Research On Mechanism Of Generating Plasma Using Liquid Phase Discharge With Laser

Plasma generated by different liquid-phase discharges has great application prospects in many fields such as biomedicine and environment.The theories and simulations about generating plasma using water discharges with laser are researched in this thesis.A fluid model of nanosecond pulse voltage discharge in water is established using the two-dimensional fluid mathematical method as simulating plasma generation by argon glow discharge at low pressures.Using COMSOL Multi-physics software,development processes of plasma channels formed in nanosecond pulse voltage discharges in water are simulated.Thereafter,a theory and its fluid model on generating plasma using laser leading-in a water discharge system with certain laser energies are studied and experimental simulations are carried out.Firstly,processes of plasma generation using argon glow discharges in a cylindrical dc reaction chamber under low-pressure are studied theoretically and simulated.A 2D fluid mathematical model of plasma in the cylindrical dc reaction chamber was established,and the corresponding mathematical equations were given.Physical characteristics of plasma generated by argon glow discharges in the cylindrical dc reaction chamber are simulated using COMSOL Multi-physics software.Those characteristics are time and space distributions of physical quantities such as electron densities,ion densities,electric potentials and electron temperatures in that reaction chamber.Discharge breakdown in the chamber and development processes from normal glow discharges to abnormal glow discharges are obtained by simulations.Compared with experimental results published,it is clearly and correctly shown by simulation results that characteristics of cathode fall regions,negative glow regions and Faraday dark regions as well as positive column regions in plasma are generated by normal glow discharges.Then,development processes of plasma channels formed by nanosecond pulse voltage discharges in water are studied theoretically and simulated.A 2D mathematical fluid model of plasma generated by water discharges between a needle electrode and a plate electrode is established,and the corresponding mathematical equations are given.Using COMSOL software distributions with time and space of electric field intensities,electron densities,hydrogen ion densities and hydroxide ion densities between the two electrodes are simulated.Effects of pulse voltage amplitudes,electrode spacing distances and conductivities on these physical quantities and streamer morphologies are studied as well.It is shown by these simulation results that breakdown voltages are proportional to electrode spaces.The larger the electrode space is,the higher the pulse voltage amplitude is needed for the breakdown.When electrode spaces are smaller and corresponding breakdown voltages are reached,purified water is ionized by high intensity fields at tip of the needle electrode and a single-streamer channel is formed.Electron densities in the channel are increased linearly with voltage amplitudes.When pulse voltage amplitudes are high enough,the single streamer channel is "bifurcated" without that linear relationship between electron densities and voltage amplitudes.When electrode spaces are much larger,"dendritic" plasma channels are easily to be formed.Compared physical characteristics of plasma channels in purified water with those in tap water,field intensities of streamer heads in plasma channels in tap water with higher conductivities are obviously higher than those in purified water,and electron densities in tap water are also much larger.At last,processes generating plasma by a laser leading-in water discharge system with certain laser energies are theoretically studied and experimentally simulated.Combining with the Drude model,a 2D fluid mathematical model of generating plasma by the laser leading-in water discharge system is established and corresponding mathematical equations are given.Using COMSOL software,processes of charged particles interacting with laser vertically shot into high density plasma channels are simulated.And effects of laser with certain energies leading-in water dielectric pulse discharge system on local electron densities are discussed.It is shown by these simulation results that electron densities of plasma channels in these local areas are increased sharply after laser energies are leaded-in the system.Area sizes and changes of electron density are related to diameters of laser spots and laser incident positions.When electron densities in channels continue to increase to a certain extent,local plasma with high-density would strongly shield the laser to enter,which limits electron densities further increased.