Petawatt lasers, because of their extremely high intensity, bring extreme astrophysical phenomena within reach of laboratory experiments, as demonstrated by recent computational research.
Recent advances in laser technology have brought us to the threshold of exploring physical regimes once considered exclusive to the most extreme astrophysical environments. With the commissioning of several high-power, high-intensity laser facilities around the world, capable of delivering peak powers in the multi-petawatt range, a new generation of laboratory experiments is now within reach – experiments that probe the frontier of light-matter interactions, relativistic plasmas, and even the conversion of light into matter and antimatter.
This article presents what can already be achieved – or is within reach – in the near term using the capabilities of current multi-petawatt laser facilities. Specifically, it highlights three interconnected phenomena that can be accessed using high-power, high-intensity laser systems: (1) the generation of extremely strong, quasi-static magnetic fields in a dense plasma, (2) the production of dense gamma-ray beams facilitated by these magnetic fields, and (3) the creation of electron-positron pairs directly from light using the laser-driven gamma-ray beams. The discussion is informed by computational plasma research performed at the University of California, San Diego, with support from the National Science Foundation and the Air Force Office of Scientific Research.
Magnetic fields stronger than a magnetar’s
In everyday laboratory settings, the strongest static magnetic fields we can generate using superconducting or resistive magnets are limited to about 100 kilogauss (10 tesla). Pulsed power technology can push this further, enabling magnetic fields up to 1 megagauss (100 tesla), though only for microsecond durations and often in destructive setups. However, in some astrophysical environments, such as the magnetospheres of neutron stars, fields can exceed multiple gigagauss. Magnetars are a type of neutron star with magnetic fields that can exceed 10 gigagauss, making them the strongest known magnets in the universe. These immense fields are not just scientific curiosities; they fundamentally alter how particles move and radiate. In such extreme environments, electrons emit high-energy photons, including gamma-rays, as they are deflected by the magnetic field. These photons can then convert into matter, creating electron-positron pairs and forming a pair plasma that fills the magnetosphere – a plasma state that has yet to be realised in the laboratory.
Multi-petawatt lasers can generate similarly extreme magnetic fields in plasma. To drive the high current densities needed for such fields, the laser must propagate through a dense plasma. However, under normal conditions, there is a fundamental upper limit: beyond the classical cutoff density, determined solely by the laser wavelength, the plasma becomes opaque and reflects the laser. Very high laser intensity provides a way around this barrier. At sufficiently high intensities, electrons in the plasma are accelerated to relativistic speeds, effectively increasing their mass. This increase raises the cutoff density – the effect known as relativistic transparency – allowing the laser to propagate through plasmas far denser than would otherwise be possible.
Relativistic transparency makes it possible to drive strong volumetric currents in dense plasmas, but doing so requires careful control of the plasma density. To support high current densities while remaining transparent to the laser, the plasma must be significantly denser than a gas, yet still less dense than a metal. This requirement places constraints on the choice of target material. One promising solution is the use of low-density foams, which can be fabricated with tuneable properties to achieve an electron density compatible with the laser intensity planned for the experiment.
Simulations using the highest laser intensities currently available indicate that magnetic fields in excess of 4 gigagauss can be generated in plasmas with electron densities around 30 times the classical cutoff density. This capability opens a path toward laboratory studies of exotic plasma phenomena, including relativistic magnetic reconnection and radiation-dominated electron dynamics – processes that, until now, have remained exclusive to extreme astrophysical environments. In terms of strength, these fields begin to approach those found in the magnetospheres of neutron stars. As new laser facilities are being developed, with plans to reach powers on the order of 100 petawatt, this approach may eventually enable access to field strengths that match or even exceed those of a magnetar.
A new class of gamma-ray sources
X-ray free-electron lasers (XFELs) and synchrotron light sources have long been the gold standard for producing high-brightness, high-energy photon beams. These facilities use some of the most energetic electron beams currently available, typically accelerated to multi-GeV energies by kilometer-scale linear accelerators. As these relativistic electrons pass through strong magnetic fields – often on the order of 10 kilogauss – they emit high-energy radiation through synchrotron processes. While extremely powerful, these conventional magnetic fields place a practical limit on the maximum photon energy that can be reached. Even with a 10 GeV electron beam, the emitted photons typically reach only a few keV in energy. These systems are already pushing the limits of what can be done using traditional accelerator and magnet technologies.
Multi-gigagauss magnetic fields generated in a plasma are so much stronger than conventional fields that they fundamentally change how one should think about reaching the photon energies required for pair production – energies that are two orders of magnitude higher than those delivered by XFELs. The key parameter governing radiation emission is the product of the electron energy and the magnetic field strength. While conventional approaches require extremely energetic electrons to compensate for relatively weak fields, the dramatic increase in field strength now allows access to multi-MeV photon energies using electrons with much lower energies. In fact, the requirement on electron energy is reduced so significantly that the electrons can be generated directly by the laser itself, removing the need for a large linear accelerator and enabling compact, laser-based gamma-ray sources.
The plasma-generated magnetic field plays a dual role in a laser-driven gamma-ray source. On one hand, it directly induces gamma-ray emission by deflecting relativistic electrons, as discussed above. On the other hand, it also facilitates the generation of these energetic electrons in the first place. In the absence of a background magnetic field, electron acceleration by the laser tends to stall due to the oscillating nature of the laser’s electric field, which reverses direction every half-cycle. However, the presence of a quasi-static magnetic field alters the electron’s trajectory in a way that maintains a favorable phase between the laser field and the electron motion. This mechanism, known as direct laser acceleration, allows electrons to gain energy continuously over multiple laser cycles, leading to substantially higher final energies. These energetic electrons then go on to emit intense, high-energy radiation as they interact with the same magnetic field that helped accelerate them.
Simulations show that this approach to gamma-ray generation is highly effective. Under currently available laser conditions, it should already be possible to convert at least several percent of the laser energy into a directed beam of multi-MeV gamma-rays. This represents a dramatic improvement over what has been achieved experimentally with other laser-based approaches. Moreover, this method offers a promising path for developing next-generation gamma-ray sources, with multiple predictions indicating that the conversion efficiency could rise into the tens of percent range as laser intensities increase at existing facilities and new, more powerful lasers come online.
Light-to-matter conversion via pair production
The ability to produce high-density, high-energy photon beams in the lab enables a dramatic next step: creating matter directly from light. This is not science fiction, but a consequence of quantum electrodynamics. The underlying mechanism is known as the Breit-Wheeler process, which describes the conversion of photons into electron-positron pairs. The idea of recreating this process in the laboratory is not new. It dates back to the pioneering SLAC E-144 experiment in 1997, where the process was observed for the first time, detecting roughly one pair for every hundred laser shots.
However, little progress has been made since the SLAC experiment. The reason is that the version of the Breit-Wheeler process used at SLAC requires extreme laser intensities, far beyond what is currently achievable, to become efficient. This is the so-called nonlinear Breit-Wheeler process, in which a high-energy gamma-ray interacts with multiple laser photons to produce an electron-positron pair.
There is, however, another option: the linear or two-photon Breit-Wheeler process. In this case, two photons collide and annihilate, producing an electron-positron pair. Since this is literally the conversion of energy into mass, the combined energy of the two photons must exceed the rest energy of the pair – a challenging threshold for conventional photon sources to reach. The probability of this process occurring – quantified by what physicists call the cross section – is also very small, which is why it has historically been overlooked in favour of the multi-photon approach. However, this apparent disadvantage also offers a significant benefit: the process does not directly involve laser photons, and therefore does not impose any stringent requirement on laser intensity.
What the linear process does require is the collision of two dense, energetic gamma-ray beams – something that has historically been out of reach. That situation is now changing. The beams produced by relativistically transparent plasmas are just a few microns in diameter and contain over 10¹² photons in the relevant energy range. A simple estimate shows that colliding two such beams head-on can produce millions of electron-positron pairs. Detailed supercomputer simulations confirm that this is indeed the case. In fact, the yield can be even higher when the beams produced by two counter-propagating lasers are allowed to collide inside the plasma, due to additional gamma-ray emission by electrons interacting with the oncoming laser pulse. These calculations assume laser parameters that are already available at existing facilities. This represents a dramatic increase in pair yield compared to earlier efforts such as the SLAC experiment, which produced fewer than one pair per shot on average.
Pair production and positron acceleration with a single laser
It might seem that using two laser pulses is a prerequisite for creating gamma-ray collisions and producing millions of electron-positron pairs directly from light. Of course, this would severely limit the range of possibilities in terms of suitable laser facilities. Using two high-power, high-intensity pulses also presents additional experimental challenges. Recent computational research, however, has revealed a surprising phenomenon that makes it possible to achieve similarly spectacular results with just a single laser beam.
Under the right conditions, a plasma irradiated by a single laser beam can self-organise into a moving photon-photon collider. In addition to emitting gamma-rays in the forward direction, the plasma also emits backward. This backward emission arises from a charge-separation effect at the front of the laser pulse. As the pulse pushes electrons forward and leaves heavier ions behind, the resulting electrostatic pull causes some of the electrons to snap back and collide with the oncoming laser field, producing backward-directed gamma-rays. These forward and backward photon populations overlap and persist as long as the laser continues to propagate, enabling head-on photon collisions without the need for a second beam. In this configuration, simulations predict the production of millions of electron-positron pairs per shot using laser parameters already available today. All of this occurs without external gamma-ray sources or multiple laser pulses, representing a major advance in experimental feasibility.
What happens to the positrons after they are born? If left in the plasma, they will eventually annihilate with electrons and go undetected. However, the same field structure that enables the photon-photon collider also acts as a positron accelerator. Since positrons are positively charged, they are pushed forward by the charge-separation field sustained by the laser. As the laser continues to propagate, a portion of the positrons is able to keep pace with the field and gain energy. Not all of them participate in this process, but those that do form a forward-directed beam that exits the plasma before annihilation can occur.
This mechanism not only aids in positron detection but also points to a path toward compact, laser-based positron sources. Unlike electrons, positrons are difficult to produce in controlled beams, yet they are in demand for a range of applications – from materials characterisation to high-energy astrophysics and antimatter research. This line of work exemplifies the broader cycle in fundamental science, where advances in laser technology drive basic research that, in turn, opens new application avenues through the discovery of novel physical mechanisms.
Outlook: A new experimental frontier
Modern laser technology is now bringing previously inaccessible regimes into reach of laboratory study. We are beginning to recreate in the lab physical phenomena typically associated with neutron stars, black hole jets, and even the early universe. Multi-petawatt lasers offer not just high power but also the extreme intensity needed to explore new regimes of plasma physics.
As these capabilities continue to develop, the field is moving from theoretical and computational studies toward experimental realisation. Many of the phenomena discussed here – ultra-strong magnetic fields, laser-driven gamma-ray bursts, and even the creation of matter directly from light – are now within reach of laboratory demonstration.
Although these advances are still in early stages, they hold promise for practical impact. They may lead to new diagnostic tools, compact particle and radiation sources, and improved models of cosmic phenomena. The intersection of dense plasmas, intense fields, and quantum electrodynamics is opening a new window into extreme physics – one defined not by what we can engineer with conventional magnets and accelerators, but by what light itself can now accomplish.
Please note, this article will also appear in the 22nd edition of our quarterly publication.
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