FE-I DENSITY-MEASUREMENTS BY LASER-INDUCED FLUORESCENCE SPECTROSCOPY

The density of Fe atoms produced by bombardment of a stainless steel target by Ar+ ions was measured by laser induced fluorescence (LIF) using a tunable dye laser. The excitation transition involved was a 5D 4→y 5D40 (λ=302.064 nm); measurements of the fluorescence signal, from the y 5D 40→a 5F5 decay (λ=382.043 nm), were made by varying the laser power incident on Fe atoms by means of grey filters. An absolute calibration of the detection system allowed the determination of the number of the emitting atoms, versus the incident power. So the experimental curve of the fraction of total pumped atoms versus the saturation parameter was obtained. On the other hand, that fraction can be calculated by using standard rate equations for LIF process involving three level atoms. The result is strongly dependent on the model for the pumping laser pulse, particularly (as expected) at low incident power level. Assuming a realistic Gaussian temporal dependence for the radiation field energy flux at frequency v, i.e., U(vt) ∼ Uv0te-t2 and calculating the fraction of atoms pumped by the incident pulse, an agreement better than assuming a square time dependence for U(v,t) was found. The measurement sensitivity limit is 109 atoms/cm3 when LIF diagnostic on FTU is considered including plasma emission background. © 1995 American Institute of Physics.