What is the High harmonic generation
In the field of spectroscopy, High Harmonic Generation (HHG) is a way of energy converting a commercial ultrafast laser with pulse durations of 10s of femtoseconds from the infrared/optical into the extreme ultraviolet or x-ray range. Using light at extreme ultraviolet and x-ray energies gives us the ability to deeply probe the electrons present in atoms, the bonds of molecules and the band structure of solids. Since HHG preserves the temporal characteristics of the original laser in the femtosecond range, we can also study how these electrons change in almost real time of chemical reactions and phase transitions. We frequently perform this in the HELIOS laboratory at Ångströmlaboratoriet in Uppsala University.
An environmental dilemma
High harmonic generation works through focusing our ultrafast laser into a cell that is held under vacuum and filled with a gas. Typically we, like many other groups around the world, use a noble gas for that because with that we get the highly intense x-ray pulses we need for our experiments. The intense laser interacts with the gas and high harmonic up-conversion occurs in a complicated so-called non-linear process. The environmental dilemma surrounds the fate of the noble gas as it continuously, over days, weeks, and months, dissipates from the cell and into the vacuum from where it is pumped away and is lost forever to the exhaust.
Noble gases are un-reactive elements of the periodic table. An implication of this is that they cannot be produced from chemical reactions and are instead extracted from the liquefaction of air (in the case of Neon, Argon and krypton), extracted from methane wells ( as is the case for Helium) and radon is a bi-product of radium alpha decay.
At Ångströmlaboratoriet in Uppsala university, Helium and Argon are amongst other gases bought in high quantities and supplied as communal gases though a gas piping system. The HELIOS lab, run by the division of X-ray Photon Science at the Department of Physics and Astronomy is a regular and extensive user of these gases, where we typically consume 0.05 standard Litres per min (SLM) of Argon and 0.2 SLM of Helium.
We decided to investigate the idea of saving on the noble gases used in the HHG process by recycling them without disturbing the non-linear conversion process. The existing system works off gas being removed from the cell containing chamber by a Turbomolecular pump, which compresses the gas into a pressure pipe called the fore vacuum line. This gas was previously removed by a Scroll Pump and released.
The original solution
The original solution was to connect the exhaust of the scroll pump back into the gas inlet and to have the system operate in a closed loop thereby conserving the majority of the initial noble gas (accepting minor losses from noble gases straying into the rest of the vacuum system). To this effect, a pressure gauge, which could measure between 0.8 mbars and 10,000 mbars was purchased with the grant money along with a digital flow meter which allowed us to evaluate how we have improved out gas consumption. The closed system concept was tested with two brands of pump, both the Scrollvac SC15D and the Edwards nxds15i-c. The system was built as follows:
The first observation when operating the new system according to the first solution was that when pump 2 was the Scrollvac SC15D we were not able to achieve as good a vacuum as when pump 2 was the Edwards nxds15i-c. With the Edwards pump we could reach a lower pressure of 2.7×10-2 mbar, while with the Scrollvac we could only reach 3.8×10-1 mbar. This over a factor of 10 in the absolute pressure suggests that the Scrollvac pump leaks more, which would lead to contamination of the recycling system. So from this point we only used the Edwards pump as pump 2.
Next we tried to run the system as planed in figure 1.0 with pump 2 circulating gas. We observed immediately that the pump seemed to leak contaminants of some sort into the vacuum loop. We investigated that and see in figure 2.0 the leak was at a rate of approximately 0.3 mbar/hour.
We attempted to further improve this by bringing our Edwards nxds15i-c pump to the Uppsala University mechanical workshop where we leak tested the pump. It was found that the pump leaked at all of its entry points (the entrance, the exhaust and the ballast), but not at the KF vacuum connectors, but at the connection between the pump and the KF nipple. The problems seemed to be that the o-rings shipped with the pump were too thin and did not form a good seal. We ordered a set of thicker o-rings (210463323F80 O-RING 28.0X2.0/FPM80) from TeknikProdukter which we installed in the connection between the pump and the KF nipple. We also replaced the ballast with the o-ring included in the ballast replacement kit, and we bought a 3/4′ pipe blank G-166 from slängservice to seal it. The pump went from being able to reach 3×10-6 mbar to 2×10-9 mbar, which is a 2000 times improvement.
Testing the improved pump
We figured that by improving the absolute pressure of the pump by 2000 and being unable to find leaks that the system could now be able to run much better. This system in figure 1 was rebuilt and we assessed the improvements to the leak rates.
Although the pump could generate a vacuum of 2×10-9 mbar we are still seeing a leak on the 10-2 mbar scale. This is likely to be due to limits in the KF vacuum component standard and this began making us believe that this closed system may not work, at least with our current KF components. We decided to try and connect the closed system to the HHG source and see what effect this contamination would have on the non-linear conversion process.
The good point about the results in figure 5.0 is that no gas was consumed during this test. However, the loss of 25% of signal (photons per second) over a 15 hour period is unideal from an experimental stand point. This loss would rush us to finish an experiment before the gas in the now closed loop would need to be changed and we would no longer be able to guarantee that all conditions stayed the same during the experiment.
A compromise to the closed system
As a modification to our original plan we decided that we could find a compromise between the gas consumption and stability by continuously pumping out some of the contaminants and allow a corresponding input of a small amount of Argon gas to compensate for the loss. The idea was to accept a slightly higher gas consumption than in the original solution but keeping the noble gas system cleaner and still consume much less noble gas than in our current implementation. The original closed system shown in Figure 1.0 was modified to that shown in figure 6.0 and 7.0.
In the new implementation, we tested the system with the light source running almost unaffected for 20 hours, after which we observed fluctuations in the gas pressure system in Ångströmlaboratoriet. If we look at the difference between the new and original solutions we see that the system was a lot more stable over a much longer period of time in the new implementation. The variation in the gas cell pressure changed by almost 2.5 mbars per hour in the closed system case compared to a total random variation of 8.6 mbars over around 53 hours in the semi pumped case. The number of HHG signal (photons per second) produced is seen in Figure 8 (a) to decrease as a response to the change in pressure at around hour 30 and re-stabilising shortly after the gas pressure varied. Overall, the signal variation over the entire 53 hours was now approximately 10 % (compared to a 25% decrease over 15 hours in the original closed-loop solution). We thus find that the system runs a lot better with the semi-pumped new system. The result of running the system in this semi-pumped configuration was a reduction of the argon gas consumption from 0.052 SLM to 0.0195 SLM, a reduction by a factor of 2.
Outlook
Overall, with our investigations within this sustainability project we were able to cut down on the amount of noble gas consumed in our high-harmonic generation (HHG) set up by about a factor of 2, keeping the signal rather stable.
Applying the developed solution to generating HHG with Helium gas will be a more ambitious task as a lot more Helium gas is required to produce HHG radiation. We started with that but reached a problem with the exhaust of the recycling pump reaching very high pressures (over 1.5 bars) and the pump begin making a lot of noise. A redesign of the system could help in resolving this, if we were to change the exhaust system to a Swagelok (one more suitable for gases and keeping volumes low) it may be possible to recycle the system more effectively for both Helium and Argon.
Another option could be to go back to the closed system and investigate whether a Swagelok system could reduce leaking there too. If the exhaust system could be brought to a level of ultra high vacuum, it could be possible to get rid of this 2.5 mbar per hour leak, and reduce it significantly. Such systems can run at 10-9 or lower and would not leak on the mbar scale. To do this we would deed to redesign the vacuum system to include a turbo pump and ideally change the system away from the KF standard to either a Swagelok or a CF standard. In these standards rubber o rings are not used and therefore the system can be backed, removing all wall contaminants which can impact the vacuum.
The implementation we realized here as well as the future directions for further improvements, that we identified with our investigations, hold great promise to considerably reducing the consumption of noble gases in our set up, which will undoubtedly be of great interest for numerous groups in the world that operate similar set ups.
This article was provided by the HELIOS lab at the Department of Physics and Astronomy, Uppsala University and the network is grateful for their contibution and hard work to recycle noble gases!
Towards Greener Research 
Thanks for sharing this story.
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