Abstrakt
Dust grains play a key role in dusty plasma since they interact with the plasma we can use them to study plasma itself. The grains are illuminated by visible light (e.g., a laser sheet) and the situation is captured with camera.
Despite of simplicity, light scattering on similar-to-wavelength sized grains is complex phenomenon. Interaction of the electromagnetic wave with material has to be computed with respect to Maxwell equations - analytic solution is nowadays available only for several selected shapes like sphere, coated sphere, or infinite cylinder.
Moreover, material constants needed for computations are usually unknown. For computation result verification and material constant determination, we designed and developed a device directly measuring light scattering profiles.
Single dust grains are trapped in the ultrasonic field (so called "acoustic levitation") and illuminated by the laser beam. Scattered light is then measured by a photodiode mounted on rotating platform.
Synchronous detection is employed for a noise reduction. This setup brings several benefits against conventional methods: (1) it works in the free air, (2) the measured grain is captured for a long time, and (3) the grain could be of arbitrary shape.