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A supersonic nozzle for a 1.5-MA gas puff Z pinch was designed and the gas flow injected out from the nozzle was measured. A miniature ionization gauge was developed to measure the density profile of the gas shell and two pressure transducers were used for determining the Mach number of the gas flow. It was found that the gas flow is not distributed symmetrically about r0, the mean radius of the exit aperture, and the radial location for the peak of the density profile moves inwards as time goes on. When the plenum pressure is 5 atm, the maximum line mass density measured was 43 μg/cm and the Mach number of the gas flow deduced from experiments is 4.3, which is close to the expectation: line mass density of about 50 μg/cm and a Mach number higher than 4. The measurements indicate that if a plenum opens into a large dead volume via a finite opening valve in a gas-puff system it will not produce a steady-state solution. Then, any agreement between the measurements and the calculations based on the steady state should not be expected.