Measurement and modeling of particle spacing in strongly coupled dusty plasmas

Vertical strings of dust grains were studied using a two-dimensional array of grains in a rf Ar plasma in a two-phase experiment. First, the sheath was characterized by fitting a sheath model to single grain behavior and Langmuir probe measurements. Then, the intergrain spacing was measured over a range of powers for spherical glass grains of diameter 10.4 mu m. A one-dimensional energy model for the arrays which includes the sheath model, gravity, and all intergrain interactions is compared with the data. Various asymmetric potentials were employed to account for possible dipolar effects. It is shown that the bulk plasma temperature greatly exceeds the effective temperature (defined in terms of the charging equation applicable in the bulk plasma) in the sheath and that existing interaction potential models do not agree with the data.