This laser turns metal into a star-like plasma in trillionths of a second

To achieve this, the team combined two advanced laser systems: an X-ray free-electron laser and the high-intensity optical laser ReLaX. Both were used at the HED-HiBEF experimental station at the European XFEL in Schenefeld near Hamburg. Their work provides new insight into how high-energy lasers interact with matter under extreme conditions. It also introduces a promising method for improving diagnostics in laser fusion research.

Tracking Ionization in Trillionths of a Second

Ionization unfolds incredibly fast, within picoseconds, or just a few trillionths of a second. Capturing such rapid changes requires even shorter laser pulses.

"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.

With these ultrashort pulses, researchers could observe how plasma forms and evolves almost in real time.

Turning a Copper Wire Into Superhot Plasma

The experiment begins with an intense burst of light striking a very thin copper wire, about one-seventh the thickness of a human hair. The energy delivered is immense, reaching about 250 trillion megawatts per square centimeter over a tiny area for an extremely brief moment. Such conditions are usually found only in extreme cosmic environments, such as near neutron stars or during gamma-ray bursts.

The copper wire instantly vaporizes, producing plasma with temperatures of several million degrees. As this happens, copper atoms lose multiple electrons and become highly ionized.

Researchers then use a second laser pulse, called the probe pulse, to examine the plasma. This pulse, generated by the European XFEL, emits an intense flash of hard X-rays. By recording how these X-rays interact with the plasma, scientists can capture a sequence of snapshots, similar to frames in a movie. This pump-probe approach allows them to follow the plasma's evolution step by step.

Measuring Highly Charged Copper Ions

The X-ray pulses are carefully tuned to interact with Cu²²⁺ ions, copper atoms that have lost 22 electrons. The photon energy of 8.2 kiloelectronvolts matches a specific electronic transition in these ions, a process known as resonant absorption.

After absorbing the X-rays, the ions emit their own distinctive 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."

A Precise Timeline of Plasma Evolution

The measurements reveal a clear sequence of events. Right after the laser hits the wire, Cu²²⁺ ions begin to form. Their numbers rise quickly and reach a peak after about two and a half picoseconds. After that, recombination begins, and the number of ions steadily declines. Within roughly ten picoseconds, these highly charged ions disappear completely.

"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.

Electron Waves Drive the Process

Computer simulations helped the researchers understand what drives this behavior. The initial laser pulse strips only a few electrons from the copper atoms. These electrons carry high energy and move through the material like a wave, knocking additional electrons free from neighboring atoms.

"They are so energy rich that they spread out like a wave and knock ever more electrons out of neighboring copper atoms," explains Cowan.

Over time, these electrons lose energy and are gradually recaptured by the ions. As recombination continues, the atoms return to a neutral state.

Implications for Laser Fusion Research

"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. Accurate simulations are essential for designing efficient and reliable laser fusion reactors in the future.