Measurements of electric charge and screening length of microparticles in a plasma sheath

An experiment is described in which microparticles are levitated within a rf sheath above a conducting plate in argon plasma. The microparticles forming a two-dimensional crystal structure are considered to possess Debye screening Coulomb potential phi(r) = (Q/4 pi epsilon(0)r)exp(-r/lambda), where Q is the electric charge, r is distance, and lambda is the screening length. When the crystal structure is slanted with an angle theta, a particle experiences a force Mg sin theta, where M is the mass of the particle and g is acceleration due to gravity, which must be equal to the Debye screened Coulomb force from other particles. By changing theta, relations for lambda(Q) are measured. The screening length lambda and Q are determined uniquely from the crossing points of several relations. The electric charge Q is also estimated from a floating potential measured with a probe. The measured lambda is nearly equal to an ion Debye length. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3112702]