Abstract

Author(s): Debus, A; Pausch, R; Huebl, A; Steiniger, K; Widera, R; Cowan, TE; Schramm, U; Bussmann, M | 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.

Highlights

  • Laser-wakefield accelerators (LWFA) [1,2,3,4,5,6,7,8,9,10,11,12,13] are driven by ultrashort, intense laser pulses traversing an underdense plasma, exciting a charge-density plasma wave

  • 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

  • The second central aspect of TravelingWave Electron Acceleration (TWEAC) is that a stable and experimentally controllable plasma cavity is achieved by having at every instant a new, unspoiled section of the laser pulse, which has not yet undergone self-phase modulation, transversely entering the plasma and, after only a short propagation distance, forming the acceleration cavity in plasma regions previously unperturbed by lasers

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Summary

INTRODUCTION

Laser-wakefield accelerators (LWFA) [1,2,3,4,5,6,7,8,9,10,11,12,13] are driven by ultrashort, intense laser pulses traversing an underdense plasma, exciting a charge-density plasma wave. Despite state-of-the-art ultrashort, petawatt-scale lasers exceeding intensity and pulse energy requirements of LWFA self-injection [18], it is this dephasing and depletion limit that currently constrain LWFA peak energies to a range of hundreds of MeV to several GeV [9,11,13] In principle, these limitations can be overcome by using multiple LWFA stages to successively accelerate one electron beam to higher energies. Extending the beam energy gained within a single stage beyond the dephasing limit is a prime objective in the development of plasma-based accelerators: In LWFAs, spatially tapering the plasma density profile within a plasma waveguide can extend Ld by precisely tailored density down-ramps [24,25], which speed up the plasma phase velocity in order to synchronize accelerating electrons with the wakefield phase Such wakefields can, in principle, be maintained indefinitely at the price of a rapidly decreasing acceleration gradient. The new approach is free of the usual dephasing, depletion, or guiding constraints of LWFA

TRAVELING-WAVE ELECTRON ACCELERATION
PARTICLE-IN-CELL SIMULATIONS OF TWEAC
CAVITY FORMATION IN TWEAC
ELECTRON PROPERTIES
LASER FIELDS ARE QUASISTATIONARY
LASER PULSE ENERGY SCALING
VIII. CONCLUSION AND OUTLOOK
TWEAC robustness to laser pointing variations
Plasma-density independence of α in translationally symmetric media
Simulation model
Findings
Numerical challenges

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