High repetition rate laser-induced plasma as a VUV radiation source

Abstract

Laser-induced plasmas are needed as sources capable of delivering intense radiation in the VUV, XUV and soft x-ray spectral range through amplified spontaneous emission (ASE). As an example transition emission on the He II line at nm is studied and investigated with respect to ASE. A laser-induced plasma (optical discharge) is produced by a focused beam of a Q-switched laser at high pulse repetition rates from 4 to 87 kHz and a pulse width of 35 ns and 200 ns respectively. The laser intensity required to ignite a laser-induced plasma is considerably reduced compared with single-shot experiments, because the electron density between two laser pulses remains on a relatively high level. Thus up to about 70% of the incoming photons are absorbed with helium and carbon dioxide as the working gas and heat the plasma. The absorption effectiveness is definitely lower in oxygen and nitrogen. The temporal and spatial distribution of electron temperature and density have been measured. In the centre of the helium plasma the electron temperature and density reach K and respectively. Strong quasi-steady transient line emission of He II lines has been observed but ASE was not found.