  In present work, we report an experimental setup that has been developed to
characterize highly under-expanded helium and nitrogen jets confined in a
vacuum chamber. The evolution of Zone of Silence (ZOS) is studied and found
that experimentally measured ZOS for helium and nitrogen jets are in agreement
with empirical relation and mathematical model. The velocity of the jet inside
ZOS is measured using a developed time of flight (TOF) probe without using a
skimmer. We are able to measures the velocity of the gas jet itself compared to
a skimmer-based method which measures the velocity of gas that is subjected to
further expansion after the skimmer. Moreover, this method is free from skimmer
interference and calibration involved with it. We observe that the measured
velocity for helium is smaller than the terminal velocity. This can be
attributed to losses which can occur due to nozzle geometry. Simulations are
carried out using available DS2V code. Experimentally observed velocities of
jets inside ZOS are compared with simulated values and are found to be in
agreement within experimental uncertainties.
