Abstract
We present experimental results of vacuum laser acceleration (VLA) of electrons using radially polarized laser pulses interacting with a plasma mirror. Tightly focused radially polarized laser pulses have been proposed for electron acceleration because of their strong longitudinal electric field, making them ideal for VLA. However, experimental results have been limited until now because injecting electrons into the laser field has remained a considerable challenge. Here, we demonstrate experimentally that using a plasma mirror as an injector solves this problem and permits to inject electrons at the ideal phase of the laser, resulting in the acceleration of electrons along the laser propagation direction while reducing the electron beam divergence compared to the linear polarization case. We obtain electron bunches with few-MeV energies and a 200 pC charge, thus demonstrating for the first time electron acceleration to relativistic energies using a radially polarized laser. High-harmonic generation from the plasma surface is also measured and provides additional insight into the injection of electrons into the laser field upon its reflection on the plasma mirror. Detailed comparisons between experimental results and full 3D simulations unravel the complex physics of electron injection and acceleration in this new regime: we find that electrons are injected into the radially polarized pulse in the form of two spatially-separated bunches emitted from the p-polarized regions of the focus. Finally, we leverage on the insight brought by this study to propose and validate a more optimal experimental configuration that can lead to extremely peaked electron angular distributions and higher energy beams.
Highlights
Owing to the progress of intense femtosecond lasers [1], new methods for accelerating particles have been developed in the last two decades
We present experimental results of vacuum laser acceleration (VLA) of electrons using radially polarized laser pulses interacting with a plasma mirror
Detailed comparisons between experimental results and full 3D simulations unravel the complex physics of electron injection and acceleration in this new regime: We find that electrons are injected into the radially polarized pulse in the form of two spatially separated bunches emitted from the p-polarized regions of the focus
Summary
Owing to the progress of intense femtosecond lasers [1], new methods for accelerating particles have been developed in the last two decades. Experimental observation of high energy gains from VLA has proven difficult to achieve [16,17,18,19] This difficulty is due to the fact that acceleration is efficient when electrons are injected with a high initial velocity at a precise phase of the laser pulse, close to a zero of the electric field, so that they remain in an accelerating phase of the laser for a long time. Electrons are injected with no initial velocity at a phase of the laser that is not optimal for electron acceleration (close to a maximum of the electric field), which leads to inefficient acceleration. It is challenging to inject the electrons at a precise phase of the accelerating structure, as it would require electron bunches with attosecond duration
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