A nine‐channel interferometer/polarimeter system is designed for the RTP tokamak (Rijnhuizen Tokamak Project, a =0.16 m, R =0.72 m, BT =2.5 T, Ip ≤ 200 kA, and plasma pulse duration 200 ms). A CO2‐pumped dual‐cavity FIR laser system is used to obtain two FIR laser beams of λ =432 μm with a frequency difference of 1 MHz. The FIR beams are expanded by a set of parabolic mirrors. Corner‐cube mixers with Schottky diodes are used as detectors. The mixers are mounted on a linear rail system, such that their positions can be easily changed. The interferometer can be operated with a maximum of 19 channels, and will be extended with a polarimeter to measure the poloidal magnetic field distribution. It will be tried to increase the accuracy of the polarimeter by modulating the polarization of the incoming beam. A scheme for polarization modulation which is based on the idea of Dodel and Kunz will be presented in this paper. This scheme has the advantage that only one set of detectors is needed to measure the electron density and current density. The complete system will be presented along with some pilot experiments concerning the polarization modulation.

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© 1990 American Institute of Physics.
1990
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