It was shown by Sibener et. al. (Ref. 65) that velocity slippage occurs when the collision frequency is too low during the nozzle expansion to yield complete momentum and energy equilibrium between the seed and carrier species. Slippage is especially typical at low pressures, high temperature expansions due to the relative low number density these systems have in the vicinity of the nozzle orifice. Sibener found that when slippage occurs it can usually be minimized by increasing the pressure behind the nozzle, which increases the number density of collisions which occur during the expansion.
Sibener also showed that seeding Oxygen in Helium resulted in a FWHM in the velocity distribution of Oxygen of about 19% (Mach 9.2), while seeding in Argon resulted in about 40% (Mach 3-4). This can be compared to about 95% for an effusive source (Maxwellian distribution).
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