U. Bielesch et al., HIGH-REPETITION-RATE LASER-INDUCED PLASMA AS A VUV RADIATION SOURCE, Journal of physics. D, Applied physics (Print), 31(18), 1998, pp. 2286-2294
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.