developed for the measurement of line integral electron density on t he horizontal midplane of the LHD. To overcome the effects of the change in the transmission path length (similar to 100 m) during long pulse plasmas (10-1800 s), we have applied two color interferometry at 140 and 285 GHz. ( C) 1999 American Institute of Physics. [S0034-6748(99)57901-X].