The impurities in the gas freeze onto the walls of the trap so that clean gas emerges to be returned to the laser. The clean processed gas is usually passed across the laser resonator optics to retard optic contamination with discharge dust.
To increase cryogenic system efficiency, a regenerative heat exchanger is used inside the gas processor to pre-cool incoming gas with outgoing gas. Most gas processors use liquid Nitrogen (LN2) as the cryogen, but helium cryopumps are sometimes used to refrigerate the cold trap to the required temperature.
| Liquid Nitrogen | Helium Cryopump |
|---|---|
| High thermal capacity | Limited thermal capacity |
| Regular LN2 replenishmentdowntime | Continuous operation |
| LN2 is fairly expensive | Expensive and complex refrigeration system |
| Mechanically simple and reliable | Less reliable than LN2 |
| Requires LN2 replenishment; LN2 becomes a consumable | No cryogen consumption; reduced consumable costs |
One concern about gas processing is liquefaction of the rare gas constituent in certain gas mixtures if the trap is too cold. In order to prevent this problem, trap temperature control is required. In order to trap the widest variety of contaminants, the trap is held at a temperature just above the liquefaction temperature of the rare gas. The following is a table of minimum trap temperatures for commonly used gas mixtures.
| Gas Mixture | Minimum Cold Trap Temperature |
|---|---|
| Arf | <77oK |
| KrF | ~105o to 110oK |
| XeCl | ~135oK |
Unfortunately, some laser gas contaminants freeze at temperatures below the liquefaction point of the rare gas and cannot be trapped. For this reason, a gas processor cannot extend gas life indefinitely. New fills are less frequent, but still eventually required.
The following table shows cold trap temperature stabilization methods using LN2:
| Active Stabilization | Passive Stabilization | |
|---|---|---|
| Push-pull control using heaters | Type 1: thermal barrier | Type 2: LN2 pressurization |
| Heaters used to elevate trap temperature to setpoint | Very simple, no controls required | LN2 pressurized to elevate boiling point to required trap temperature |
| High LN2 consumption 1-2 Liters/hour | Low LN2 consumption | Low LN2 consumption: <0.5 Liter/hour |
| Temp control system complex, potentially unstable | Relies on temp gradient; trap temp varies with laser gas flow rate | Excellent temp control at varying gas flow rates |
| Microprocessor temp control system required | No control system required | Requires pressurization control system |
| Simple vacuum bottle cryogen containment | Simple vacuum bottle cryogen containment | DOT pressure-rated cryogen containment |