Nitrogen Purging – Purposes and Effects of Ozone
Authors: Robert Mech, Xiaohua Ye
The purpose of purging Energetiq’s Laser-Driven Light Source (LDLS®) units with dry nitrogen is to negate the impacts of oxygen, ozone, and trace organics that are present in the air on LDLS operation and the workplace environment. First, the presence of oxygen in the LDLS lamp head results in the generation of ozone (O3) during operation. Ozone is generated when UV light from a wavelength less than 300 nm breaks down molecular oxygen (O2) and one of the resulting free oxygen atoms combines with another oxygen molecule to form O3. Ozone is a known human health hazard and can be detected by its sharp, distinctive smell, or with an ozone-monitoring system. The side effects of ozone inhalation are largely respiratory, including irritation of the lungs and throat, especially for those with pre-existing respiratory conditions. OSHA sets the time-weighted average permissible exposure limit (TWA-PEL) for ozone over an 8-hour workshift at 0.1 ppm and recommends using respirators at higher concentrations of up to 5 ppm. Additionally, ozone also absorbs UV in the broad Hartley band between 200-300 nm and therefore decreases LDLS light output, especially near the 250 nm range. For wavelengths that are shorter than 200 nm, the presence of oxygen decreases light output via absorption in the Schumann-Runge bands.
Key Effects of Purging
- Prevention of ozone generation
- Longer expected lifetime for bulbs and optics
- Slight radiance decrease in VIS and IR
Purging with dry nitrogen also ensures that trace organics which are found in regular room air are removed from the vicinity of important optics. These organics may be oxidized by ozone and dissociated by UV light and can deposit onto optical surfaces, such as the laser-focusing lens and Xe bulb surface, in the lamp head. These deposits discolor the components, reducing optical transmission and bulb lifetime.
For these reasons, Energetiq recommends purging all LDLS units with 4.8 N nitrogen and a pressure of 20 PSIG at the inlet. For applications using light wavelengths that are shorter than 200 nm, the operator should consider purging the beam path as well. For example, optics can be placed in a lens tube that has an inlet for nitrogen, ensuring that the beam path does not permit the generation of ozone along the optical path. For applications not requiring the UV portion of the spectrum, optics can be used to eliminate UV radiation, including bandpass filters and window materials. In these applications, purging is still recommended to prevent ozone generation inside the lamp head.
Impacts of Purging on LDLS Performance
Purging has effects on lamp performance which must be understood when designing an application. Purging a lamp head with a continuous flow of dry nitrogen at room temperature cools the lamp head slightly, thus resulting in a slight decrease in light output in the visible and infrared. However, at wavelengths shorter than 300 nm, purging increases light output, as it negates the loss in the UV caused by ozone and oxygen. See Figure 1 for example data with an EQ-77 LDLS.
Figure 1: EQ-77 Radiance – Purge/No Purge Comparison
Since nitrogen purging prevents the generation of ozone in the lamp head, radiance while purging is significantly greater near the center of the Hartley band at ~250 nm, where ozone absorbs radiation. For applications below 200 nm, the Schumann-Runge absorption bands of molecular oxygen will result in a radiance decrease if the user is not purging the unit. Thus, purging with nitrogen slightly lowers VIS and IR light output but significantly boosts output across the UV, especially in ozone and oxygen absorption bands.
The extent to which VIS and IR radiance decrease while purging is dependent on the flow rate of nitrogen entering the lamp head. A higher flow rate of room temperature gas will result in a greater radiance decrease. If a user wishes to change the nitrogen flow rate into the lamp head, it can be done by modifying parameters in the “choked flow” equation. The flow rate Q of nitrogen is given by:
Where Cd is the discharge coefficient (which usually must be experimentally determined for a given flow restrictor), A is the cross-sectional area of the flow restrictor (0.008” in many LDLS applications), ΔP is the pressure difference between the upstream and downstream ends of the flow restrictor, and ρ is the fluid density. In LDLS applications, the flow rate may be changed by modifying either the upstream pressure (thus changing ΔP), or the flow restrictor, which would change A and Cd.
2. Light Output Stability
Although purging at the specified flow rate does not affect light output stability of LDLS units, purging at an increased flow rate may affect noise in LDLS units. Consider irradiance stability data under nitrogen purging for a free-space output EQ-77 unit. Processing parameters are not relevant here – only the relative fluctuation between data with purging and data without purging:
Figure 2: Purging and Noise in Free-Space EQ-77 LDLS Units
As seen in Figure 2, no meaningful difference in light output fluctuation is seen in free-space units between setups that use purging and those that do not use purging. This is consistent for all free-space LDLS models. However, consider the difference in noise when an 0.008” flow restrictor is used to route nitrogen into the lamp head of a fiber-coupled EQ-99X-FC LDLS unit as shown in Figure 3 below:
Figure 3: Purging and Noise in Fiber-Coupled EQ-99X-FC LDLS Units
The observed fluctuations increase by an order of magnitude, a phenomenon which is consistent across fiber-coupled LDLS units. To avoid this, the flow restrictor should be installed upstream of the gas inlet of the lamp head as shown in Figure 4 below:
Figure 4: Difference in Location of Flow Restrictor Between Purging Setups
According to the fluid dynamics of Bernoulli’s principle, nitrogen exits the small orifice of the flow restrictor at high velocity. In purge setup A shown above, this high-velocity nitrogen ejects immediately upon the fiber-coupled optics and can result in mechanical vibrations of these fiber-coupling optical components. This is the same purge setup used for the data shown in Figure 3. In purge setup B, a hose with a cross-sectional area much larger than the flow restrictor is used to decrease the velocity of nitrogen entering the lamp head while keeping the flow rate of nitrogen the same. This causes greatly reduced mechanical vibration of the fiber-coupled optics and solves the problem of purging-induced noise. See the following fluctuation data:
Figure 5: Purging and Noise in Fiber-Coupled LDLS Units Where a Hose Is Mounted Downstream from the Flow Restrictor
Purge setup B results in no noise increase while purging. The differences between fluctuation levels in the measurements shown are best explained by environmental factors, such as ambient temperature changes.
Purging during operation is important for LDLS performance and lifetime as well as workplace safety. Without a dry nitrogen purge, oxygen, ozone, and photon-dissociated trace organics can negatively affect long-term LDLS performance and result in workplace hazards. Purging results in a slight radiance decrease in the VIS and IR wavelengths, and it will not affect noise in any free-space units or fiber-coupled units that use the correct purging apparatus.
For more information on ozone and LDLS products, refer to the Energetiq technical note “Operation of Laser-Driver Light Sources Below 300 nm: Ozone Mitigation” by H. Zhu and P. Blackborow. For more information regarding safe ozone levels in the air and means of controlling ozone levels in workplace atmospheres, consult OSHA.