The mechanism of pressure generation in hydrospark was theoretically analyzed, assuming the polytropic relationship between the pressure generated by hydrospark and the travel distance of the ball in a specially designed shooting type hydrospark device. It appears that the pressure was produced by hydrospark owing to the expansion of high pressure vapor generated between electrodes.
In the authors' experiments, the expansion ratio was about 300 and the polytropic index for the vapor generated between electrodes was 1.24 for the electrode distance of 3.85mm and the discharge energy of 4000 Joules.
The composition of gas generated between electrodes in hydrospark was 99:1 to 9:1 for liquid vapor vs. metal vapor of electrodes.
The strain rate of metal deformation by hydrospark forming was theoretically analyzed, though not in detail.