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When laser flashes hit matter, electrons are knocked off their orbits around the atomic nuclei. This can generate extremely hot plasmas composed of charged particles – ions and electrons. As they report in the journal Nature Communications, researchers at HZDR have now observed this ionization process in more detail than ever before. To do so, they combined two state-of-the-art lasers: the X-ray free-electron laser and the high-intensity optical laser ReLaX at the HED-HiBEF experiment station at the European XFEL in Schenefeld, near Hamburg. Their findings not only deliver fundamental insights into the interaction of high-energy lasers and matter under extreme conditions.
Ionization takes place extremely quickly – in picoseconds, within a few trillionths of seconds. In order to monitor this process in detail, laser pulses must be significantly shorter. “These are exactly the conditions provided by the two lasers that have pulse durations of just 25 and 30 femtoseconds – that is, trillionths of a second,” explains Dr. Lingen Huang, head of experimentation in HZDR’s Division of High-Energy Density.
Initially, an extremely intense flash of light strikes a delicate copper wire that is only about one-seventh the thickness of a human hair. The pulse intensity is approximately 250 trillion megawatts per square centimeter – concentrated on a tiny surface for an extremely short time. Values like this are otherwise achieved only under exceptional conditions, such as in extreme astrophysical environments like the immediate vicinity of neutron stars or during gamma-ray bursts.
The wire vaporizes instantly, creating plasma at a temperature of several million degrees. In the process, the copper atoms lose many of their electrons – they become ionized multiple times. The so-called pump pulse, which produces the plasma, is followed at short, variable intervals by a second flash of light, the probe pulse. This is an extremely brilliant flash in the hard X-ray spectrum, generated by the European XFEL. Its interaction with the plasma is recorded by a detector. Just as with a camera, snapshots of the process are produced. By using this pump-probe method, whereby the first pulse kick-starts the process and the second “probes” it after a time delay, the researchers can monitor the dynamics within the plasma step by step.
The energy of the X-ray pulses is precisely tuned so that it is primarily absorbed by Cu²²⁺ ions, that is, by copper atoms that are missing 22 electrons. The photon energy of 8.2 kiloelectronvolts corresponds exactly to a specific electronic transition in these ions. Physicists refer to this as resonant absorption.
After absorption, the copper ions emit their own characteristic X-ray radiation. “In our pump-probe experiment, we exactly measure the temporal development of this stimulated X-ray emission,” says Huang. “Because it shows us how many Cu²²⁺ ions are present in the plasma at any given time.”
First the laser pulses, then the heating electron waves
The results reveal that the process has a distinct time structure: immediately after the laser has hit the copper wire, the first Cu²²⁺ ions develop. The number increases rapidly and reaches a maximum after about two and a half picoseconds. Due to recombination processes, the number of ions then decreases again. Within just about ten picoseconds, Cu²²⁺ ions can no longer be detected. “No one has ever looked at this type of ionization so precisely before,” says Prof. Tom Cowan, former director of the Institute of Radiation Physics at HZDR.
Assisted by sophisticated computer simulations, the physicists working with Cowan and Huang also know the reasons for this ionization process. The first intensive laser pulse acts as a trigger and robs the copper atoms of just a few electrons. “They are so energy rich that they spread out like a wave and knock ever more electrons out of neighboring copper atoms,” explains Cowan. But gradually the electrons sort of run out of steam. They are recaptured by the copper ions. At the end of this recombination process, the copper atoms have become neutralized once again.
“This experiment demonstrates how powerful our lasers are and paves the way for future laser fusion facilities,” concludes Dr. Ulf Zastrau, who is responsible for the HED-HIBEF experiment station at the European XFEL – because laser fusion is also based on extremely hot plasmas that are heated up by lasers and the resulting electron waves. “Thanks to our new concrete findings, we can now focus on continuing to refine our simulations of these processes,” explains Zastrau. And they are essential for precisely designing a laser fusion reactor.
Publication:
L. Huang et al.: Probing ultrafast heating and ionization dynamics in solid density plasmas with time-resolved resonant X-ray absorption and emission, Nature Communications, 2026. (DOI: 10.1038/s41467-026-71429-5)
Additional information:
Dr. Lingen Huang
Institute of Radiation Physics at HZDR
Phone: +49 351 260 2231 | Email: lingen.huang@hzdr.de
Media contact:
Simon Schmitt | Head
Communications and Media Relations at HZDR
Phone: +49 351 260 3400 | Mobile: +49 175 874 2865 | Email: s.schmitt@hzdr.de
Dr. Bernd Ebeling | Group Leader and Press Spokesperson
Communication Group at European XFEL
Tel.: +49 40 8998 6921 | Email: bernd.ebeling@xfel.eu
The Helmholtz-Zentrum Dresden-Rossendorf (HZDR) performs – as an independent German research center – research in the fields of energy, health, and matter. We focus on answering the following questions:
• How can energy and resources be utilized in an efficient, safe, and sustainable way?
• How can malignant tumors be more precisely visualized, characterized, and more effectively treated?
• How do matter and materials behave under the influence of strong fields and in smallest dimensions?
To help answer these research questions, HZDR operates large-scale facilities, which are also used by visiting researchers: the Ion Beam Center, the Dresden High Magnetic Field Laboratory and the ELBE Center for High-Power Radiation Sources.
HZDR is a member of the Helmholtz Association and has six sites (Dresden, Freiberg, Görlitz, Grenoble, Leipzig, Schenefeld near Hamburg) with almost 1,500 members of staff, of whom about 700 are scientists, including 200 Ph.D. candidates.
Dr. Lingen Huang
Institute of Radiation Physics at HZDR
Phone: +49 351 260 2231 | Email: lingen.huang@hzdr.de
L. Huang et al.: Probing ultrafast heating and ionization dynamics in solid density plasmas with time-resolved resonant X-ray absorption and emission, Nature Communications, 2026. (DOI: 10.1038/s41467-026-71429-5)
https://www.hzdr.de/presse/probing_hot_plasma
The XFEL photon energy was carefully tuned to match a specific electronic transition in highly charg ...
Source: B. Schröder
Copyright: B. Schröder/HZDR
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