Inside stars and planets, extreme conditions exist where pressures soar to millions of bars and temperatures reach several million degrees. Advanced techniques allow for the reproduction of such states of matter in the lab, but only for fleeting moments and within a minuscule volume. Until now, achieving these conditions required the most potent lasers globally, like the National Ignition Facility (NIF) in California. However, such powerful lasers are scarce, limiting experimental opportunities. Recently, a team of researchers from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), in collaboration with the European XFEL, has made significant strides by creating and observing extreme conditions using a much smaller laser. The breakthrough technology centers around a copper wire that is thinner than a human hair, as reported in the journal Nature Communications.
Up to now, scientists have been directing extremely high-energy laser pulses at a material sample, typically a thin sheet. This intense interaction causes the material’s surface to heat up rapidly, creating a shock wave that travels through the sample. This shock wave compresses and heats the material, momentarily simulating conditions found inside planets or in stellar shells. This brief window is adequate for researchers to analyze the phenomena using advanced measuring techniques, like the ultra-powerful X-ray pulses from the European XFEL located in Schenefeld, Germany, near Hamburg.
At this cutting-edge X-ray laser facility, HZDR is at the forefront of an international user consortium known as HIBEF — Helmholtz International Beamline for Extreme Fields. This group manages a laser at the High Energy Density (HED-HIBEF) experimental station, which generates ultra-short bursts of energy with a relatively low energy output of around one joule. However, the duration of just 30 femtoseconds allows for an incredible output of 100 terawatts. The research team used this laser at HED-HIBEF to target a thin copper wire, just 25 micrometers thick. “We then utilized the powerful X-ray beams from the European XFEL to monitor the events occurring within the wire,” explains Dr. Alejandro Laso Garcia, the lead author of the study. “This unique combination of short-pulse laser and X-ray laser is unprecedented globally. It was solely due to the exceptional quality and sensitivity of the X-ray beam that we were able to witness an unforeseen effect.”
Concentrated shock waves
Throughout several measurement series, the scientists adjusted the timing between the laser pulse impact and the X-rays passing through to capture a detailed “X-ray film” of the sequence. “Initially, the laser pulse interacts with the wire, generating a localized shock wave that propagates through the wire like an explosion, ultimately leading to its destruction,” elaborates HIBEF department head Dr. Toma Toncian. “Before this occurs, some of the high-energy electrons produced by the laser strike race along the wire’s surface.” These fast-moving electrons rapidly heat the wire’s surface, producing additional shock waves that converge towards the wire’s center. For a brief moment, all the shock waves collide in the center of the wire, producing extremely high pressures and temperatures.
The measurements revealed that the density of copper at the center of the wire briefly surged to eight to nine times that of regular, cold copper. “Our computer simulations indicate that we achieved a pressure of 800 megabars,” states Prof. Thomas Cowan, director of the HZDR Institute of Radiation Physics and founder of the HIBEF consortium. “This is equivalent to 800 million times the pressure of the atmosphere and 200 times the pressure found inside the Earth.” The observed temperature was also extraordinarily high by Earth’s standards, reaching 100,000 degrees Celsius.
Perspectives for nuclear fusion
These conditions mirror those found in the corona of a white dwarf star. “Our technique could also replicate conditions similar to those in the interiors of massive gas planets,” Laso Garcia emphasizes. This applies not only to well-known giants like Jupiter but also to numerous distant exoplanets discovered in recent years. The research team is also exploring wires made of alternative materials such as iron and plastic. “Plastic consists mainly of hydrogen and carbon,” remarks Toncian. “And both of these elements are found in stars and their corona.”
The new measurement technique holds promise beyond astrophysics, benefitting another research domain as well. “Our experiment vividly demonstrates how we can create very high densities and temperatures across a variety of materials,” notes Ulf Zastrau, who leads the HED group at the European XFEL. “This represents a significant advancement for fusion research.” Around the globe, several research teams and startups are engaged in the development of a fusion power plant utilizing high-performance lasers.
The concept revolves around delivering intense laser pulses to a frozen hydrogen fuel capsule from multiple angles, igniting it and yielding more energy than was originally supplied. “With our method, we would be able to closely observe the internal processes of the capsule when struck by the laser pulses,” Cowan describes future experiments. “We anticipate that this could greatly influence fundamental research in this area.”
