Prospects and limitations of wakefield acceleration in solids

TL;DR

This paper explores the potential of X-ray driven wakefield acceleration in solids, demonstrating through simulations that quantum effects are significant but manageable, enabling extremely high-gradient acceleration with reduced radiation reaction effects.

Contribution

The study provides the first detailed simulation analysis of X-ray driven wakefield acceleration in solids, highlighting the role of quantum effects and radiation reaction at nanometer wavelengths.

Findings

Similarity scaling laws hold at 5 nm wavelength with high relativistic amplitudes.

Quantum parameter χ reaches around 1 at moderate amplitudes, indicating significant quantum effects.

Reduced radiation reaction energy loss at X-ray wavelengths compared to optical wavelengths.

Abstract

Advances in the generation of relativistic intensity pulses with wavelengths in the X-ray regime, through high harmonic generation from near-critical plasmas, opens up the possibility of X-ray driven wakefield acceleration. The similarity scaling laws for laser plasma interaction suggest that X-rays can drive wakefields in solid materials providing TeV/cm gradients, resulting in electron and photon beams of extremely short duration. However, the wavelength reduction enhances the quantum parameter $\chi$, hence opening the question of the role of non-scalable physics, e.g., the effects of radiation reaction. Using three dimensional Particle-In-Cell simulations incorporating QED effects, we show that for the wavelength $\lambda=5\,$nm and relativistic amplitudes $a_0=10$-100, similarity scaling holds to a high degree, combined with $\chi\sim 1$ operation already at moderate $a_0\sim 50$, leading to photon emissions with energies comparable to the electron energies. Contrasting to the generation of photons with high energies, the reduced frequency of photon emission at X-ray wavelengths (compared to at optical wavelengths) leads to a reduction of the amount of energy that is removed from the electron population through radiation reaction. Furthermore, as the emission frequency approaches the laser frequency, the importance of radiation reaction trapping as a depletion mechanism is reduced, compared to at optical wavelengths for $a_0$ leading to similar $\chi$.