Cracking open strong field quantum electrodynamics

Left: The proposed scheme to probe SF-QED using present-day and near-future Lasers. A plasma mirror shaped with radiation pressure converts a laser pulse (red), into Doppler-boosted harmonics(purple), and focuses them onto a secondary target. They reach extreme intensities. These dimensions range from tens to hundreds (millionths) of a meter to a few to many tens of thousands of microns. Right: Berkeley Lab was responsible for developing the simulation code that was used in the research. This simulation image shows intense Doppler-boosted light waves (red and blue), which plow through the solid target of gray, generating high-energy photons that (orange) decay into pairs (green and positrons) following further interaction with the incoming pulses. Only photons that are not yet broken down into pairs are displayed. Credit: Luca Fedeli/CEA
A new theoretical and computer-modeling study has suggested that the world's strongest lasers may finally be able to understand the mysterious physics behind extreme phenomena such as pulsar magnetospheres and gamma radiation bursts.

Researchers from France's Alternative Energies and Atomic Energy Commission and Lawrence Berkeley National Laboratory (Berkeley Lab), comprise the international research team behind this study. Their findings are published in the prestigious journal Physical Review Letters.

Henri Vincenti of CEA was the head of the research team and he proposed the main physical idea. Jean-Luc Vay, Andrew Myers and the Computational Research Division at Berkeley Lab, developed the simulation code that was used in the research. Vincenti was previously a Marie Curie Research Fellow at Berkeley Lab and is still an ATAP affiliate. Vincenti's CEA team led the theoretical and numerical work.

A modeling study by the team shows that PETAWATT (PW)-class lasers can be juiced to higher intensities through light-matter interactions. This could help unlock the secrets of the strong-field regime of quantum electrodynamics. One petawatt is equal to ten to fifteen power, which is followed by 15 zeroes or a quadrillion Watts. Petawatts are the measurement of the output of today's most powerful lasers.

Cameron Geddes, ATAP Division Director, stated that this is a strong demonstration of how sophisticated simulation of complex systems can open up new avenues for discovery science through integrating multiple physical processes. In this case, it was the laser interaction with a target followed by subsequent production of particles in another target.

Lasers can probe some of the most guarded secrets in nature

QED, a cornerstone in modern physics, has withstood the rigors of experimentation over many decades. However, to probe SF-QED you need electromagnetic fields that are many orders of magnitude stronger than those available on Earth.

Side routes have been explored by researchers to SFQED. For example, powerful particle beams from accelerators were used to observe the interaction of particle particles with strong fields found in aligned crystals.

PW-class lasers deliver the strongest electromagnetic fields in a laboratory for a more direct approach. The world's strongest laser, a 10-PW laser, can be focused down to just a few microns to reach intensities close 1023 watts per sq. centimeter. The electric field values associated with SF-QED can reach as high as 1014 V per meter. However, SF-QED study requires higher field amplitudes that can be achieved using lasers.

Researchers have proposed to use powerful electron beams in order to break this barrier. These beams can be found at large accelerators or laser facilities. The amplitude of the laser field seen by electrons when a relativistic beam collides with a high-power pulse of laser light can be increased by orders-of-magnitude, allowing for new SFQED regimes.

Although such experimental methods can be challenging because they require the synchronization of high-power laser pulses and relativistic electron beams at micron and femtosecond scales in space and time, some experiments have been conducted successfully. Several more are being planned at PW-class laser facilities around the globe.

A combination of a high-power pulse laser (red and blue), a plasma mirror (not seen) and a secondary target, could produce conditions that allow for the investigation of Strong Field Quantum Elektrodynamics effects far beyond our current experimental capabilities. Credit: Luca Fedeli/CEA

Use a plasma mirror that is moving and curved to get a direct view

A complementary method was proposed by the research team: a compact scheme that increases the intensity of high-power laser beams. This method is based on the well-known concept light intensification, as well as their theoretical and computer-modeling studies.

This scheme involves increasing the intensity of a laser pulse using a relativistic plasma mirror. This mirror is formed when an ultrahigh-intensity laser beam hits a solid target optically polished. The high laser amplitude causes the solid target to become fully ionized. This creates dense plasma which reflects incident light. The intense laser field also moves the reflecting surface. The Doppler effect converts a portion of the reflected pulse to a shorter wavelength as a result.

This plasma mirror is naturally curvature due to the radiation pressure from laser. This allows the Doppler boosted laser beam to be focused on smaller areas, which can result in extreme intensity increases of up to three orders. Simulations show that a secondary target would provide clear SFQED signatures in real experiments.

Berkeley Lab is integral to the international team-science effort

This study used Berkeley Lab's many scientific resources, including the WarpX simulation code. It was created under the auspices of U.S. Department of Energy’s Exascale Computing Program to model advanced particle accelerators. WarpX's unique capabilities allowed for the modeling of the intensity boost as well as the interaction between the boosted pulse and the target. All previous simulation studies could only explore proof-of principle configurations.

Berkeley Lab Laser Accelerator, a laser accelerator of petawatt class with a repetition speed of a pulse per sec, might be used to verify the team's method for probing SFQED. Berkeley Lab researchers are currently building a second beamline, which could also be used in experimental studies of SFQED. The proposed new laser kBELLA could allow future high-rate studies. It will bring high intensity at a repetition rate of kilohertz to the facility.

WarpX's discovery of novel, high-intensity plasma-plasma interaction modes could bring benefits that go beyond the exploration of SF-QED. These include a better understanding of and design for plasma-based accelerators, such as the ones being developed at BELLA. They are smaller and cheaper than traditional accelerators of similar energie and could be game-changers in a variety of applications, including the expansion of high-energy Physics and the use of penetrating Photon Sources for precision imaging and implanting ions into semiconductors to treat cancer and develop new pharmaceuticals.

Vay expressed satisfaction at the Berkeley Lab team’s contribution to the study, saying that "it is gratifying being able to contribute towards the validation of new and potentially very impactful ideas through the use of our novel algorithmic codes." This is the beauty of team science.

Continue reading The creation of curved relativistic lenses to reflect high-power laser pulses

L. Fedeli et. al., Probing StrongField QED using Doppler-Boosted PETAwatt-Class Lasers. Physical Review Letters (2021). Information from the Journal: Physical Review Letters L. Fedeli et. al., Probing StrongField QED using Doppler-Boosted Petawatt Class Lasers, (2021). DOI: 10.1103/PhysRevLett.127.114801