SINGLE-BEAM LASER-INDUCED FLUORESCENCE TECHNIQUE FOR PLASMA TRANSPORTMEASUREMENTS

A technique for measuring ion transport using laser-induced fluorescen ce has been developed and tested in an argon plasma. It uses only one broadband beam thus being simpler than some previous techniques becaus e no detection beam is required. First, a 5 mu s laser pulse centered on 611 nm stimulates a transition from the metastable state in Ar(II) 3d (2)G(9/2) to 4p F-2(7/2)0. A 4p F-2(7/2)0 to 4s(2)D(5/2)transition rapidly results with emission at 461 nm. Upon cessation of the laser p ulse, the 461 nm light in the detection volume does not return to its background level immediately because the 3d (2)G(9/2) level is partial ly depleted. The time history of the 461 nm signal in returning to ste ady-state background intensity provides a means of determining ion tra nsport because the recovery signal is due to processes including ion e xcitation, diffusion, convection, and thermal motion. Measurements of the ion velocity distribution yield the contributions of thermal and c onvective effects to ion transport. By varying the laser beam diameter and the detection volume the plasma ion spatial diffusion coefficient D, and the time, tau(p) it takes for processes other than transport t o bring the 461 nm emission back to the steady-state background level are determined. For example, in one set of plasma conditions D=0.58+/- 0.16 m(2)/s and tau(p)=59+/-7 mu s were found. (C) 1996 American Insti tute of Physics.