Quasisteady gas injection into vacuum has been carried out by using a fasthyphen;acting valve of piston configuration. The pulsed motion of the piston was analyzed from stroboscopic photographs. There is no scatter in valve operation due to Ohyphen;ring friction. The pulse width of the jet of several milliseconds and the mass flow rate during steady state can be varied independently. The rise time and the quasisteadiness of the pressure in the transient gas flow were measured by a newly developed fast ionization gauge of triode construction. The guage has a constant sensitivity from 3times;10minus;3to 2 Torr and a sufficient time resolution, and it introduces little obstruction of the gas flow. The pressure distribution measured in the jet expanding into vacuum agrees well with the rarefied gasdynamic prediction.
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