Page 34 - High Power Laser Handbook

6 G a s , C h e m i c a l , a n d F r e e - E l e c t r o n L a s e r s Carbon Dioxide Lasers 7

Figure 1.4 Left: A typical α-discharge in a 6-mm interelectrode gap. Right:
A typical γ-discharge in a 6-mm interelectrode gap.

dissipation in the sheaths and their current densities. The two types can
easily be distinguished by their intensity and luminosity distribution
along the discharge length (see Fig. 1.4). The γ-discharge is also called
high-current discharge because it has an order of magnitude higher cur-
rent density than the α-discharge, which is called low-current discharge.
The gas used in CO lasers is usually a mix of CO , N , and helium
2
2
2
(He). To improve certain aspects of the laser’s performance CO, xenon
(Xe), and other gases are added as needed. The high thermal conduc-
tivity of helium, which is about six times higher than the thermal
conductivity of N and CO , reduces the gas temperature, because the
2
2
temperature gradient between the laser gas and the cooled electrode
surface is inversely proportional to the thermal conductivity. The
thermal conductivity of helium is κ = 0.17 W/mK (watts per meter-
He
kelvin) at 100ºC. Helium’s energy levels are all above 20 eV; thus, for
a gas mix that is optimized for high laser power—namely, electron
energy levels in the 1–3 eV range—helium does not significantly
influence the discharge. Thermal conductivity is basically determined
by the amount of helium in the gas mix, which results in improved
heat removal and a reduction of the lower laser level’s thermal popu-
lation. In addition, the width of the gain profile is temperature depen-
dent and increases with decreasing temperature. Helium also
stabilizes the discharge since diffusion processes and thermal con-
ductivity are important to stabilize the discharge by evening out local
inhomogeneities.
Gas mixtures for diffusion-cooled lasers typically contain Xe.
Three to five percent concentrations of Xe increase the laser’s output
power and efficiency. The increased efficiency results from the effect
Xe has on the electron energy distribution in the discharge. Xenon has
a relatively low ionization energy of 12.1 eV, which is about 2–3 eV
less than that of the other gas components. Therefore, the number of
electrons with energies above 4 eV decreases, and the number of elec-
7
trons with energies below 4 eV increases. The change in electron
energy distribution has a favorable effect on the vibrational excitation
of CO and N , as discussed earlier in this chapter.
2
Water (H O) has a strong influence on laser performance. In low-
2
power, sealed-off lasers, H O is often added to the laser gas mix to
2
suppress the dissociation of CO molecules into CO and oxygen. In
2