When a powerful laser flash hits a material, electrons can be knocked out of positions around the nucleus. This process creates plasma, a very hot state made up of charged particles known as ions and electrons. Researchers at Dresden-Rossendorf-Helmholtzzentrum (HZDR) have reportedly captured this ionization process in unprecedented detail. nature communications.
To achieve this, the team combined two advanced laser systems: an X-ray free electron laser and a high-intensity optical laser ReLaX. Both were used at European XFEL’s HED-HiBEF experimental station in Schönefeld, near Hamburg. Their research provides new insights into how high-energy lasers interact with matter under extreme conditions. We also present promising methods for improving diagnostics in laser fusion research.
Track ionization in trillionths of a second
Ionization is incredibly fast, occurring within picoseconds, or just a trillionth of a second. Even shorter laser pulses are required to capture such rapid changes.
“These are precisely the conditions provided by two lasers with pulse durations of just 25 femtoseconds and 30 femtoseconds, or trillionths of a second,” explains Dr. Lingen Huang, head of experiment at HZDR’s High Energy Density Division.
These ultrashort pulses allow researchers to observe how plasma forms and evolves in near real time.
Turn copper wire into super hot plasma
The experiment begins by shining intense light onto a very thin copper wire, about one-seventh the thickness of a human hair. The energy provided is enormous, amounting to about 250 trillion megawatts per square centimeter in a small area for a very short period of time. Such conditions are typically found only in extreme space environments, such as near neutron stars or during gamma-ray bursts.
The copper wire evaporates instantly, creating a plasma with temperatures in the millions of degrees. When this happens, the copper atom loses multiple electrons and becomes highly ionized.
Researchers then examine the plasma using a second laser pulse, called a probe pulse. This pulse, produced by Europe’s XFEL, emits a powerful flash of hard X-rays. By recording how these X-rays interact with plasma, scientists can capture a series of snapshots, similar to frames in a movie. This pump-and-probe approach allows us to follow the evolution of the plasma step by step.
Measurement of highly charged copper ions
The X-ray pulses are carefully tailored to interact with Cu²²⁺ ions, which are copper atoms that have lost 22 electrons. The photon energy of 8.2 kiloelectron volts corresponds to specific electronic transitions in these ions, a process known as resonant absorption.
After absorbing the X-rays, the ions emit their own X-ray radiation.
“Our pump-probe experiment precisely measures the temporal development of this stimulated X-ray emission,” Huang says. “Because it tells you how many Cu22+ ions are present in the plasma at any given time.”
Precise timeline of plasma evolution
Measurements reveal a clear sequence of events. As soon as the laser hits the wire, Cu22+ ions begin to form. The number increases rapidly and peaks after about 2.5 picoseconds. Then recombination begins and the number of ions steadily decreases. Within approximately 10 picoseconds, these highly charged ions disappear completely.
“No one has ever observed this kind of ionization so precisely before,” says Professor Tom Cowan, former director of the HZDR Radiation Physics Laboratory.
Electron waves drive the process
Computer simulations have helped researchers understand what causes this behavior. The first laser pulse removes only a few electrons from the copper atom. These electrons carry high energy and travel through the material like waves, knocking additional electrons off neighboring atoms.
“They’re so energetic that they spread out in waves and knock more electrons out of neighboring copper atoms,” Cowan explains.
Over time, these electrons lose energy and are gradually recaptured by ions. As recombination continues, the atoms return to their neutral state.
Impact on laser fusion research
“This experiment demonstrates how powerful our laser is and paves the way for future laser fusion facilities,” concluded Dr. Ulf Zastrau, head of the HED-HIBEF experimental station at European XFEL. This is because laser fusion is also based on a very hot plasma that is heated by a laser and the resulting electron waves.
“Thanks to new concrete discoveries, we can now focus on continually improving the simulation of these processes,” Zastrow explains. Accurate simulation is essential for designing efficient and reliable laser fusion reactors of the future.
