Capturing relativistic wakefield structures in plasmas using ultrashort high-energy electrons as a probe

Abstract

A new method capable of capturing coherent electric field structures propagating at nearly the speed of light in plasma with a time resolution as small as a few femtoseconds is proposed. This method uses a few femtoseconds long relativistic electron bunch to probe the wake produced in a plasma by an intense laser pulse or an ultra-short relativistic charged particle beam. As the probe bunch traverses the wake, its momentum is modulated by the electric field of the wake, leading to a density variation of the probe after free-space propagation. This variation of probe density produces a snapshot of the wake that can directly give many useful information of the wake structure and its evolution. Furthermore, this snapshot allows detailed mapping of the longitudinal and transverse components of the wakefield. We develop a theoretical model for field reconstruction and verify it using 3-dimensional particle-in-cell (PIC) simulations. This model can accurately reconstruct the wakefield structure in the linear regime, and it can also qualitatively map the major features of nonlinear wakes. The capturing of the injection in a nonlinear wake is demonstrated through 3D PIC simulations as an example of the application of this new method.

Femtosecond Probing of Plasma Wakefields and Observation of the Plasma Wake Reversal Using a Relativistic Electron Bunch

“…This linear portion is critical for preserving the emittance of the accelerating electrons. Although the wake measured in this experiment is in the linear regime, this probing technique can be extended to give qualitative features of highly nonlinear wakes [27].…”

“…The transit time of the probe through the wake is 66 fs. However, the time resolution is mainly determined by the length of the probe beam (1-3 fs rms) for a linear wake as explained in [27].…”

“…Therefore the density modulation δn/n 0 is related to the electric field E x,y,z of the wake by [27],…”

Mapping electromagnetic fields structure in plasma using a spin polarized electron beam

“…Certainly it also means that we can only get the electric fields information in the current probe case. Since the probe is relativistic and the wake size is limited, the time for transmission through the wake field is too short to gain enough transverse kicking 31 . We then assume that the velocity of the probe beam does not change by the wakefields while it travels through the wake.…”

Visualization of relativistic laser pulses in underdense plasma

“…Whether laser-driven wakefield acceleration (LWFA) or beam-driven plasma wakefield acceleration, the evolution of the pump and of the wakefield structure in the plasma is of critical importance for the resulting properties of the accelerated particles, e.g., electrons or positrons. Whereas diagnostics for characterizing the accelerated particle bunch [8][9][10][11][12], their associated magnetic fields [13,14], the wakefield's plasma-density distribution [15][16][17], and quasielectrostatic fields [18,19], as well as the laser or particle driver in vacuum [20,21] are available, measurement of the driver in situ has been limited to nonspatially resolved indicators in terms of the pump's local intensity distribution [22,23].…”