Staged power-law spectrum of electrons
A relativistic laser pulse propagating in a near-cirtical density plasma (0.1–10\({\text{n}}_{\text{c}}\)) induces a plasma channel along the propagation path of the laser pulse instead of a periodic structure wakefield. Both electrons and protons are accelerated inside the plasma channel. There are two simulations (please refer to Methods for specific parameters) with switching the on/off RR effect to study the accleration process of charged particles in an intense laser field. As shown in Fig. 1, a clear plasma channel forms in both cases. Electrons are expelled away along the laser axis and transveral direction by strong pondermotive force, and a plasma channel forms after driving the laser pulse. As the RR effect is switched on, a great number of energetic electrons accumulate inside the plasma channel due to the RR trapping effect, as shown in Fig. 1(a). Protons also accumulate at the laser axis due to the transversal electric field, as plotted in Fig. 1(b), forming a dense quasi-neutral plasma beam21. As the laser pulse further propagates, both the electron and proton beam densities greatly increase over the critical density (see Fig. 1(d) and (e)). This trapped plasma bunch has a transverse size of 20 µm and longitudinal length of 50 µm, which is much longer than the laser pulse. The first half of the electron bunch overlaps with the laser pulse. They are directly accelerated and modulated by the intense laser field, resulting in a periodic distribution of electron density. These electrons oscillate in the laser field, emitting high-energy γ-photons. The production of a high-energy electron beam and γ-photon beam causes serious energy consumption of the local pulse field and thus leads to a blurry distribution of Ey at the back part of the laser pulse, as shown in Fig. 1(f). When the RR is turned off, the plasma channel is almost empty, and there are few electrons and protons inside the channel. Moreover, the laser energy loss is much smaller than that in the case of RR on, as plotted in Fig. 1(f) and (i). It is clear that the RR trapping effect greatly changes the electron density distribution and thus the acceleration process of charged particles bunching inside the plasma channel as well as the \({\gamma }\)-photon beam production when the laser intensity is beyond 1023 W/cm2.
The RR effect has a fundamental influence on the particle acceleration process when the RR force is comparable to the laser pondermotive force and traps electrons at the propagation axis of the laser pulse. Figure 2(a) presents the normalized energy spectrum of energetic electrons in different time steps as the laser intensity a0 = 500. The maximum energy of electrons quickly increases beyond the GeV level and then stops at 5 GeV, leading to a broad energy spectrum. The energy spectrum has a nonthermal distribution function with a constant power-law slope of ~ 1.5 during the whole acceleration process. The electron spectrum in the RR-off case has a power-law index of ~ 2, which agrees well with that in the non-QED regime8. At t = 374 T0 in Fig. 2(b), the power-law index remains constant over the simulation time, but the cutoff energy of high-energy electrons extends beyond 10 GeV, which is much larger than that in the RR-on case. This confirms that the QED effect leads to a large drop in high-energy electrons because these high-energy electrons efficiently channel energy into the radiated γ-photon beam. The corresponding spectra for γ-photons with energy Eγ > 1 MeV are plotted in Fig. 2(c), which is synchrotron-like with maximal energy up to 3 GeV.
A series of simulations have been performed to study the electron acceleration with different laser intensities from a0 = 50 to a0 = 1000, where other laser and plasma parameters remain constant (see Methods). When the laser intensity of a0 = 50 is far from the near-QED regime, the electron spectrum has a power-law distribution with an index of ~ 2, as shown by the black dotted-dashed line in Fig. 3(a), which is similar to the previous result in the non-QED regime8. Both the energetic electron number and energy increase by one order of magnitude when the laser intensity increases to a0 = 100, but the power-law index is still ~ 2. As the laser intensity further increases above 1023 W/cm2 (a0 = 400\(\)and 500), the maximum energy of electrons increases to 5 GeV with a sharp knee near the cutoff energy, and the power-law index of its spectrum decreases from ~ 2 to ~ 1.5. The QED effect starts to play a significant role in the electron acceleration process and stops rapidly increasing the electron cutoff energy since energetic electrons emit a great number of \({\gamma }\)-ray photons. This effect becomes clearer as the laser intensity further increases beyond 1024 W/cm2. Both the electron energy and number have a small increase compared to a0 = 500, as shown in Fig. 3(c), since a large part of the laser energy is depleted into high-energy γ-photons rather than energetic electrons. Thus, the power-law index of the electron spectrum further decreases to ~ 1.2.
The maximum energy evolution of energetic electrons is present for different laser intensities in Fig. 4(a). In the case of a0 = 50, the maximum electron energy increases rapidly beyond 1 GeV at 100 T0 and then slowly increases at approximately 3 GeV until the laser pulse field starts to decrease due to energy loss after 300 T0. When the laser intensity increases to a0 > 500 in the near-QED regime, the maximum energy of accelerated electrons rapidly increases until 150 T0 and then remains constant, which is completely different from the non-QED regime. Electron energy stops increasing in the near-QED regime since electrons radiate a great number of gamma-ray photons during their acceleration process, which again confirms that QED effects strongly modulate the electron acceleration process. Figure 4(b) shows that the maximum energy of electrons increases linearly with \({a}_{0}\) instead of \({a}_{0}^{2}\) and that the QED effect changes this trend.
Nonthermal Electron Accelerating Mechanism
The generation of an acclerated electron bunch with its spectrum of power-law distribution functions in an intense laser field has a large difference from that in the non-QED regime. These electrons undergo three main different acceleration mechanisms: direct laser acceleration (DLA), nonlinear multiphoton scattering and spatial charge-separation field acceleration, where the QED effect significantly contributes to the laser plasma interaction process.
Figure 5 presents the longitudinal electron density and electron momentum at y = 0 inside the plasma channel. The laser pulse occupied the range of x = 230 and 255 µm, where a large number of electrons are accelerated via the strong pondermotive force of the intense laser field, i.e., the DLA mechainism. These energetic electrons have a high density spike above 10 nc located x = 256 µm in front of the plasma channel. DLA is an effective mechanism for accelerating electrons to several GeVs22,23. The accelerated electrons have a periodic laser field structure. For electrons initially at rest, longitudinal and transversal momentum gains in the first half laser cycle are py = a and px = a2/2 and then turn to zero in the remaining half cycle. Electrons quiver frequently in the laser field and thus lead to stochastic emitting radiation as well as being trapped inside the plasma channel. A large part of the laser energy is converted into high-energy γ-photons. Thus, an electron accleractron by the DLA process of relativistic electrons is always accompanied by efficient γ-ray beam radiation.
In our simulations, the quantum discrete process in the PIC simulation is realized based on the Monte algorithm, and each radiation event is calculated within an optical depth according to the quantum probability24. The most energetic photons are radiated at a certain angle of 0.22 rad along the laser propagation axis. Radiation processes exert transient RR froce \({f}_{d}\) in the opposite direction of photon emission to radiated electrons. When the laser intensity exceeds 1022 W/cm2, \({f}_{d}\) is comparable to the laser pondermotive force and thus significantly participates in the electron aceleration process, bringing random electron trajactories. Each radiation event is accompanied by one multiphoton process, \({\text{e}}^{-}+\text{n}{{\gamma }}_{\text{L}}\to {\text{e}}^{-}+{\gamma }\), where \({{\gamma }}_{L}\) and \({\gamma }\) represent laser photons and γ-photons24,25. This is a pure quantum phenomenon, and the quantity of \({f}_{d}\) is exactly proportional ℏ. Thus, a radiating electron will suddenly change its momentum during γ-photon emission, which brings both random electron direction of motion and kinetic energy. Collective electron bunches that suffer stochastic effects will lead to exotic phenomena, e.g., boarding energy spreading of electrons 27 and stochastic heating of electrons in a standing laser field 28,29. Electrons have a larger probability of emitting γ-photon radiation as their energy increases in the laser field. Thus, stochastic effects of quantum emission prevent the energy spectrum from high-energy expansion and induce bending of power-law spectra at approximately 2 GeV with laser intensity above a0 = 500 in Fig. 3(b), while the electron spectrum has no sharp end at the cutoff energy in the non-QED regime.
The RR trapping effect leads to both electron and proton accumulation inside the plasma channel as the laser intensity exceeds 1023 W/cm2, forming a high-density plasma bunch, as shown in Fig. 1(a, b) and (d, e), and creates a large-amplitude charge-separation field. The longitudinal electric field Ex at y = 0, marked by the black solid line in Fig. 5(a), has both a positive field of 100 TV/m located between 230 µm and 255 µm for decelerating electrons (or accelerating protons) and a negative field of -25 TV/m at x = 220 µm for accelerating electrons (or decelerating protons). One notes that Ex lacks the periodic structure of the laser wakefield and is not sensitive to the plasma wavelength because the driving laser pulse has a much larger temporal and spatial scale than the plasma wavelength of \({\lambda }_{p}=1.1 {\mu }\text{m}\) under the condition of ne = 0.5 nc. A large number of electrons is accelerated/decelerated by the local charge-separation field. Their longitudinal momentum px reaches a peak value of 3000 at approximately 220 µm, as plotted in Fig. 5(b). Moreover, Ex is sensitive to the plasma bunch as well as the laser pulse evolution and thus leads to electrons participating in time-dependent acceleration and deceleration phases. These electrons cannot have continuous energy gain and bring a broad spectrum. However, the maximum momentum of these electrons is much lower than that of electrons accelerated in the DLA regime, which is above 9000 (see Fig. 5(b)).
Proton Acceleration In Near-qed Regime
For driving laser intensities lower than 1025 W/cm2, a proton cannot be accelerated to relativistic energy within a short light period. The principle of proton acceleration in the plasma channel is similar to that of electrons accelerated by a wakefield, and they can be accelerated by a positive longitudinal electric field in front of the bubble channel 30,31. The RR trapping effect significantly alters the space-charge field distribution as the laser intensity reaches a0 = 500 in our simulation. Longitudinal field Ex has a positive value of 100 TV/cm in the front part of the plasma channel, as plotted in Fig. 5 (a), where the proton bunch confined within the plasma channel can be mainly accelerated. However, the velocities of these protons at the end of the acceleration field are not high enough to catch up with the laser pulse and then move to the deceleration phase. Moreover, the accelerating space-charge field Ex inside the plasma channel changes over time with the plasma bunch and laser field, which also brings an unstable acceleration field. Thus, protons accelerated in the time-dependent charge-separation field also result in a nonthermal spectrum distribution, as shown in Fig. 6, instead of a quasi-monoenergetic spectrum.
The maximum energy gain for protons in the plasma channel is approximately \({E}_{p} \tilde a{m}_{e}{c}^{2}\), which is estimated based on the Coulomb electrostatic potential resulting in radial explosion in the non-QED regime 22. For laser intensities of a0 = 50 and 100, the maximal proton energies in our simulation are 23 MeV and 50 MeV, respectively, and agree well with the estimation of\({E}_{p}\). When the laser intensity increases to a0 = 1000, the maximal proton energy is 5 GeV, which is larger than the \({E}_{p}\) estimated by more than one order of magnitude and almost catches up with the cutoff energy of electrons. Such high energy gain for protons mainly benefits from the longitudinal space-charge field, originally from electron trapping induced by the RR effect. Proton spectra with different laser intensities, as plotted in Fig. 6, show that the cutoff energy of accelerated protons strongly extends with increasing laser intensity. Additionally, the power-law index of the proton spectrum is ~ 1.5\(\)in the case of a0 = 50 and a0 = 100. However, it decreases to 0.7 with a sudden decrease near the cutoff energy as the intensity increases above 1023 W/cm2. The evolution of the proton energy spectrum with increasing laser intensity is similar to that of electrons. The ultraintense laser pulse drives a strong charge-separation field due to the RR effect, which accelerates protons to high energy and leads to a step-like power-law proton spectrum.