CO2 laser scintillation interferometer for the measurement of density fluctuations in plasma confinement devices

Density fluctuations in magnetically confined plasmas have been measured from the phase scintillations impressed onto a 10.6-μm wavelength laser beam probe, using a Mach--Zender interferometer operating in the optical near field. By careful design, the effects of vibration and air-borne sound waves, which limit the instrument's sensitivity, have been reduced to a level such that the minimum detectable signal is 10-6 rad in the frequency range 10 kHz--1 MHz, corresponding to a plasma density fluctuation level (δN/N) of 10-4