Page 35 - High Power Laser Handbook

8 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 9

most high-power diffusion-cooled lasers with metal electrodes, the
amount of water vapor in the system is so large that it has a negative
effect on the laser’s performance; thus, it must be removed. H O or
2
hydrogen (H ) contents above a critical level have a large impact on
2
the relaxation of the upper laser level.
Water forms monolayers on the surfaces of the electrodes and the
vacuum vessel. Removing these monolayers requires baking at high
temperatures and low pressures. To prevent the moisture level in a
laser system from increasing, water adsorbers, such as zeolites, can
be added to the vacuum system of diffusion-cooled lasers. In fast-
flow lasers, a small percentage of the laser gas is continuously
replaced to prevent contamination and degradation.
If failures due to issues such as mirror damage are neglected, the
lifespan of sealed-off CO lasers will mainly be determined by the
2
long-term stability of the CO partial pressure and the contamination
2
of the laser gas by leaks and outgassing of the materials used in the
construction of the laser. Analogously, the gas exchange frequency of
near sealed-off lasers depends on the same factors. 8,9
The partial CO pressure changes over time due to dissociation of the
2
CO molecules. 10,11 If no special preparations are taken, the initially
2
reached equilibrium among CO , CO, and oxygen will be pushed further
2
and further to the CO side, until efficient laser operation is no longer pos-
sible. The initial equilibrium is influenced by such factors as the gas mix,
pressure, rf input power, and electrode materials. Typically 50 to 70 percent
of the CO is dissociated when the initial equilibrium is reached.
2
The dissociation of the CO molecule is triggered by electron
2
impact in the gas discharge: 12
+
+
−
CO + e ⇔ − COO e − 5,5 eV (1.1)
2
CO + e ⇔ − COO − − 3,85 eV (1.2)
+
2
There are a few ways to stabilize the CO partial pressure:
2
• Prevent the oxygen generated in the gas discharge from being
consumed by either oxidation or adsorption processes.
• Use catalysts, such as gold, to accelerate the back reaction of
CO and O to CO . 11
2 2
• Use gas additives, such as H O and H . 2
2 2
• Use CO donors. 13
2
• Use a pre-dissociated gas mix.
To conserve the CO partial pressure, it is critical to use the proper
2
materials in constructing the laser. According to the law of mass
action, the CO partial pressure will decline if the oxygen partial pres-
2
sure is reduced. Reduction of the oxygen partial pressure can be