Real-time density computer for two-color interferometers

An electronic circuit has been developed which performs real-time density computation on the quadrature phase signals of the two-color CO2 interferometer for the RFX experiment. Cartesian to polar coordinate conversion is obtained by a look-up table, and a ''phase fine tuning'' table allows us to introduce corrections to the systematic phase nonlinearity errors of the interferometer, by which the overall accuracy of the diagnostic is made better than 1°, equivalent to a line density of 6×10^17 m^-2 for a CO_2 system. The phase total range is ±512 fringes and the maximum allowed rate of change is 125×10^3 m^-2 s^-1 equivalent to a density rate of change of 2×10^25 m^-2 s^-1. A real-time signal proportional to the plasma density to be used for density feedback is provided by a 50 kHz, 12 bit converter. The system is a triple width CAMAC module and the two-color wavelength ratio is remotely programmable as a 20 bit constant, hence it is suitable in general for any two-color interferometer. The density data are also stored with a sampling rate of 10-250 kHz in a 128 K×24 bit memory buffer, which is interfaced via a CAMAC bus to the data acquisition system.