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Types
Types of CO2 Lasers
- Sealed tube. The structure of these is very similar to that of sealed HeNe and Ar/Kr ion lasers but with features (bore diameter, etc.) geared to much longer C02 wavelength.
- Axial gas flow. The gas mixture is pumped in one end of the tube and out the other. This provides fresh gas to replace the (CO2) depleted due to dissociation of the gas molecules. He and N2 are added to the mixture to boost efficiency.
- Transverse gas flow. Instead of flowing down the tube, the gas flows across it providing the highest power ratings for continuous CO2 laser operation.
The most common types of excitation are a direct electrical discharge (usually DC) and radio frequency (RF).
Sealed CO2 Lasers
Commercial sealed CO2 lasers have much in common with sealed HeNe lasers. Like HeNe lasers, they use DC electrical current excitation requiring a very high voltage for starting and somewhat less (but still high) voltage while operating. However, specific requirements differ:
- The typical sealed CO2 tube has an operating voltage of between 3 and 12 KV at 2 to 15 mA DC. Some may require 20 KV at 20 mA or more. (The corresponding values for HeNe tubes are: .6 to 5 KV at 3 to 8 mA DC.)
- A high starting voltage is required to initiate the discharge. For HeNe laser tubes, the actual value is typically 3 to 5 times the operating voltage with the manufacturer's specifications being somewhat higher to assure reliable starting. I have not been able to find equivalent info for sealed CO2 lasers.
- The value of the negative resistance of the CO2 discharge is between -200K and -600K ohms. (The corresponding value for HeNe tubes is about -50K ohms). The effective ballast resistor value is selected to be 30 to 50% higher than this to assure stability.
Therefore, requirements for the power supply to be used with sealed CO2 and HeNe lasers are very similar. However, due the significantly larger negative resistance characteristic of the CO2 laser, there is more incentive to push part of the effective ballast resistance into the control of a switchmode inverter rather than a simple power wasting resistor.
For example, the dissipation in a 300K ballast resistor at 10 mA, would be 30 W. Depending on your actual needs, this may still be acceptable since it should simplify the power supply design not to have to deal with the negative resistance in the current regulator feedback loop itself. However, at the high end of the range where an 800K ballast is required at 20 mA of operating current, the corresponding power dissipation would be - ready? - 320 W! This is probably a bit more than is desirable.
Finding inverter schematics for HeNe lasers is tough enough. Finding them for C02s is virtually impossible. The only CO2 power supplies of any kind I have are based on neon sign transformers for use with home-built CO2 lasers. They have no regulation but may be an alternative at least for initial testing.
The good news is that if you were to design an inverter type of power supply with an oversize or multiple flyback transformers, I think you would find that it inherently had a high effective series resistance - possibly enough so only a minimal external ballast would be needed.
The starting voltage is no problem - that can use any of the approaches for starting higher power HeNe tubes.
One thing that is significantly different for a CO2 laser compared to the HeNe variety is efficiency: The electrical to optical efficiency of a typical sealed CO2 laser is around 5 to 8 percent compared to less than .1 percent for a HeNe laser. However, the efficiency of large (flowing gas) CO2 lasers can exceed 20 percent.
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