Design & development of 140 GHz D-band phase locked heterodyne interferometer system for real-time density measurement

Abstract

A 140 GHz phase-locked heterodyne interferometer system has been designed, developed and installed at Aditya Tokamak for real time density measurement. The interferometer uses a novel approach of zero-cross detection using in-phase and quadrature (IQ) signals with a single balanced mixer. The transmitter and receiver systems are phase locked by a 100 MHz crystal oscillator and the output signals are mixed in a single balanced mixer which produces highly stabilized intermediate frequency (IF) frequency of 2.0 GHz. This signal is further amplified by low noise amplifier (LNA) and power amplifier with built-in automatic gain control (AGC) of response time 5 ms. The AGC keeps the IF signal amplitude stable for 20–70 dB attenuation. This IF signal is further down converted to 100 KHz IQ signals using a synthesizer which is phase locked by the same 100 MHz crystal oscillator used for phase locking the transmitter and receiver oscillators. The 100 KHz IQ signals are digitized by built-in 12 bits ADCs and processed though FPGA programming for real time phase-density calculations. The developed system has minimum phase sensitivity of 0.07 radian and a time resolution of 5 μs with 10 KHz resolution for real time density measurement. The density evolution of plasma discharge and effect of gas puff on density built up is successfully measured in real time and presented in this paper.