EXPERIMENTAL-OBSERVATION OF VERY LOW-FREQUENCY MACROSCOPIC MODES IN ADUSTY PLASMA

Images of a cloud of grains in a dusty plasma reveal a pair of very lo w-frequency modes, termed here the filamentary and great void modes. T he plasma was a radio-frequency discharge formed between parallel-plat e graphite electrodes. A cloud of 100 nm carbon particles was produced by accretion of carbon atoms produced by sputtering the graphite. The cloud was illuminated with a laser sheet and imaged with a video came ra. The great void mode was a spoke-shaped region of the cloud that wa s free of dust and rotated azimuthally in the discharge. The filamenta ry mode had the appearance of turbulent striations, with a smaller amp litude than the great void. The filamentary mode sometimes appeared as a distinctive vortex, curling in the poloidal direction. Both modes h ad a very low frequency, on the order of 10 Hz. Two possible causes of the modes are discussed. The low phase velocity of the modes may be c onsistent with a dust-acoustic wave. Alternatively, the great void may be an ionization wave that moved the dust about, since a modulation i n the glow was seen moving at the same speed as the void. II is argued that existing theories of waves in dusty plasmas assume weakly collis ional plasmas, which may be unsuitable for explaining experimental res ults in laboratory dusty plasmas, since they are often strongly couple d. (C) 1996 American Institute of Physics.