Accelerated UV Aging and Degradation Studies with SERE

EMA’s Space Environment and Radiation Effects lab (SERE) is a unique commercial test facility that makes space evaluations possible on Earth. It contains an array of energy sources from 5keV to >2MeV over a large working area. The combination of sources allows EMA to produce realistic spectra associated with various orbits.

SERE is also equipped with dynamic shuttering and control capabilities creating a completely customizable environment. This provides the ability to evaluate complex orbits, eclipse conditions, and accelerated lifecycles in a single test. SERE is also equipped with video, spatial arc detection, and several sensor modalities.

SERE is located in Pittsfield, Mass. at EMA’s lab inside of the Berkshire Innovation Center.

In this White Paper, we take a closer look at SERE’s accelerated UV source.

Fig. 1. Space Environment and Radiation Effects test chambercommercial test chamber, also known as SERE.

UV Source

A high output UV source has been constructed for material aging and degradation studies. Its construction consists of four UV-A micro discharge lamp bulbs combined into a single housing and reflector that is mounted to a vacuum viewport. The source is intended to be onto a load lock chamber for close sample working distance that can be changed with the linear translation stage.

The lamps were acquired from Spectro-UV® and are ML-3500C MAXIMA™ ultra-high intensity UV-A blacklight lamps intended for UV curing purposes. Four of them were purchased for our source, where they were stripped of the handheld housing they came in and retrofitted to be held within a 3D printed housing. They were wired into a bundle using connectors to attach the power supplies to the bulbs. The housing is attached to the CF flange with six threaded rods that also hold the reflector housing. The viewport used is a 10”CF flange window using Corning HPFS 7980 fused silica that has nearly 99.8% transmission from 200-400 nm.

Fig. 2. (Left) Exploded CAD view of the UV source, including the housing and mounting hardware, bulbs and holder, reflector, and viewport connecting to the load lock nipple. (Right) Photo of the source attached to the load lock chamber with the sources on.

Thermal Testing

Since the viewport manufacturer does not recommend direct thermal radiation exposure of the window, a thermal test was done to determine if the temperature gradient exceeded that of the specifications given for the viewport. Two type-K thermocouples were used as temperature probes that were taped to the atmosphere side of the window, one in the middle of the window and the second put directly in front of one of the bulbs. The chamber was pumped down, and the bulbs were turned on one by one in roughly 30-minute steps. Figure 2 shows the results for temperature rise, along with the temperature difference in one-minute intervals. The peak temperature seen with all sources turned on is roughly 90- 95°C. The window manufacturer strongly suggests a temperature ramp of no more than 2- 3°C, and the greatest ΔT seen is when bulb two was turned on, producing a ΔT of 2.2. Turing off the bulbs also produces a large ΔT. Repeated cycling of the source was not done.

Fig. 3. (Top) Plot showing the temperature rise of the probes when the bulbs were turned on one by one. (Bottom) The temperature different between one minute intervals of the temperature rise data.

Spectral Analysis

The radiometric output of the UV source was characterized using the StellerNet® Silver Nova spectrometer that was first calibrated to the SL4 lamp. Calibration involves preparing the measurement setup, in this case it uses the integrating sphere (IS) with its aperture put at plane with the SL4 source. Calibration was done with an integration time of either 1,200 ms or 6,000 ms. If the spectrometer is calibrated with a sufficient integration time, there doesn’t seem to be a large influence on the measured irradiance even when changing the integration time afterward. While taking data of the SL4 source in the at-plane setup, the calibration file is loaded, where the calibration file is supposed to be the true irradiance of the SL4 source for a given configuration. This allows the software to generate correction factors for the measured data to match the calibration file data. Figure 4 shows the comparison of the calibration file data to the measured SL4 data in the same configuration, and it shows there is some deviation in the UV and IR regions.

Fig. 4. Comparison of the SL4 calibration file and the SL4 measurement just after calibration.

To use the spectrometer for radiometric measurement of the UV source, attenuation of the beam would need to be done to prevent over saturation of the detector. Neutral density filters (NDF) were used in series mounted in front of the integrating sphere to reduce the source intensity. Data was taken of the source with the integration time set to 6,000 ms, with the integrating sphere 14- 3/16” from the inside of the window. To properly scale this data to an unattenuated value, accurate determination of the optical density is needed. The NDFs used are from Edmund Optics, that provide OD curves for their filters, with this data taken and combined to give a total OD of the filter stack. The NDFs used were a 2.0 and a 0.4 optical density, with the combined wavelength dependent OD shown in Figure 4 from 200- 400nm. This waveform was used to scale the measured data to an unattenuated value using

Figure 6 shows the scaling comparison using either a constant 2.4 OD or the wavelength dependent OD shown in Figure 5, with the results as expected with a lower spectral irradiance seen in the 100- 220 nm and 300- 400 nm range due to the OD being less than 2.4 in these regions.

Fig. 5. The wavelength dependent total optical density.

Fig. 6. UV source measurement scaling comparison, using either a constant OD or the wavelength dependent OD seen in Figure 5.

To find the total irradiance, integration of the spectral irradiance is done between 200-400 nm and was done for both the AM0 spectrum and the UV source to determine what total irradiance acceleration factor is for the UV source compared to the solar spectrum. Results are shown in Table I, that gives an acceleration factor of 274 to that of AM0.

Table 1. Total irradiance comparison (200- 400 nm)

Comparing the UV source spectrum to the AM0 spectrum is done in Figure 7, that has the unattenuated UV data plotted with the AM0 spectrum that has been scaled by a factor of 274 for a variance comparison by wavelength.

Fig. 7. UV source data plotted with AM0 scaled x273 for comparison.

Parts List

Conclusion

This White Paper introduces EMA’s new UV material aging and degradation equipment on the Space Environment and Radiation Effects test chamber. Learn more about SERE by clicking here. To start testing, reach out to EMA here.