“If a critical mass is reached, a new star will burn”

— Sergey Alekseevich, in your last work, together with your Russian and foreign colleagues, you managed to simulate a supernova explosion in the laboratory. What is the significance of such experiments for scientists?

— To be precise, the experiment is not about modeling the supernova explosion itself, but about the passage of a shock wave from such a flash through a cloud of plasma. Experiment and work itself belong to the direction of laboratory astrophysics. It’s a very strange combination of two far-flung extremes – massive astrophysical phenomena in time and space, and their investigations in laser plasma fired at miniature microtargets. Most astrophysical objects are plasma. Almost any space object can be considered plasma, with the exception of the sun, dust clouds – asteroids, which are the surface layers of stone planets.

And the hydrodynamics of a plasma object, the physics of such processes, turns out to be well scalable while maintaining a certain set of invariants. If the ratio of two or more hydrodynamic quantities, for example, the Mach number, Strouhal number, or Reynolds number, is preserved in the transition from a large astrophysical object to a small laboratory object, then the course appears. similar to hydrodynamic processes.
And if we take such a laboratory object as a laser plasma created in matter due to the action of a powerful nanosecond laser pulse, such a plasma would be a very good scale object for a large number of astrophysical objects:

young stars, binary star systems, plasma jets, even supernova explosions.

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Sergei Pikuz

From personal archive

– What questions of astrophysics can be answered in this way?

— The main difficulty in observational astronomy is that you look at an object and see its instantaneous frame. And from this framework we cannot trace what has happened in the past, what will happen in the future – we can only assume. In the lab, under more or less controlled conditions, we can trace all this evolution from start to finish and thus test the existing or propose a new theoretical model of an astrophysical phenomenon.

For example, an important parameter for explaining the evolution of galaxies and star clusters is the probability and velocity of stars igniting from protostar nebulae. For the formation of a star, a density fluctuation in a rare medium is necessary – an embryo, which will become the center of gravity. If during the gravitational process a critical mass accumulates to ignite a thermonuclear reaction, then a new star will burn.

Thus, the Sun is a third-generation star born from matter that was once part of stars of previous generations.
One of the important mechanisms for the emergence of stellar embryos may be the passage of a shock wave from a supernova explosion through the medium. The role of such a mechanism can be tested in a laboratory experiment in plasma produced by high-power nanosecond laser pulses.

What is an experimental setup?

— This experiment was carried out in the framework of international cooperation with the Higher Polytechnic School at the LULI2000 laser plant in France. In the vacuum chamber we have several solid targets on which powerful laser beams are focused.
One or two nanosecond laser pulses with 500 J energy generated irradiated carbon rods and a stream of plasma heated to several million degrees. The plasma diffusion region was filled with helium and the concentration of gas molecules was chosen based on the problem of scaling to the density of matter in the interstellar medium. As the plasma dissipated at a supersonic speed, they created a compressed region – a shock wave – in the gas.

In contrast, a foamed plastic ball was placed in the path of the shock waves, simulating density fluctuations in the interstellar medium.
Another laser focuses on a titanium wire, which causes a bright X-ray source of short, picosecond duration. We need X-rays to illuminate the entire volume of the interaction of the plasma wave and the obstacle ball, to get a shadow radiographic picture of what’s there. The short picosecond duration of the X-ray flash allows us to obtain frame-by-frame images at different points in time, and as a result, a beautiful and informative “film” of how the shock wave is formed, how it interacts. a barrier and how the “embryo” evolved from such an effect.

What happens to the ball when the plasma enters, does it evaporate?

— The impact of a shock wave on the ball should lead to its compression, breaking of symmetry as well as the development of turbulences in the plasma flow. The processes we observe take place in timescales of hundreds of nanoseconds, during which time the ball, on the contrary, contracts, increasing in density. And only later, when the energy of the shock wave is converted into heat, the ball begins to dissipate. After all, the ball evaporates, but not when we think about it.

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– What conclusions did you reach?

“We have shown the extent to which the passage of a plasma shock wave (simulating the shock wave from a supernova) can compress a relatively low-density medium, thereby increasing the probability of a stellar embryo emerging from such a group. In the experiment, we were able to obtain visual radiographic images of the medium at various stages of compression. .

We were able to show that the increase in impact due to the overlap of the two shock waves increases the degree of compression of the substance by 30%.

Further interpretation of the obtained data will make it possible to draw conclusions about the accuracy of the models describing the rate of star formation in various star clusters near supernova remnants.
– You managed to conduct an experiment before the events in Ukraine. How will they affect further cooperation with Europeans?

— The experiment was carried out at the beginning of 2021. We are not just on this matter, but all of our science is based on international cooperation, not just with Europe. Of course, these are extremely difficult times for our activities. Everything about our possibility of further work on foreign facilities, even the preparation of joint publications, is a big question. On the side of the scientific and political leadership of foreign partners, there is strong opposition, a demand to freeze cooperation and sometimes to break relations.

Everything now depends quite strongly on the country, on a particular organization or institution. European organizations react particularly harshly, while in other continents decisions are taken more rationally. At the same time, I expect that everything will become homogeneous in the near future and that common solutions will be developed in the international scientific community, and that very negative solutions will be developed for us.

– Have you been denied access to European facilities?

– The custom LULI2000 setup used in the article discussed in Ecole Polytechnique is now really closed to us. Moreover, apparently, the ban applies not only to employees of Russian institutions, but also to Russian citizens in general. A long-term solution, unfortunately I don’t expect the restrictions to be lifted in the next few years. This was a very important research point for us. My closest colleagues and I participated in 3-5 experiments per year in this setup.

The situation of the German partners is also critical. Despite the fact that for several decades Russia simultaneously participated in the creation and financing of several large and unique experimental complexes in Germany, now access to them is also closed, all experiments with Russian participation have been canceled. The Helmholtz Society, which is an operator or a key participant in these mega-science projects, prohibits its employees from contacting scientists working in Russian organizations and planning new experiments.

– And even publish in joint research?

– Helmholtz – yes. Perhaps over time the situation will become easier, but at the moment some of our colleagues in Germany are prohibited from publishing co-authored articles with scientists from Russian institutes, regardless of when the results were obtained and at what facilities.

In general, it is especially characteristic for our field of science – laser plasma physics, studies of matter in the extreme state – to conduct work in the framework of wide international cooperation. In this way, it is possible to work most efficiently and have a small scientific group publish several dozen articles a year in good journals. The disconnection in international cooperation is a huge problem for us and will inevitably lead to a “downfall” in the level of work we do.

— Are there similar facilities in Russia?

– Currently no, although Russia has sufficient competence in the production of laser equipment. Even under favorable conditions, the creation of such an installation will take at least five years and will cost a total of 1.5-2 billion rubles. At the same time, precisely such a task – the construction of a multi-kilojoule laser complex and an experimental platform for modeling astrophysical phenomena – is now actively being addressed as part of the MEPhI University’s strategic development project. We also place great hope in the effectiveness and support of the recently established National Center for Physics and Mathematics, where the topic of laboratory astrophysics has been selected as one of the priorities. First of all, with the provision of modern diagnostic equipment, difficulties will inevitably arise. Let’s see what we can do.