Magnetic pinching of relativistic particle beams: a new approach to strong-field QED physics

Quantum electrodynamics (QED) is a foundation of modern physics, yet access to the strong-field QED regime in the laboratory remains a formidable challenge. Currently, high-power lasers at the multi-petawatt level and above are generally believed to be an important approach to test QED physics. Here, we present a different approach by use of an electron beam self-pinched to near-solid-density. The beam self-pinching is realized while it transports through a properly designed hollow cone target, where strong azimuthal magnetic fields are generated by the beam-induced plasma return currents at the inner surface of the cone target. In this way, the beam diameter can be reduced by more than an order of magnitude down to submicron and its density is increased by hundreds of times. The produced ultradense electron beams can unlock a new regime of QED-dominated beam-plasma interactions, for example, more than 60% of the beam energy can be converted into GeV gamma-rays with unprecedented brilliance when such a beam passes through a thin solid foil. Moreover, with proper parameter design, this beam-focusing scheme can also be applied to positron beams and thus may find applications in broad areas, such as particle colliders and strong-field physics.