Traveling-Wave Electron Acceleration: Laser-plasma acceleration without limits from dephasing and pump-depletion

We show how to simultaneously eliminate both dephasing and depletion constraints of laser-plasma accelerators. For this we introduce the Traveling-wave electron accelerator (TWEAC) approach, in which the wakefield driver is not provided by a single laser pulse, but instead by a region of overlap of two obliquely incident, ultrashort laser pulses with tilted pulse-fronts in the line foci of two cylindrical mirrors, aligned to coincide with the trajectory of subsequently accelerated electrons. Such a geometry of laterally coupling the laser into a plasma allows for the region of overlap to move with the vacuum speed of light, while its field is continuously being replenished by the successive parts of the laser pulse. Supported by 3D particle-in-cell simulations, we show that this results in quasi-stationary acceleration conditions for an electron bunch along the total acceleration length, which allows to break both the dephasing and depletion limit of LWFA.

Particularly, and in contrast to LWFA and PWFA, a single TWEAC-stage can arbitrarily be extended in length to higher electron energies without changing the underlying acceleration mechanism. Additionally, the TWEAC geometry greatly facilitates reducing beam transport distances between the laser-plasma accelerator and subsequent insertion devices, such as undulators, plasma lenses or colliding laser pulses, to below millimeters. After analyzing stability of acceleration and possible limits of the scheme, we present energy scaling laws for both laser as well as electrons and detail experimental design considerations.

In the future, we expect the new TWEAC technique to greatly reduce the need for staging in order to attain higher electron energies beyond 10GeV, possibly reaching for the energy frontier of high-energy physics. For lower GeV-scale electron energies, TWEAC at high plasma densities and 10TW-class laser systems could enable compact accelerators at kHz-repetition rates.