Researchers have made a groundbreaking advancement by combining two sophisticated laser systems: an X-ray free-electron laser and the high-intensity optical laser known as ReLaX. This innovative approach was executed at the HED-HiBEF experimental station located at the European XFEL in Schenefeld, near Hamburg. Their findings offer profound insights into the interaction of high-energy lasers with materials under extreme conditions, presenting a novel method for enhancing diagnostics in laser fusion research.
Capturing Ionization in Trillionths of a Second
Ionization occurs at remarkable speeds, often within picoseconds or trillionths of a second, necessitating even shorter laser pulses to capture these fleeting moments. "These conditions are achieved with our two lasers, which have pulse durations of just 25 and 30 femtoseconds," states Dr. Lingen Huang, head of experimentation at HZDR's Division of High-Energy Density. This ultrafast capability enables researchers to observe the formation and evolution of plasma in near real-time.
From Copper Wire to Superhot Plasma
The experiment initiates with a powerful light burst targeting an ultra-thin copper wire, approximately one-seventh the thickness of a human hair. This intense energy, peaking at about 250 trillion megawatts per square centimeter over a minuscule area, mimics conditions typically found in extreme cosmic environments, such as those near neutron stars or during gamma-ray bursts.
The copper wire vaporizes instantly, creating plasma with temperatures soaring into the millions of degrees. During this process, copper atoms lose numerous electrons, becoming highly ionized.
A subsequent laser pulse, referred to as the probe pulse, is utilized to investigate the plasma. This pulse, produced by the European XFEL, emits a powerful flash of hard X-rays. By analyzing how these X-rays interact with the plasma, scientists can capture a series of snapshots, akin to frames in a cinematic sequence, allowing them to trace the plasma's evolution step by step.
Analyzing Highly Charged Copper Ions
The X-ray pulses are meticulously adjusted to engage with Cu²²⁺ ions, which are copper atoms that have lost 22 electrons. The photon energy of 8.2 kiloelectronvolts aligns with a specific electronic transition in these ions, facilitating a process known as resonant absorption. Upon absorbing the X-rays, the ions emit their unique X-ray radiation.
"Our pump-probe experiment enables us to precisely measure the temporal development of this stimulated X-ray emission," explains Huang. "This data reveals the number of Cu²²⁺ ions present in the plasma at any given moment."
Mapping the Timeline of Plasma Evolution
The measurements illustrate a distinct sequence of events. Following the laser's impact on the wire, Cu²²⁺ ions start forming, with their numbers peaking after approximately two and a half picoseconds. Subsequently, recombination occurs, leading to a gradual decline in ion numbers, with complete disappearance within about ten picoseconds.
"This level of precision in observing ionization is unprecedented," remarks Prof. Tom Cowan, former director of the Institute of Radiation Physics at HZDR.
Electron Waves as Catalysts
Computer simulations have shed light on the mechanisms driving this phenomenon. The initial laser pulse strips a few electrons from the copper atoms, which then move through the material as a wave, liberating more electrons from neighboring atoms. "These energy-rich electrons spread out like a wave, dislodging additional electrons from adjacent copper atoms," Cowan elaborates.
As time progresses, these electrons gradually lose energy and are reabsorbed by the ions, allowing the atoms to revert to a neutral state.
Future of Laser Fusion Research
"This experiment highlights the immense power of our lasers and sets the stage for future laser fusion facilities," concludes Dr. Ulf Zastrau, responsible for the HED-HIBEF experiment station at the European XFEL. Laser fusion, which relies on extremely hot plasmas heated by lasers and the resulting electron waves, stands to benefit significantly from these findings. "Our new insights will enable us to refine simulations of these processes, which are crucial for designing efficient and reliable laser fusion reactors in the future."