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The Large Helical Device (LHD) experiments has successfully started after the eight-years of construction period (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997). The major and minor radi of the plasma are 3.6 -3.9 m and 0.6 -0.65 m, respectively. The magnetic field up to 3 T is generated by a pair of superconducting helical winding of pitch parameters of m/l = 2/10 and three sets of superconducting poloidal coils. The plasma is generated by ECRH of 84 and 82.6 GHz, and heated up by NBI and ICRF. The plasma parameters obtained to date are: the electronjion temperature of 1.0 -10/5.0 keV, the he-averaged electron density of i 1.5 x lo2" m-' and the maximum stored energy of 1.2 MJ.For the electron density measurements of the LHD plasma, four kinds of diagnostics have been used: a multi-channel FIR laser interferometer, a two color mm wave interferometer, a CO2 laser imaging interferometer and a 3-channel tangential CO2 laser polarimeter. A 13-channel far infrared laser interferometer has been routinely operated for the precise measurements of the electron density profile. The optical configuration is of the Michelson interferometer type with a heterodyne detection system. The light source 0 a hlghly stable twin 118.8-km CH3OH laser pumped by a cw CO2 laser. The spatial and time resolutions are 90 mm and 1 p, respectively. The overall accuracy of the system is about 1/100 of a fringe, corresponding to a minimum measurable line-averaged density of 5.6 x 1OI6 at the central chord. This interferometer routinely provides density profile almost every shot except i.n the case of high-density plasma by ice pellet injection. When the large sized pellet is injected into the plasma steep density gradient is formed in the peripheral region, which sometimes causes the fringe jumps at the corresponding channels. In order to solve this problem, a CO2 laser imaging interferometer has been developed. The imaging system is employed by using three slablike beams and multi-channel detector arrays to measure fine structure of the density profile.In this conference, details of the each density diagnostics will he presented.A tangential far infrared interferometerfpolarimeter system developed for the measurement of 2-D electron density profile and toroidal magnetic field profile on the National Spherical Torus Experiment (NSTX). The system is operating with the three-wavelength configuration that simultaneously produces interferometric phase shift and Faraday rotation from polarimetry from one detector for each channel.Three CHIOH lasers (I =119 mm) ncluding Stark tuned laser cavities are optically pumped by a CO2 laser tuned at 9.7 mm. For a high time resolution measurement, modulation frequency in the heterodyne interferometer is operated at -7 MHz via the Stark-tuned laser as the local oscillator in this system. Unwanted phase changes introduced by mechanical vibrations originated from various sources were salved by extensive works including a specially designed vibration free stand for retro-reflector. The system is designed to have ...

The Influence of paramagnetism and diamagnetism will significantly alter the vacuum toroidal magnetic field in spherical torus. Therefore, plasma parameters dependent upon B T such as the q-profile and local b value needs an independent measurement of B T (r,t). The multi-chord Tangential Far Infrared Interferometer/ Polarimeter (FIReTIP) system [1] currently under development for National Spherical Torus Experiment (NSTX) will provide temporally and radially resolved toroidal field profile [B T (r,t)] and 2-D electron density profile [n e (r,t)] data. Two channel interferometer will be operational this year and full system will be ready by 2002.

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The Influence of paramagnetism and diamagnetism will significantly alter the vacuum toroidal magnetic field in spherical torus. Therefore, plasma parameters dependent upon B T such as the q-profile and local b value needs an independent measurement of B T (r,t). The multi-chord Tangential Far Infrared Interferometer/ Polarimeter (FIReTIP) system [1] currently under development for National Spherical Torus Experiment (NSTX) will provide temporally and radially resolved toroidal field profile [B T (r,t)] and 2-D electron density profile [n e (r,t)] data. Two channel interferometer will be operational this year and full system will be ready by 2002.