Measurement of grain charge in dusty plasma Coulomb crystals

We report the measurement of the grain charge on individual dust grains of a dusty plasma Coulomb crystal. These dust grains, grown in a radio frequency plasma from constituent gases, are formed by the aggregation of nanometer sized crystallites. The Coulomb crystal formed from the dust grains appears to be made of mutually repulsive columns of grains confined by the walls of the electrode. A direct measurement of the grain charge was performed using a Faraday cup. The average measured value of the grain charge using this technique was found to be 2/spl times/10/sup 5/ e. This value of the grain charge is approximately consistent with estimates based on the plasma parameters. The corresponding electrostatic interaction energy between the dust grains is found to be 200 times greater than the dust thermal energy.