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 35 to 250 ms. 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 100 GHz (lambda(0)=3 mm). In this paper, a low-cost approach for real-time diagnostic using a digital signal programmable intelligent computer embedded system is presented, which allows the measurement of the phase with a 1% fringe accuracy in less than 6 micros. 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 82 kHz (limited by the analog to digital converter), and the data are transmitted for each millisecond although this last parameter could be much lower (around 12 micros--each value calculated is transmitted). In the future, this same system is expected to control plasma actuators, such as the piezoelectric valve of the hydrogen injection system responsible for the plasma fueling.