A first principles model for laser triggered spark columns

A computer model (SPRAD) for a laser triggered spark column has been developed which, from first principles, describes the hydrodynamic and radiative expansion of a narrow (approx. =50 μm) laser produced plasma column into a wide (approx. =1 mm) arc capable of conducting 10 kA. The expansion of the arc is primarily a result of convective transport of the hot ionized core in a radial expansion wave. Non-convective expansion of the arc at the inner edge of the shock proceeds by an electron avalanche initiated by electrons produced by photoionization and by thermal ionization resulting from adiabatic heating of the neutral gas. The high neutral density shock wave at the front of the expansion limits the growth of the plasma column by confining the hot region in a shell of cold gas with a low E/N. In gas mixtures containing mostly hydrogen, the inner core of the spark column is fully ionized and the electron temperature can exceed 15 eV. In noble gas and hydrogen mixtures, the electron temperature is only a few eV