01 Apr 2019

Experimenting at ELI: productive and pleasant

What kinds of user experiments are typically conducted at ELI? And how do the users look back on their experience of using the facilities at either of the ELI facilities? Marcel Mudrich from Aarhus University in Denmark has visited ELI-ALPS twice so far; first in August 2018 and again in January this year. ‘I was very impressed with the exceptional welcoming atmosphere, the user friendliness, and the dedication of both the supporting staff and the scientists.’

Mudrich got informed about the possibility of experimenting with the mid-InfraRed laser at ELI by his former colleague Giuseppe Sansone, who is a Scientific Advisor at ELI-ALPS. They jointly conducted two rounds of experiments, together with Frank Stienkemeier from the University of Freiburg.

The two experiments were aimed at gaining a fundamental understanding of the ignition and charging of nanoplasmas formed out of doped rare gas droplets and clusters. ‘The mid-IR laser at ELI-ALPS has some quite unique properties in terms of the wavelength, pulse energy, and repetition rate, which were important for our experiment. So we moved the dedicated droplet set-up that we built oirselves from scratch ourselves from Germany to Hungary during the summer of 2018, and left it there until our second round of experiments last January,’ Mudrich says.

Efficient ignition of nanoplasmas
In the first round of experiments, the team studied the behavior of dopant-containing helium droplets exposed to mid-IR pulses. Last January, they returned to do similar experiments with neon. ‘Our niche field is to study dopant effects and compare them in different gases. Where under our experimental conditions helium is a liquid with superfluid properties, heavier rare gases like neon take up a crystalline, solid form. In the superfluid helium, the dopants can move around without any friction and are able to aggregate into unusual cluster structures, which turn out to be beneficial for the ignition process. In fact, we demonstrated that doping helium nanodroplets with calcium and xenon atoms leads to a cascading effect, that contributes to a very efficient ignition of the plasma even at lower laser intensities. In the second set of experiments, we wanted to compare that behavior with that of dopants in neon. Because of its solid crystalline structure, the dopants will stay on the surface, and we wanted to see how that influences the ignition process.’

Help to become productive
Mudrich was, in his own words, ‘generally positively surprised about the welcoming, helpful environment at ELI. Since we went there with a small team, we really benefited from their help with setting up the apparatus. This enabled us to become very productive in the few days we were there.’ During his stay, ELI-ALPS was still in the setup phase, so some things will probably change in the near future, he expects. ‘For example, the electronics workshop was not that well-equipped yet, and some regulations were perhaps still too strict. We weren’t allowed to access support systems such as cooling water supply unattended. Though we ourselves didn’t experience any problems with that, since everyone was available whenever we needed them, I can imagine that when all systems are operational and schedules are tight, such strict rules might cause delays.’
The research project these experiments are part of will run for another three years, Mudrich says. ‘But since the PhD student involved is currently writing up his thesis, we are now looking for new personnel. As soon as we have new people, we can start making plans for new experiments, and look for the most suitable laser facility to conduct them at. I had a very productive and pleasant time at ELI, so I would love to come back sometime.’

Results of the helium experiment: Typical single-hit electron velocity-map images (left column) and the correlated ion time-of-flight spectra. The data were recorded at the same experimental conditions, and sorted according to signal intensity.