In the Chalmers team’s concept, electrons and positrons (green) trapped in a petawatt-power laser field (surfaces in red, orange and yellow) are oscilated to produce cascades of high-energy gamma-ray photons (pink). The team’s concept relies on careful control of laser pulse duration and peak power, as well as density of charged particles, to maximize gamma ray production and energy. [Image: Arkady Gonoskov]
The advent of lasers of petawatt peak powers, at facilities such as those of the European Extreme Light Infrastructure (ELI), has physicists licking their chops for a previously unavailable, extremely bright source of high-energy gamma-ray photons for new kinds of experiments. But just how “high” can “high-energy” be?
Previous simulations have suggested that as laser peak powers reach lofty petawatt levels, the laser field itself can start to run into fundamental limits. Those limits are tied to strong-field quantum electrodynamic (QED) effects, which can, through complex feedbacks, eventually sap the energy of the laser field driving them. As a result, it’s generally been assumed that efficient gamma-ray production from these new petawatt-peak-power lasers would be limited to energies well under a billion electron volts (GeV).
Now, researchers from Sweden, Russia and the United Kingdom have re-crunched the numbers, and suggested that this fundamental limit might not be so fundamental after all (Phys. Rev. X, doi: 10.1103/PhysRevX.7.041003). The team’s modeling suggests that, by tweaking the laser pulse intensity and duration in the right way, it’s possible to tune the system to minimize the energy-depleting effects and maximize the creation of gamma rays. This, says the team, would allow the radiation from the high-power laser to be “converted into a well-collimated flash of GeV photons.”
Thus far, the scenario, requiring lasers with peak powers on the order of 10 PW, has been proved out only on the computer. But the authors hope to see it verified in practice as such powerful lasers start come on line with the maturing of the ELI and other projects—a development that, they maintain, “could enable a new era of experiments in photonuclear and quark-nuclear physics.”
QED’s double-edged sword
One reason for doubts about maximum attainable energy has to do with the previously inaccessible physics of strong-field QED that petawatt-peak-power lasers will suddenly put on the table. On the plus side, the strong fields of 10-PW-plus lasers, interacting with and accelerating particles in an electron–positron plasma, can cause those particles to radiate a large fraction of their energy as energetic gamma-ray photons. That, in turn, has raised considerable anticipation that these soon-to-be-launched high-peak-power lasers could provide a source for high-energy gamma rays for new kinds of experiments.
But there’s a catch. As the flux of gamma-ray photons produced by these light–matter interactions increases, a significant share of those high-energy photons would themselves interact with the laser field to create a cascade of electron–positron pairs, through the QED process of pair production. The result would be an increasingly dense plasma cloud in the laser field that would rapidly pull energy out of the field itself, quickly erasing its ability to create additional gamma-ray photons and preventing its use as a sustainable a gamma-ray source above a certain energy threshold.
The ART of gamma-ray creation
The team behind the new research—led by physicist Arkady Gonoskov of Chalmers University of Technology, Sweden, along with colleagues at Chalmers, the Russian Academy of Sciences, Lobachevsky State University in Russia, and the University of Plymouth in the U.K.—sought to get around that limit. To do so, they looked in detail at the interaction of the electron–positron cascade with another process in these high-energy laser fields, so-called anomalous radiative trapping (ART).
In ART, using a complex set of parabolic mirrors, 12 laser pulses can be focused into a dipole standing wave that traps electrons and positrons. The trapped particles are then oscillated in the wave in such a way that they gain substantial energy and have a high probability of emitting a substantial part of that gained energy in a single gamma-ray photon.
As with other approaches to gamma-ray creation, the increasing gamma-ray flux from ART leads to a pair-production cascade and a growing plasma cloud of electrons and positrons. But using advanced 3-D QED particle-in-cell (PIC) numerical simulations, the Gonoskov team was able to establish that, at laser powers above around 7 PW, it’s possible to keep that cascade from putting a lid on the laser field’s energy for gamma-ray production.
The trick, according to the team is to tune the ART setup’s pulse duration, peak power and initial particle density to maximize the field intensity, and thus the gamma-ray production, just before the plasma effects from the cascade start to reduce the energy of the generated photons. This, according to the researchers, allows “a maximal number of particles to interact with the most intense part of the laser pulses, and emit a large number of high-energy photons.”
From simulation to reality?
In their comprehensive PIC simulation, the researchers found that an experiment using 12 laser pulses with a total peak power of 40 PW could result in a well-collimated gamma-ray beam with an energy greater than 2 GeV, and “the unique capability of achieving high peak brilliance in an energy range unachievable for conventional sources.” As such, it could offer “a powerful tool for studying fundamental electromagnetic processes, and will open qualitatively new possibilities for studying photonuclear processes.”
Putting the that promise to the test outside of numerical experiments, of course, must await the full production implementation of petawatt-scale lasers in ELI and elsewhere. In a press release accompanying the study, Gonoskov noted that the team’s concept “is already part of the experimental program proposed for one such facility: the Exawatt Center for Extreme Light Studies in Russia,” currently under construction.