Research Of Discharge Mechanism Of Underwater Plasma Acoustic Source

The underwater plasma acoustic source(UPAS)has many advantages such as wide frequency band,high repetition rate and better safety,and it is being applied in the underwater security area in recent years.In this dissertation,the related aspects of discharge mechanism of UPAS were studied systematically and profoundly,such as establishment of the platform for experimental study of underwater discharge,the responses of discharge on a series of parameters,the physical transformation of discharge channel.These efforts are of great theoretical and realistic significance for both basic and applied research of the UPAS.The detailed contents include the followings:1.The electric-acoustic transition in UPASA physical model for electric-acoustic transition in PUAS was used,and the generation of the discharge shock wave was simulated.The result indicates that,the thermal radiation is the primary mode of the energy dissipation.The initial energy of shock wave obtained from numerical simulation and the experiment result is well consistent.2.The platform for experimental study of UPASThe pulse power system was introduced.Two kinds of air swith were proposed for different experimental envirement,and experiment results were analyed.A trigger system for the camera was proposed,and the camera could work synchronously with light production and transportation from water gap.The HSFC and the shielding methods were introduced.A charge-discharge circuit for the typical driven capacitor(10nF/100KV)was proposed.The working mechanism of this circuit was introduced,and its electric behavior was simulated by pSpice.Based on the simulation results,the rules of choosing the electric parameters for this circuit were discussed.3.Experimental studies of underwater discharge(1)Two different electrode configurations were used.Research of the responses of discharge on the configuration effect of electrode and the dissipation area were studies.(2)Research of the water conductivities was performed.Increasing the conductivity,the prebreakdown time reduces,and the randomicity is small.With the increasing of the conductivity,dissipation energy increse,the electric field strength reduces,and the speed of the streamer reduces,until stop.However,due to the energy dissipation,there is a threshold for the conductivity of water,and therefor a limited minimum value for the prebreakdown time,where the prebreakdown time and randomicity are smallest.(3)Influence of the water gap on prebreakdown time was investigated.Reducing the water gap,the mean value of prebreakdown time decrease,and the standard deviation decrease as well until smaller than unit when the gap smaller than 10 mm.On the other hand,when the conductivity of water is smaller than some threshold,the influence of water gap on the prebreakdown time can be neglected,but the the influence on dissipation voltage can not be ignored.(4)The waveform of shock wave was analysed.Because of the influences of circuit currnt and the plasma,the shock wave includes four parts: the front of shock wave,the primary shock wave,the second shock wave and the end of shock wave.4.The optical diagnosis of UPAS(1)In order to capture the photographs of streamer propagation,the influence of the randomicity of pre-breakdown duration was taken into consideration.By increasing the conductivity of water,the randomicity reduces effectively.(2)The optical diagnosis was applied to the spark discharge.The streamers do not directly move to the opposite electrode,but form a bush-like figure.With increasing the number of branches,the velocity of streamer propagation slows down.The trajectory of the initial channel between electrodes is not straight.However,with channel expanding,its shape transforms into a straight column.When the plasma disappears,a large bubble filled with vapor is formed.(3)The optical diagnosis was applied to the corona discharge.The mode of streamer generation for corona discharge is same as spark ones.But the distamce of streamer is shorter and there is not a discharge channel between two electrodes.When the movement of streamer stop,streamer neither develop nor disappear.There is not a process of expansion for the streamers,directly,the streamer transforms to plasma.(4)The details of the discharge channel were investigated by experiments.Using the HSFC,for the first time,we take a series of pictures of streamer development,and the generation of discharge channel.For the first time,we take the pictures of cathode emission.And finally,we explain the the reason of formation of the front of shock wave.