Ultrabright GeV Photon Source via Controlled Electromagnetic Cascades in Laser-Dipole Waves

Electromagnetic cascades have the potential to act as a high-energy photon source of unprecedented brightness. Such a source would offer new experimental possibilities in fundamental science, but in the cascade process radiation reaction and rapid electron-positron plasma production seemingly restrict the efficient production of photons to sub-GeV energies. Here, we show how to overcome these energetic restrictions and how to create a directed GeV photon source, with unique capabilities as compared to existing sources. Our new source concept is based on a controlled interplay between the cascade and anomalous radiative trapping. Using specially designed advanced numerical models supported with analytical estimates, we demonstrate that the concept becomes feasible at laser powers of around 7 PW, which is accessible at soon-to-be-available facilities. A higher peak power of 40 PW can provide $10^9$ photons with GeV energies in a well-collimated 3-fs beam, achieving peak brilliance $9\times {10}^{24}\mathrm{ph}{\mathrm{s}}^{-1}{\mathrm{mrad}}^{-2}{\mathrm{mm}}^{-2}/0.1\%\mathrm{BW}$.

Popular Summary

Almost any physical investigation involves the use of light, or photons, as a probe. High-energy photons are required to probe fundamental quantum processes, and interactions between lasers and matter can generate such photons; a charged particle can absorb laser photons and, in giving up some of its own energy, re-emit the photons at a much higher energy. This has led to great interest in using intense laser facilities to create high-energy photon sources, but there are seemingly unavoidable physical restrictions on their energy. In particular, the generation of high-energy photons leads to the start of an avalanche, or cascade, of electron-positron pairs that, while creating more photons, rapidly draws too much energy from the system, making the source unsustainable. We show that there is in fact a way to overcome such restrictions and create a source of giga-electron-volt (GeV) photons with unprecedented brightness.

The key to our concept is the use of particle-trapping phenomena to initiate a new scenario in which particle cascades and highly nonlinear particle dynamics induce and support each other but, crucially, in a controllable manner. By matching the intensity and duration of the laser pulse, it is possible to induce a cascade that is held at a subcritical level, avoiding significant depletion effects while sustaining photon production. In this scenario, laser radiation is converted into a well-collimated flash of GeV photons. The resulting source has parameters exceeding those provided by existing laser-based sources by several orders of magnitude.

Our concept is feasible for upcoming laser facilities and could enable a new era of experiments in photonuclear and quark-nuclear physics.

Figure 1

Schematic illustration of the concept of anomalous radiative trapping and photon emission. In the top left we show how to generate a dipole wave using a set of off-axis parabolic mirrors (yellow) and 12 laser pulses (dark red). (See Ref. [16] for details and the reason for the choice of 12 mirrors.) The field structure of the dipole wave is shown in the left half of the horizontal plane (there is axial symmetry about the $z$ axis). We also show a trajectory of one of the seeding electrons taken from a simulation with $P=200$ PW. Photons of energy above 3 GeV are shown together with their emission events (magenta spheres). While some of these photons contribute to cascade development (gray spheres), others form a brilliant source emitted along the $z$ axis, also shown in the top left.

Figure 2

Simulation results. (a) Rate of particle population growth (red, right-hand scale) and the dependency of the backreaction threshold (blue, left-hand scale), both as a function of power $P$. The analytical estimate for the backreaction threshold is shown with a solid blue line, and agrees very well with the numerical results shown as blue points. The accurate numerical approximation to the data points obtained from simulation is shown with a solid red curve, and this is used to derive the estimate Eq. (4). (b) The expected number of photons with energy above 2 GeV, as calculated based on Eq. (5) for a Gaussian pulse of total peak power $P$ and duration $\tau$. The analytically obtained optimal condition Eq. (4) is shown with the black dashed curve: this clearly runs through the center of the optimal region.

Figure 3

The functions (a) $X(P,N_{e{e}^{+}})$ and (b) $Y(P,N_{e{e}^{+}})$ interpolated from the values calculated from the data of simulations with constant power $P$ at the instances of $E=0$ (crosses).

Figure 4

The results of 3D PIC simulations for a dipole wave formed by 12 slightly dephased laser pulses of 15-fs duration (FWHM for intensity) and total peak power of 40 PW. The electron density $>10^{20}{\mathrm{cm}}^{-3}$ and electric field strength in the plane $y=0$ (shifted downward) are shown for several instances: (a) the initial target, (b) the target affected by the leading edge of the pulses, (c) ART beginning to trap particles and start the cascade, (d) the cascade development, and (e) the generation of GeV photons. The overall angular-resolved spectrum of emitted photons is shown in (f). Panel (g) shows the temporal evolution of the field strength in the center (blue line), the total power of generated photons (green line), and the flux of photons with energy $>2\mathrm{GeV}$ (red line).

Figure 5

Comparison of the proposed source with existing sources, in terms of (a) peak brilliance and (b) average flux. The plots represent schematically summarized properties of existing (blue) as well as upcoming or planned (solid gray) facilities. The gray hatched areas show the prospects of our proposed source, estimated based on the results of 3D PIC simulations.