Abstract
Compact electron accelerators are paramount to next-generation synchrotron light sources and free-electron lasers, as well as for advanced accelerators at the TeV energy frontier. Recent progress in laser-plasma driven accelerators (LPA) has extended their electron energies to the multi-GeV range and improved beam stability for insertion devices. However, the subluminal group velocity of plasma waves limits the final electron energy that can be achieved in a single LPA accelerator stage, also known as the dephasing limit. Here, we present the first laser-plasma driven electron accelerator concept providing constant acceleration without electrons outrunning the wakefield. The laser driver is provided by an overlap region 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 the accelerated electrons. Such a geometry of laterally coupling the laser into a plasma allows for the overlap region to move with the vacuum speed of light, while the laser fields in the plasma are continuously being replenished by the successive parts of the laser pulses. Our scheme is robust against parasitic self-injection and self-phase modulation as well as drive-laser depletion and defocusing along the accelerated electron beam. It works for a broad range of plasma densities in gas targets. This method opens the way for scaling up electron energies beyond 10 GeV, possibly towards TeV-scale electron beams, without the need for multiple laser-accelerator stages.
- Received 5 March 2018
- Revised 4 March 2019
DOI:https://doi.org/10.1103/PhysRevX.9.031044
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
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Popular Summary
Compact electron accelerators are key components for next-generation probes of matter at ultrafast and ultrasmall scales. To that end, laser-wakefield accelerators (LWFAs) trap and accelerate electrons up to GeV energies in a few centimeters using a laser-driven plasma wave. However, the laser pulse and the plasma wave propagate slightly slower than the speed of light, so that the electrons outrun the plasma wave, which imposes a fundamental limit on the maximum energy gain. Reaching higher electron energies requires multiple LWFA stages in sequence, which is very challenging to control. Here, we propose a novel, scalable, laser-plasma accelerator scheme without these limitations, extending the maximum electron energy—possibly indefinitely.
We introduce the traveling-wave electron accelerator (TWEAC), in which the wakefield driver is provided by a region of overlap of two oblique ultrashort laser pulses with tilted pulse fronts in the focus of two cylindrical mirrors, aligned to coincide with the trajectory of subsequently accelerated electrons. Such a geometry allows for the region of overlap to move at the vacuum speed of light while its field is continuously replenished by the successive parts of the laser pulse. Supported by 3D simulations, we show that this results in quasistationary acceleration conditions for an electron bunch along the total acceleration length that circumvent the limits of LWFA.
We expect the TWEAC technique to greatly reduce the need for staging to attain electron energies beyond 10 GeV, possibly reaching for the energy frontier of high-energy physics. For lower GeV-scale electron energies, TWEAC at high plasma densities and 10-TW-class laser systems could enable compact accelerators at kHz-range repetition rates.
