Differential measurement of the absolute ion yield from laser-produced C plasmas

Abstract

The ion flux produced by an obliquely incident Nd Q-switch pulse on a graphite target has been analyzed with regard to its kinetic energy, charge, and angular distribution. The laser intensity has been varied in a range between 109–5·1010 W/cm2, appropriate for many low-irradiance applications. It is observed that for ions of charge state n the emission cone of the number of ions scales with cos2n+1. The angular emission probability of the kinetic energy of the individual ions is found to be independent of their charge and scales as a cosine function. Due to the asymmetrical heating of the expanding plasma by the obliquely incident laser pulse, the maximum of emission is rotated away from the target normal toward the incoming laser, depending upon the ion's charge and the laser energy. The measured kinetic energy spectra are determined by the recombination during the plasma expansion: There are no low-energetic highly charged ions and no high-energetic lowly chargedions. If the laser energy (intensity) is enhanced, it is observed that the additional heating essentially serves only to increase the velocity of the higher charged ions; the energy of the individual singly charged ions is not altered.