Phase Shift Measurement of Microwave Interferometer for Plasma Diagnostics

Phase shift measurement with a microwave interferometer has been analysed to estimate the error in the measurement caused by mismatchings and unbalances of microwave elements in the detection system. When the most reasonable adjustments are made to balance the phase sensitive detection system. the error Δθ is estimated to be proportional to the degree of mismatch and unbalance γ, the intensity ratio of a transmitted wave to a reference wave ξ and the ratio of the wave attenuation through the plasma 1-α and is given as Δθ=¼γξ(1-α). This relation has been confirmed with simlation experiments. In an usual case where γ\lesssim0.4 and ξ=0.3, the error is about 0.05 radians, if the transmitted wave is attenuated to one tenth due to the presence of a plasma.