|Ph.D Student||Daniel Shafer|
|Subject||Investigation of Converging Strong Shock Waves in Different|
|Department||Department of Physics||Supervisor||Full Professor Krasik Yakov|
|Full Thesis text|
The presented thesis is devoted to the study of converging strong shockwaves (SSW), produced by underwater electrical wire explosion (UEWE), and the phenomena that accompany this explosion, namely, cumulative shock waves, water flows and high-velocity jets. This subject relates to the warm dense plasma which has fundamental (astrophysics, transport coefficients of the material at extreme conditions, equation of state), and applied interests (inertial confinement fusion, shock wave generation). During UEWE, the metal wire material undergoes several phase transitions, namely melting, evaporation and the formation of plasma. These phase transitions occur during short time ( s) and lead to the rapid expansion of the wire in dense, viscous and expansion impeding water, resulting in the conditions of extremely large pressure (?Pa) and relatively low temperature (of several eV) of the wire material and generation of SSW in water.
Although the study of converging SSW has already been performed for about 10 years in our Plasma Laboratory, there are still many unanswered questions about the dynamics of the generated SSW during their convergence and the physical conditions obtained during cumulation. The study of these issues in our laboratory previously included experiments with the explosion of cylindrical and spherical wire arrays. The presented research deals with annular, “X-pinch” and conical geometries, which are formed by ring-shaped wires, wires crossed at an acute angle and conical wire arrays, respectively.
It was shown that UEWE of ring-shaped wire generates toroidal SSW,
whose center-facing front is converging with a constant velocity of 1.8?105 cm/s. In spite of the expectation that the process of cumulation will manifest itself at the final stages of the convergence, no acceleration of the SSW was observed. This apparent contradiction was explained by the model showing that the SSW is cumulating in the ring’s plane and is decaying in the perpendicular plane; these competing processes prevent cumulation on the one hand, but on the other also do not allow the complete decay of the SW, hence one obtains the constant implosion velocity of the annular SW in water.
In experiments with the UEWE of wires in the X-pinch configuration, the generation of two fast copper jets with velocity of ~2.5x105 cm/s was observed. The process of cumulation in that case was achieved by the thermal expansion, and not by the magnetic forces, as in the case of X-pinch configuration in vacuum.
Finally, the investigation of the explosions of conical wire arrays has shown that a state of continuous cumulation is achieved in this configuration. This results in ejection of fast jets into the air above the array, whose velocities reach as high as Vμs~1.2?105 cm/s or Vsub-μs~1.8?105 cm/s in the microsecond or sub-microsecond experiment, respectively. In these experiments, the shape and the velocity of the water jets and the SW that accompany them in air, were found to depend on the apex angle of the conical wire array, on the energy deposition rate and on the thickness of the water above the array.