<title>Electron density measurements of laser-induced surface plasma by means of a beam deflection technique</title>

Abstract

A laser-induced surface plasma is generated by a Q-switched CO2 laser oscillator amplifier system. At a pulse repetition rate of 20 kHz, laser pulses at 10.6 ?m with 70 mJ pulse energy, 0.2 ?s half-width, 250 kW peak power, an average power of 1.4 kW and TEM00 beam quality are available. At a higher repetition rate of 100 kHz, the peak power and the pulse energy diminish to 15 kW and 14 mJ respectively, and the half-width is with 0.6 ?s three times larger. The spatial and temporal distribution of the electron density in a laser-induced plasma near an aluminum surface is measured by a beam deflection technique. As a laterally probing beam a cw 4 W CO2 laser tuned at 10.2 ?m wavelength is used, whose beam is focused through a small volume in front of the target surface. The electrons of the laser-induced plasma refract the cw CO2 laser beam, and the resulting beam deflection is determined by means of a partially absorbing CaF2 wedge in connection with a fast infrared detector. Simultaneously the plasma absorption of the probing laser beam is measured. A simultaneous detection of the beam deflection and the actual laser power of the oscillator amplifier system gives the possibility to estimate the velocity of the expanding plasma to 103 m/s. Two different kinds of laser- induced discharges were observed. Depending on the conditions the laser-induced plasma leaves the surface or remains attached. Surface leaving plasmas and plasmas attached to the metal surface may be distinguished by the sign of the deflection angle. The electron density in the laser-induced plasma which is responsible for the energy deposition and therefore for the cutting and welding quality can be influenced by the energy and pulse form of the laser pulses and working gases. The electron density reaches 1.1023 m-3 in case of helium as working gas and the dimensions of the helium plasma are about 1.5 mm using laser pulses at 20 kHz repetition rate. The values of the electron density and the dimensions of the plasma are 3 times higher in case of argon resulting in an beam absorption of 80 % in the plasma compared to about 20 % in case of helium. Using laser pulses at 100 kHz repetition rate the electron density in the plasma is reduced by a factor of two due to the lower peak power of the laser pulses compared with the laser pulses at 20 kHz repetition rate.