Pressure dependence of enhanced x-ray line emission from laser-produced plasmas expanding in an ambient gas

An experimental study of space-resolved x-ray emission from a laser produced magnesium plasma expanding in a helium gas background has been performed at different gas pressures up to 125 mbar. A strong enhancement in MgXI He-α resonance (1s2 1So - 1s2p 1P1 at 9.17Å) and intercombination (1s2 1So - 1s2p 3P1 at 9.23Å) line emission is observed in the blast wave region extending upto ~6 mm from the target. Results of measurements of pressure dependence of peak x ray line intensity, onset distance, size of the emission region, and intensity ratio of the two lines are described and the role of background gas to explain the observed behavior is discussed. At optimum background pressure of about 40 mbar there is an order of magnitude increase in the x-ray line intensity compared to that for a gas pressure of few mbar. Charge exchange and three body recombination rates have also been analyzed for their relative contribution towards this intensity enhancement.