Fast beam pinching via a plasma lens yields extremely brilliant gamma-rays

Bright sources of high-energy gamma-rays are applied in a broad range of areas, from fundamental research in astrophysics, particle and nuclear physics, to high-resolution imaging in chemistry, biology, medicine, materials science and industry. These applications can benefit greatly from compact gamma-ray sources with low divergence, short pulse duration, high energy and high peak brilliance. At present, synchrotrons and X-ray free-electron lasers (XFELs) can deliver brilliant X-ray pulses, but their photon energies are generally limited to the range of a few keV to hundreds of keV. There is significant interest in extending photon energies well beyond the MeV level. So far, however, an efficient approach to producing high-energy gamma-rays with peak brilliance comparable to modern XFELs has remained elusive: existing plasma-based schemes, such as betatron radiation, typically suffer from limited field strength and low beam density, with photon energies of only a few keV to hundreds of keV, peak brilliance on the order of 10²³ photons s⁻¹ mm⁻² mrad⁻² per 0.1% bandwidth, and energy conversion efficiencies generally around 0.001%. Based on inverse Compton scattering, some facilities are now capable of delivering gamma rays with photon energies reaching the 20 MeV level; however, the efficiency remains low. 

In the present study, published in Research 9, 1330 (2026), a novel mechanism called the beam fast pinching radiation burst (FPRB) is proposed to efficiently produce collimated ultra-bright gamma-ray beams with photon energies extending from the MeV to the GeV level. A 5GeV nC-class electron beam is injected into a 400μm-long plasma with a linearly increasing (up-ramp) density profile, which acts as an effective plasma lens. During propagation, the transverse focusing force experienced by the beam increases continuously, so that the beam is violently pinched as a whole and compressed to a sub-micrometer diameter of about 0.2 μm, enhancing its density by hundreds of times. The focused dense beam, in turn, drives the plasma wake to unprecedented transverse fields of up to tens of TV/m in the high-density plasma region, enabling the interaction to enter the radiation-dominated regime. Under such extreme conditions, the quantum parameter  can exceed 0.2, triggering intense emission of high-energy photons with a cutoff energy of more than 2 GeV. As a result, the electron beam energy is converted into gamma-ray emission with an efficiency of about 31% in a typical case and up to 46.3% with optimized beam parameters, compared with only about 0.3% in a uniform plasma. The resulting gamma-ray beam combines a submicrometer spot size (~0.2 μm), a low divergence of about 3 mrad, a short duration of about 5 fs, and a photon number of 1.8×10¹⁰ at 1 MeV, corresponding to a peak brilliance of about 10²⁸ photons s⁻¹ mm⁻² mrad⁻² per 0.1% bandwidth. This is about five orders of magnitude higher than existing gamma-ray sources, reaching a level close to that of modern XFELs.

Another interesting feature of this scheme is that, due to the intense focusing of the multi-GeV electron beam and the extremely strong effective field it generates, the radiation reaction and the strong-field quantum effects play a crucial role. The substantial energy loss and spectral broadening induced by radiation naturally relax the requirement on electron-beam monochromaticity. Moreover, the scheme is robust to beam parameters: it works well for electron beams with relatively large energy spreads (e.g., 10%) and for lower charges down to the hundreds-of-pC level, which considerably lowers the experimental requirements.  Comparable nC-class, GeV-scale electron beams are now attainable using advanced accelerator techniques, for example, through the planned experiments at SLAC's FACET-II facility in the US, or via PW-class laser-plasma accelerators, which have recently demonstrated beams with energies up to 10 GeV and charges of approximately 1.7 nC. The required up-ramp plasma structures can be generated in experiments, for example, by prepulse laser heating of a thin solid film or by supersonic gas jets with a blade. These features suggest that the proposed scheme can be realized in a relatively simple configuration and tested in existing or upcoming high-power laser and advanced accelerator facilities.

Sources: https://spj.science.org/doi/10.34133/research.1330