Anomalous Group velocity and Plasma Dispersion in the Laser Wakefield Accelerator through a new Relativistic Theory

TL;DR

This paper introduces a novel relativistic theory for Laser Wakefield Accelerators that accounts for pulse evolution, revealing extraordinary plasma physics phenomena, including near-equal group and phase velocities and a new dispersion branch, validated by simulations.

Contribution

A new relativistic theory incorporating pulse evolution effects in LWFA, extending the Quasi-Static Approximation and revealing novel dispersion phenomena.

Findings

Local group and phase velocities are approximately equal.

Group velocity evolves over time, remaining above the linear value.

A new dispersion branch with a linear relation to light speed emerges.

Abstract

A new cold relativistic theory is proposed to describe the Laser WakeField Accelerator (LWFA) in the presence of pulse evolutions, capable of being utilized to study the group velocity and the plasma dispersion. This new capability is mainly due to exploiting the concept of the real Lorentz-boost Pulse Co-Moving (LPCM) frame, in spite of previous studies. The theory is reduced to the well-known Quasi-Static Approximation (QSA) in the absence of the pulse evolutions, and shows excellent agreement with Particle-In-Cell (PIC) simulations in terms of its new results. The obtained results show the extremely extra-ordinary nature of the fully nonlinear plasma physics of LWFA. It is turned out that the local group and phase velocities of the light are approximately equal. The obtained group-velocity evolves in time according to ( and are parameters depending on wake amplitude and initial group velocity) at early stages, showing non-explicit density dependency and remaining above the linear value over a long period of the propagation. The obtained equations for the carrier-mode, on the other hand, consistently suggest the emergence of a new dispersion branch with the linear relation ( is the light speed). Regarding these remarks, we expend on the plasma dispersion in details with the aid of simulations, confirming the observed anomalies and the emergence of the new branch. In addition, a detailed description of the spectral evolutions in the dispersion plan is provided and it is shown that the dispersion anomalies tend to cease at long pulse lengths.