Spatially and temporally resolved study of laser-induced absorption waves produced by a 5 μs/1J TE-CO 2 laser

Abstract

The phenomenon of laser supported absorption (LSA) of CO2 laser radiation is investigated with spatially and temporally resolved plasma spectroscopy in Argon as ambient gas at pressures of 1 -iO Pa. A TB-CO2 laser with a gas mixture of C021N2/He = 114/4 at a total pressure of half an atmosphere is used. The pulse length of the Th-C02 Laser is about 6 ts at a mean power of some 100 kW. LSA-phenomena and material ablation processes are compared with that of a con ventional TEA-CO2 short pulse laser. It could be shown that longer pulses of less mean power make material ablation more efficient ifthe absorption wave becomes transparent within the pulse time ofthe laser. Maximum electron density of 2.5 .1017 and a temperature of 3 eV were measured 1.5 is after the begin of the irradiation for an expanding aluminium plasma into vacuum. 1.ThE LASER SYSTEMBy using a non conventional gas mixture and total pressures of about half an atmosphere in a modified TEA-C02-Laser, see fig. 1, pulse-durations in the range of some hundred nanoseconds to 10 p.s could be produced. As the pulse-length we define the time within which 85 % ofthe total pulse energy has been emiued by the laser. On the one hand the CO2 molecule is excited directly by electron impact and on the other by energy transfer from the excited nitrogen molecule. The rate ofenergy transfer' is 1.4 x iO mbar1 s .The laser pulse consists of a 50 -100 ns peak of high amplitude corresponding to the direct excitation mechanism and a 1 -10 ts part of lower amplitude due to the energy transfer from the nitrogen molecule. 1) By lowering the partial pressure of CO2 as well as the total pressure the pulse length is prolonged, the laser efficiency however decreases. The total pulse energy amounts 500 mJ to 3J at pulse lengths of 10 ts to some 100 ns, respectively, see fig 2. A stable resonator is used, consisting of a gold coated copper mirror with 10 m focal length and a plane ZnSe output coupler with a reflectivity of 60 %. In multimode operation the measured beam divergence was approximately 3 mrad. At a CO2 partial pressure of less than ca. 25.102 Pa (about 8 % of the total pressure) the threshold gain condition is no longer fuffilled. For pulse durations exceeding 10 psthere must be used an output coupler with higher reflectivity or a folded resonator.2'3) The efficiency ofthe laser would be much lower then. The electrical circuit is a simple charge transfer circuit. The capacity of the main discharge capacitor bank is 60 uP. The switch element is a pseudo spark switch.4 IGNITION OF A PLASMA AND ABSORPTION WAVESInvestigations were made on the plasma formation and absorption waves. In air there is a plasma generated at irradiation with pulsed CO1 lasers above an threshold power density of W/cm2 which decreases in the vicinity of surfaces to about i07 W/cm"6. From this initial plasma a laser supported absorption wave (LSAW) starts and moves towards the incoming laser beam. As the duration of the laser pulse is in our case one order of magnitude longer than that of the TEA CO ...

<title>Contribution to the beam plasma material interactions during material processing with TEA CO2 laser radiation</title>

“…1 ,600 ns Allil lines are detected during processing of aluminum at a power density of 5 10 W/cm2 (Fig.6) . Under the condition that the plasma is optical thin for the observed Al-lines (electron density is below 1017/cm3) the electron temperature can be calculated for this Al-plasma [17,18] . Such a calculation for the Ti-plasma is not possible because the relevant data are not transferable.…”