Jetting and Ignition due to Laser-Based Deposition of Energy in a Gas

Ignition in the interior of a flow can be achieved by deposition of energy by a focused laser. The induced hydrodynamics affect ignition and can result in a jet by which hot gas is convected out of the breakdown region to a distance several times the size of the plasma kernel. High-resolution numerical simulations of a instantaneous deposition of thermal energy in a gas provide a hydrodynamic description of the cause of this jetting phenomenon. Jet strength and direction are sensitive to the initial geometry of the energized region. Changes in curvature of its bounding surface alter the evolving misalignment of density and pressure gradients, leading to early-time production of vorticity. A sufficiently intense energy deposition produces a ring-like vortex that causes involution of the kernel and propagates along the laser axis as a jet of hot gas that is above the autoignition temperature for common mixtures. Recent experiments confirm that this can be a mechanism of ignition. The geometry of the kernel can be modified to magnify mismatch in the signs of vorticity produced and ultimately change the direction of jetting. The dependence of net vorticity production with respect to geometry, energy input, and Reynolds number is also presented.