Plasma measurements using ponderomotive forces to perturb the translational motion of particles

The ponderomotive forces associated with the interference pattern of two crossing laser beams can effectively perturb the translational motion of atoms, molecules and electrons. The ponderomotive force sets up a density perturbation among the dipoles moving synchronously with the interference pattern. Possiblities of using ponderomotive-driven density perturbations for plasma diagnostics are discussed. For example, coherent Rayleigh scattering is a process in which a laser beam is Bragg-scattered off a traveling atom or molecule number density wave created by ponderomotive forces. The strength of the signal is directly related to the number of atoms or molecules whose velocities are closely synchronized with the traveling wave, the thermal velocity distribution. We show that coherent Rayleigh scattering is applicable to the measurement of neutral temperature in weakly ionized plasmas. Experimental results are given for glow discharges. We also discuss the possibility of acoustic detection of energy deposition by optical lattice generated with two crossing laser beams. Again, ponderomotive forces perturb the translational motion of gas particles and energy is transferred from laser beams into the gas. Numerical calculation shows that the power deposition is a function of the laser beams' frequency difference. And such a power deposition curve bears the information of the gas particles' Maxwell-Boltzmann distribution, therefore, gas temperature can be derived from such a measurement.