An infrared vibration compensated interferometer for the MST experiment

The paper describes a two-color CO2/HeNe laser (10.6/3.4 ?m) interferometer which has been designed and built at the IGI laboratory in Italy and installed on the MST Reversed Field Pinch (R=2 m, a=0.52 m, I=0.6 MA) at University of Wisconsin Madison. The system is very compact and is mounted on a 1.2×0.6 m wood board for a total weight of ?60 kg. The infrared vibration compensation system allows us to use a one-window access (the beam is reflected back by a mirror mounted in the vacuum vessel) and to operate without a stiff mechanical structure embracing the machine. Several new features, such as the use of an interferometric local oscillator, the on-line calibration algorithm, and the four-passage scheme, allow a line-integral density accuracy of 1×10^18 m^-2.