Improvement of the dual C02 laser interferometer

A new phase comparator named VRPC (Very high Resolution Phase Comparator) has been developed to improve the phase and density resolutions of a dual CO2 laser interferometer in JT-60U tokamak. The designed time discrimination of the VRPC (39.0625 ps) can provide a phase resolution of 1/12800 of a fringe for a 2 MHz interference beat signal, which corresponds to an effective density resolution of 1.32 × 1017 m−2. The first operation of the VRPC for plasma discharges has been executed. The phase resolution is improved significantly in comparison with that of a previous comparator which has a resolution of 1/100 of a fringe. The density resolution is also well improved to extract noise components which were comparable to a bit noise of the previous comparator. The VRPC has the capability to enable the dual C02 interferometer to measure precise density changes in tokamak fusion plasmas.