HIGH-REPETITION-RATE LASER-INDUCED PLASMA AS A VUV RADIATION SOURCE

Laser-induced plasmas are needed as sources capable of delivering inte nse radiation in the VUV, XUV and soft x-ray spectral range through am plified spontaneous emission (ASE). As an example transition emission on the He II line at lambda = 164.0 nm is studied and investigated wit h respect to ASE. A laser-induced plasma (optical discharge) is produc ed by a focused beam of a Q-switched CO2 laser at high pulse repetitio n rates from 4 to 87 kHz and a pulse width of 35 ns and 200 ns respect ively. The laser intensity required to ignite a laser-induced plasma i s 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 nitrog en. The temporal and spatial distribution of electron temperature and density have been measured. In the centre of the helium plasma the ele ctron temperature and density reach 1.7 x 10(5) K and 2.25 x 10(24) m( -3) respectively. Strong quasi-steady transient line emission of He II lines has been observed but ASE was not found.