Real-time digital heterodyne interferometer for high resolution plasma density measurements at ISTTOK

With the implementation of alternating discharges (ac) at the ISTTOK tokamak, the typical duration of the discharges increased from 35to250ms. This time increase created the need for a real-time electron density measurement in order to control the plasma fueling. The diagnostic chosen for the real-time calculation was the microwave interferometer. The ISTTOK microwave interferometer is a heterodyne system with quadrature detection and a probing frequency of 100GHz (λ0=3mm). In this paper, a low-cost approach for real-time diagnostic using a digital signal programable intelligent computer embedded system is presented, which allows the measurement of the phase with a 1% fringe accuracy in less than 6μs. The system increases its accuracy by digitally correcting the offsets of the input signals and making use of a judicious lookup table optimized to improve the nonlinear behavior of the transfer curve. The electron density is determined at a rate of 82kHz (limited by the analog to digital converter), and the ...