Enhancement of extreme ultraviolet emission from a lithium plasma by use of dual laser pulses

We demonstrated enhancement of extreme ultraviolet (EUV) emission at 13.5nm from a lithium plasma by use of dual laser pulses. A single laser pulse produced a lithium plasma condition for the EUV emission far beyond its optimum. Utilization of dual laser pulses, however, enhanced the EUV emission energy, and its maximum in-band EUV conversion efficiency (CE) in a measured solid angle was observed to be 2.4% at a pulse separation time between 20 and 50ns. The EUV CE became 1.8 times as large as that produced by a single laser pulse, which was one of the highest values ever reported. Enhancement of the EUV CE was attributed to the decrease of the plasma temperature and density to their optimum values.