One shot, game changed: How RAVEN captured a petawatt laser and supercharged fusion research

Ultra-intense lasers can accelerate electrons to near-light speeds within a single oscillation (or 'wave cycle') of the electric field, making them a powerful tool for studying extreme physics. However, their rapid fluctuations and complex structure make real-time measurements of their properties challenging. Until now, existing techniques typically required hundreds of laser shots to assemble a complete picture, limiting our ability to capture the dynamic nature of these extreme light pulses.

The new study, jointly led by researchers in the University of Oxford's Department of Physics and the Ludwig-Maximilian University of Munich, Germany, describes a novel single-shot diagnostic technique, named RAVEN (Real-time Acquisition of Vectorial Electromagnetic Near-fields). This method allows scientists to measure the full shape, timing, and alignment of individual ultra-intense laser pulses with high precision.

Having a complete picture of the laser pulse's behaviour could revolutionise performance gains in many areas. For example, it could enable scientists to fine-tune laser systems in real-time (even for lasers that fire only occasionally) and bridge the gap between experimental reality and theoretical models, providing better data for computer models and AI-powered simulations.

The method works by splitting the laser beam into two parts. One of these is used to measure how the laser's color (wavelength) changes over time, whilst the other part passes through a birefringent material (which separates light with different polarization states). A microlens array (a grid of tiny lenses) then records how the laser pulse's wavefront (shape and direction) is structured. The information is recorded by a specialized optical sensor, which captures it in a single image from which a computer program reconstructs the full structure of the laser pulse.

Lead researcher Sunny Howard (PhD researcher in the Department of Physics, University of Oxford and visiting scientist to Ludwig-Maximilian University of Munich) said: "Our approach enables, for the first time, the complete capture of an ultra-intense laser pulse in real-time, including its polarization state and complex internal structure. This not only provides unprecedented insights into laser-matter interactions but also paves the way for optimizing high-power laser systems in a way that was previously impossible."

The technique was successfully tested on the ATLAS-3000 petawatt-class laser in Germany, where it revealed small distortions and wave shifts in the laser pulse that were previously impossible to measure in real-time, allowing the research team to fine-tune the instrument. These distortions, known as spatio-temporal couplings, can significantly affect the performance of high-intensity laser experiments.

By providing real-time feedback, RAVEN allows for immediate adjustments, improving the accuracy and efficiency of experiments in plasma physics, particle acceleration, and high-energy density science. It also results in significant time savings, since multiple shots are not required to fully characterize the laser pulse's properties.

The technique also provides a potential new route to realize inertial fusion energy devices in the laboratory - a key gateway step towards generating fusion energy at a scale sufficient to power societies. Inertial fusion energy devices use ultra-intense laser pulses to generate highly energetic particles within a plasma, which then propagate into the fusion fuel. This 'auxiliary heating' concept requires accurate knowledge of the focused laser pulse intensity to target to optimize the fusion yield, one now provided by RAVEN. Focused lasers could also provide a powerful probe for new physics - for instance, generating photon-photon scattering in a vacuum by directing two pulses at each other.

Co-author Professor Peter Norreys (Department of Physics, University of Oxford), says: "Where most existing methods would require hundreds of shots, RAVEN achieves a complete spatio-temporal characterization of a laser pulse in just one. This not only provides a powerful new tool for laser diagnostics but also has the potential to accelerate progress across a wide range of ultra-intense laser applications, promising to push the boundaries of laser science and technology."

Co-author Dr Andreas Döpp (Faculty of Physics, Ludwig-Maximilians-University Munich and visiting scientist to Atomic and Laser Physics, University of Oxford) adds: "Shortly after Sunny joined us in Munich for a year it finally 'clicked' and we realized the beautiful result underpinning RAVEN: that because ultra-intense pulses are confined to such a tiny space and time when focused, there are fundamental limits on how much resolution is actually needed to perform this type of diagnostic. This was a game changer, and meant we could use micro lenses, making our setup much simpler."

Looking ahead, the researchers hope to expand the use of RAVEN to a broader range of laser facilities and explore its potential in optimizing inertial fusion energy research, laser-driven particle acceleratorsandhigh-field quantum electrodynamics experiments.

This study was conducted in collaboration with Ludwig-Maximilian University of Munich, the Max Planck Institute for Quantum Optics, and the John Adams Institute for Accelerator Science. The work was supported by the UKRI-STFC and funding bodies in Germany and the European Union.