Threshold of the 2ωpe instability in a laser produced plasma

3 2 ω L emission from various plane targets irradiated by a 20 J, 5 ns neodymium laser pulse has been investigated. The onset of 3 2 ω L emission and an increase in intensity by five orders of magnitude is observed at a laser intensity of ≌2×10 13 W cm -2 , the threshold intensity predicted for excitation of the 2ω pe instability.