A non-transferred atmospheric pressure argon arc jet is ejected into water from an anode nozzle to study thermohydraulic characteristics. The plasma temperature dropped sharply around 10-13 mm downstream measured from the nozzle. Since the arc
diameter was found to be almost constant, the arc velocity can be traced from the mass-flux conservation, which was found subsonic over the whole volume in the water. The velocity dropped sharply in the aforementioned region, where its velocity approximately agreed with the sonic velocity in the water. It is considered that energy dissipation occurred near the sonic velocity of water. Even
if the plasma is subsonic, the flow characteristics may depend on the sonic velocity of the surrounding water.