Ionisation path formation in gases using a laser with retractable focus

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Abstract
A novel scheme for producing long paths of ionisation in gases by causing gas breakdown in a rapidly retracted focal region of a CO2 laser beam is described. Avalanche breakdown first occurs at the focal point of a laser optical system where intensity IF is above the clean gas threshold. The focus of the beam is retracted at a velocity u such that breakdown and complete absorption of the laser beam occur only in the focal volume. Analysis of clean gas breakdown times in N2, H2, He and air using a Boltzmann code for calculating the electron distribution function with the appropriate cross sections has been performed as a function of laser intensity. The results are applied to a diffraction-limited laser beam which has a Gaussian profile near the focus. For air (energy per unit volume deposited 10 J cm-3, IF=1.3*1010 W cm-2, mirror diameter 10 cm, range 10 m) the total energy required is found to be in the range 20-50 J and pulse time tau 1 mu <or approximately=s. Effects of impurities and aerosols which may initiate breakdown at much lower fluxes are also discussed. Applications of long ionisation paths in gases are for initiation of discharges over long paths and for guiding particle beams in particle-beam fusion experiments.