The phase contrast method as an imaging diagnostic for plasma density fluctuations (invited)

An imaging diagnostic for the observation of plasma density fluctuations is presented. It is based on the phase contrast method, and is used on the TCA tokamak to investigate fluctuations associated with plasma turbulence and driven waves in radio frequency heating experiments. The diagnostic uses a 23-cm-wide CO2 laser beam transmitted through the plasma, and produces an image of the plasma where the small phase shifts (‖φ‖∼10−5 to 10−3) due to refractive perturbations are revealed as corresponding intensity variations. A wide range of fluctuation wavelengths between 0.2 and about 20 cm is accessible to observation, with a sensitivity better than 10−5 rad for a 1-MHz bandwidth.