Gasdynamic focusing in an underexpanded jet.

Gasdynamic focusing is demonstrated to concentrate a sample stream along the flow axis in an underexpanded supersonic jet. The focusing is characterized, using the technique of laser-induced fluorescence, for an I(2) seeded He expansion at pressure ratios between 2 and 6. The spatial fluorescence profile is monitored via array detection as a function of the sample and sheath volume flow rates and pressures. For source pressures between 10 and 100 Torr, the results obtained can be qualitatively understood using the model of hydrodynamic focusing. The observed deviations from this model are discussed in terms of gaseous diffusion and jet expansion.