Extreme conditions prevail inside stars and planets. The pressure reaches hundreds of thousands of bars, and it could possibly be several million degrees hot. Sophisticated methods make it possible to create such states of matter within the laboratory — albeit just for the blink of a watch and in a tiny volume. To this point, this has required the world’s strongest lasers, corresponding to the National Ignition Facility (NIF) in California. But there are only a number of of those light giants, and the opportunities for experiments are correspondingly rare. A research team led by the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), along with colleagues from the European XFEL, has now succeeded in creating and observing extreme conditions with a much smaller laser. At the guts of the brand new technology is a copper wire, finer than a human hair, because the group reports within the journal Nature Communications.
To this point, experts have been firing extremely high-energy laser flashes at a fabric sample, often a skinny foil. This causes the fabric on the surface to heat up suddenly. This creates a shock wave that races through the sample. It compresses the fabric and heats it up. For a number of nanoseconds, conditions arise like those in the inside of a planet or within the shell of a star. The tiny time window is sufficient to review the phenomenon using special measuring techniques, corresponding to the ultra-strong X-ray flashes of the European XFEL in Schenefeld near Hamburg, Germany.
Here, at Europe’s strongest X-ray laser, the HZDR leads a world user consortium called HIBEF — Helmholtz International Beamline for Extreme Fields. Amongst other things, this consortium operates a laser on the High Energy Density (HED-HIBEF) experimental station, which generates ultra-short pulses that do not need particularly high energy — only about one joule. Nonetheless, at 30 femtoseconds, they’re so short that they achieve an output of 100 terawatts. The research team used this laser at HED-HIBEF to fireplace at a skinny copper wire, just 25 micrometers thick. “Then we were capable of use the strong X-ray flashes from the European XFEL to watch what was happening contained in the wire,” explains Dr. Alejandro Laso Garcia, lead creator of the paper. “This mixture of short-pulse laser and X-ray laser is exclusive on the earth. It was only due to the prime quality and sensitivity of the X-ray beam that we were capable of observe an unexpected effect.”
Concentrated shock waves
In several series of measurements, the scientists systematically varied the time interval between the impact of the laser flash and the X-rays shining through. This made it possible to record an in depth “X-ray film” of the event: “First, the laser pulse interacts with the wire and generates a neighborhood shock wave that passes through the wire like a detonation and ultimately destroys it,” explains HIBEF department head Dr. Toma Toncian. “But before that, among the high-energy electrons created when the laser hits, race along the surface of the wire.” These fast electrons heat up the surface of the wire quickly and generate further shock waves. These then run in turn from all sides to the middle of the wire. For a temporary moment, all of the shock waves collide there and generate extremely high pressures and temperatures.
The measurements showed that the density of the copper in the course of the wire was briefly eight to nine times higher than in “normal,” cold copper. “Our computer simulations suggest that now we have reached a pressure of 800 megabars,” says Prof. Thomas Cowan, director of the HZDR Institute of Radiation Physics and initiator of the HIBEF consortium. “That corresponds to 800 million times atmospheric pressure and 200 times the pressure that prevails contained in the earth.” The temperature reached was also enormous by terrestrial standards: 100,000 degrees Celsius.
Perspectives for nuclear fusion
These are the conditions which can be near those within the corona of a white dwarf star. “Our method is also used to attain conditions like those in the inside of giant gas planets,” emphasizes Laso Garcia. This includes not only well-known giants like Jupiter, but additionally a lot of distant exoplanets which were discovered over the past few years. The research team has now also set its sights on wires fabricated from other materials, corresponding to iron and plastic. “Plastic is principally fabricated from hydrogen and carbon,” says Toncian. “And each elements are present in stars and their corona.”
The brand new measurement method shouldn’t only be useful for astrophysics, but additionally for one more field of research. “Our experiment shows in a powerful way how we are able to generate very high densities and temperatures in a wide selection of materials,” says Ulf Zastrau, who heads the HED group on the European XFEL. “This can take fusion research a very important step further.” Several research teams and start-ups all over the world are currently working on a fusion power plant based on high-performance lasers.
The principle: Strong laser flashes hit a fuel capsule fabricated from frozen hydrogen from all sides and ignite it, with more energy coming out than was put in. “With our method, we could observe intimately what happens contained in the capsule when it’s hit by the laser pulses,” says Cowan, describing future experiments. “We expect that this could have a big impact on basic research on this area.”
