Experimentally it has been found that a high power laser pulse focussed on a Helium gas at atmospheric pressure creates a plasma that over time assumes a torus shape. This process could see...Show moreExperimentally it has been found that a high power laser pulse focussed on a Helium gas at atmospheric pressure creates a plasma that over time assumes a torus shape. This process could see applications in plasma chemistry and is a first step towards a self-confined magnetohydrodynamics plasma. However, the torus shape is eventually destroyed as cold gas flow from the center of the torus splits the plasma. To study this process the gas dynamics shortly after the laser pulse are reduced to one dimension by applying cylindrical symmetry. The resulting equations are solved numerically. By fitting the one dimensional simulation results to experimental data, a three dimensional starting condition is proposed. Using this starting condition, a three dimensional axi-symmetric simulation is performed which is capable of reproducing both torus formation and splitting. These simulations show that the primary process responsible for torus formation is a low pressure area that is dragged behind the strong shock fronts moving perpendicular to the laser axis. Arguments for the local thermodynamic equilibrium of the plasma are presented. This justifies the application of the Saha equation to find the electron density from the simulated pressures and temperatures. Finally, passive modifications of the plasma environment are considered to prevent the plasma from splitting and provide flow confinement of the plasma for up to 100 μs.Show less
