Topics that shape the future
- Closer look at research results -

The crucial difference between shock waves on Earth and shock waves in space is that the constituent particles of the colliding group are plasma (a gas of ionized charged particles, such as electrons and protons moving around freely) in space, and its density is very low. Compared to the density of air on Earth, the density of plasma in space is about one trillionth, or even one ten millionth of that. If it is that low, it seems that the constituent particles will not collide with each other. "Shock waves are created because of collisions, but they don't seem to collide. But shock waves exist in space. Why is that?" With this question in mind, shock waves that form in the dilute plasma in space are called "collisionless shock waves." This term conveys the mystery of the phenomenon by combining "collisionless," which means "no collision," and "shock waves that are created because of collisions." Even though they are unlikely to collide, it is true that shock waves exist in space, so it is thought that we humans just have not unraveled their mechanism, but despite various research efforts to date, it has not yet been fully elucidated. During this collision process, some of the constituent particles gain energy and become cosmic rays. Cosmic rays are one of the basic building blocks of the universe, and elucidating the origin of cosmic rays is one of the major themes in astrophysics, but the process itself is not well understood. In order to get closer to this mystery, I have been conducting theoretical research, observational research, and large-scale simulation research using supercomputers, aiming to elucidate the physical processes that occur in shock waves, and in the process, I came across a field of research called "laboratory astrophysics."

Astronomical phenomena occur at the edge of space, far away from Earth, so it is impossible to travel to the site. However, if we could create similar conditions to astronomical phenomena in experimental facilities on Earth, we would be able to control even the smallest of conditions at hand, and we would be able to obtain a much larger amount of experimental data than we could obtain from space observations. This idea was the origin of laboratory astrophysics. This is attracting attention in the field of astrophysics as the third new research methodology after space observations, theory, and simulations. Advances in laboratory astrophysics may help elucidate previously unexplained phenomena and physical processes.

Since 2014, we have been working with Osaka University, Kyushu University, Toyama University, and others to conduct shock wave generation experiments to elucidate the physical processes of shock waves using the large laser "GEKKO Ⅻ" at the Institute of Laser Engineering, Osaka University, which has one of the world's highest power outputs. We proposed an experiment to generate collisionless shock waves to the institute's joint use and collaborative research, which is open to the public every year, and the proposal was accepted and we began the experiment. Professor Shuichi Matsukiyo of Kyushu University and I, as the principal investigators, were each given one week of machine time (*) to conduct the experiment, and we accumulated know-how together. In the experiment, in order to reproduce the same conditions as in outer space in the laboratory, the device is filled with nitrogen gas and a strong magnetic field is applied. In this state, when a large laser is irradiated on an aluminum plate target, a blast of aluminum that has been turned into plasma spreads. This blast compresses the surrounding nitrogen that has been turned into plasma, generating shock waves similar to those in outer space. This has the advantage of being able to measure the parameters (conditions) of the shock wave with high precision compared to conventional shock wave generation methods. By being able to actively control the parameters and ensuring reproducibility, there is a possibility that research in this field will progress significantly.

In addition to myself, Assistant Professor Shuta Tanaka and undergraduate and graduate students also participated from Aoyama Gakuin University in the experiment, and we were responsible for everything from designing the experimental equipment (such as the shape of the magnetic field generating coil, the design of the aluminum plate target, and the placement of the measuring equipment) to acquiring experimental data, analyzing the data, and discussing physical interpretations. Researchers and students from Kyushu University, Hokkaido University, Toyama University, and Osaka University also participated in the experiment, and a team of about 20 people in total acquired experimental data. At the theoretical interpretation stage, researchers from Nagoya University, Tohoku University, and the University of Tokyo also joined the team.
In an experiment conducted in 2019, we succeeded in generating space plasma shock waves in a laboratory. Although similar research had been conducted in Europe and the United States, this was the first time that space plasma shock waves had been generated in a laboratory on Earth under conditions identical to those in outer space. In 2022, the results of the data analysis obtained from this experiment were published in two papers in the American scientific journal "Physical Review E." The lead author of one paper is Professor Matsukiyo of Kyushu University, and the lead author of the other paper is me.

*Machine Time: The period during which the large laser "Gekkou XI" can be used.