Calibrators for astrophysical spectrographs need to maintain a stable spectrum over their anticipated operational lifetime. In practice, emission sources often show signs of aging. We conducted a dedicated observation run to study the behavior of the MIP lamp during long-time usage. We discuss the results of our measurements in comparison with information on HCLs, and also report previous aging experiences with the MIP lamp.
5.4.1 Accelerated aging test
We conducted an accelerated aging test with the N2 discharge cell. Before the aging test, the N2 discharge cell had been operated between September 2014 and June 2015 for about 105 hours in documented measurements which each lasted between 5 and 26 hours.
We started the aging test on 6 August 2015, keeping the lamp running continuously, and recording spectra twice a day. At each observation time, we recorded spectra of the N2 cell, the C2H2 cell and the combined light from both sources. We used the same experimental setup as for the RV measurements presented in Sect. 5.3. For the first six measurements (first 60 hours of operation), the MIP lamp operated with a stable reflected MW power of
≤1 W. The reflected power, as measured at the MW generator, is the power not absorbed by the gas discharge, but instead reflected back to the generator. After about 72.75 hours, the reflected power increased above 15 W which triggered an automatic shutdown to prevent damage to the MW generator. Several attempts restarting the lamp were not successful.
Although the gas inside the cell glowed when an active high frequency coil was positioned near the cell, a discharge was not sustained by the microwaves. The gas cell was removed from the experimental setup for inspection. No damage was visible at the gas cell or at any other part of the hardware.
We then heated the gas cell in an oven at 300◦C for three hours and, afterward, could restart the discharge. The spectrum of the discharge recorded after the restart exhibited two obvious changes compared to the measurements that had been taken before the lamp shut down: the overall flux was lower and some strong lines appeared. Most of these new strong lines could be assigned to oxygen. The oxygen probably comes from the cell material (quartz glass) and was evaporated during the heating process. Approximately 10 hours after the restart, the lamp shut down again with the same characteristics as described before. We discuss this behavior in Sect. 5.4.2.
In Fig. 5.20, we show the RV curve for the accelerated aging test. We refer to Sect. 5.3.4.1 for a detailed description of the two panels. The RV shifts of the N2 discharge and the C2H2 absorption reference are measured relative to two templates that were recorded at the beginning of the time series. The first six data points represent the spectra that were recorded during the first 60 hours of the aging test. The lamp shut down at the time indicated by the dashed line (about 72.75 hours after the first spectrum). The last data
8This section has been submitted for publication to the scientific journalAstronomy & Astrophysics as Sect. 5 “Aging behavior” in “Near-infrared wavelength calibration with molecular discharge spectra of nitrogen and CN”.
5.4 Aging behavior
point represents the measurement after heating the gas cell and restart of the discharge.
The vertical dashed line at 113.5 hours indicates the second shut down. The stability of the line positions corresponds to an RV scatter of 2.2 m/s. It is noteworthy that the stability is preserved even after the heating process and the restart of the discharge.
−6
−4
−2 0 2 4 6
vrad−vrad[m/s]
N2discharge cell C2H2absorption cell
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time since first spectrum [hours]
−6
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−2 0 2 4 6
δvrad[m/s]
σ=±2.2 m/s
Figure 5.20: Radial velocity measurements during the accelerated aging test. Top panel: The shifts of the template spectra for the discharge cell (blue squares) and for the reference gas cell (red triangles) to match the spectra of the time series with the combined light from both sources. The respective mean is subtracted. The dashed vertical lines indicate the times when the lamp shut off.
Lower panel: The relative RV shift of the two sources.
We also investigated the change in line intensities. In the spectrum of the nitrogen gas discharge, we can distinguish emission lines from atomic and molecular nitrogen. In Fig. 5.21, we show the evolution of the average line intensities for these two species. The values are calculated in the following way: in each of the seven recorded spectra, we determine the line intensities using the line fitting procedure described in Sect. 3.4.2 and the intensity of a line is divided by the intensity of the same line in the first spectrum.
Then, we calculate the average intensity ratio for the lines from each species. Before the first shutdown of the lamp (indicated by the left dashed vertical line in Fig. 5.21), the molecular nitrogen lines remain at a nearly constant intensity. The intensities of the N I lines, however, increase linearly over time. In the last spectrum, lines from both species show intensities lower by a factor of 0.6. This drop in intensity can also be influenced by a slightly different optical alignment of the discharge cell after its removal and reintegration into the experimental setup. We observe a larger scatter in the intensity for the molecular lines in the last spectrum, but there remains a clear difference between the data points from molecular and atomic lines. At this point, we can only speculate whether the change
in the relative intensities between the two species is a symptom of aging, whether it is related to the cause of the lamp’s shutdown, and whether it can be used to predict the remaining lifetime of the lamp.
0 20 40 60 80 100 120
time since first spectrum [hours]
0.4 0.6 0.8 1.0 1.2 1.4 1.6
meanlineintensity(relativetofirstspectrum)
N I
molecular nitrogen
Figure 5.21:Evolution of mean line intensities of atomic and molecular nitrogen relative to the corresponding line intensities in the first spectrum. The dashed vertical lines indicate the times when the lamp shut off.
5.4.2 Discussion of aging behavior
A potential advantage of the MIP lamp is its electrodeless design. This mode of operation eliminates the sputtering process which is one source of aging in HCLs. Nevertheless, the MW powered discharge shut down after about 72.75 hours of continuous operation in the accelerated aging test. We believe that a different aging effect is observed in this test run, which has also been reported for HCLs: the so-called “clean-up” effect that describes the absorption of the gas by the surfaces inside the lamp (e.g., Kerber et al., 2007). In the electrodeless discharge lamp, the gas can be absorbed by the inner walls of the glass tube.
This would lead to a decrease in gas pressure until the pressure is too low to sustain the discharge. If gas absorption by the glass enclosure at the plasma region is the limiting factor for the operation, a larger gas cell with a larger gas reservoir might extend the lifetime.
The heating of the gas cell, as described in the previous section, leads to outgassing of material from the glass. The evaporated material does not only include the original gas components, i.e., nitrogen molecules, but also particles from the glass tube. As the glass