An apparatus for rapidly establishing at least one preselected gas pressure in a process chamber comprising: (a) a chamber defining an interior space adapted to be maintained at a reduced pressure; and (b) a gas supply means for supplying at least one burst of gas to the chamber for rapidly establishing the at least one preselected gas pressure in the chamber, the gas supply means including: (i) a source of the gas; (ii) a supply ballast fluidly connected to the gas source for receiving the gas from the source; (iii) at least one burst ballast fluidly connected to the supply ballast via a metering valve for receiving the gas from the supply ballast; and (iv) an on/off valve fluidly connected to the at least one burst ballast and the chamber for supplying the process chamber with the gas from the at least one burst ballast.