Plasma photonic spatiotemporal synchronization of relativistic electron and laser beams

We present an ultracompact plasma-based method to measure spatial and temporal concurrence of intense electron and laser beams nonintrusively at their interaction point. The electron beam couples with a laser-generated seed plasma in dependence of spatiotemporal overlap, which triggers additional plasma production and manifests as enhanced plasma afterglow. This optical observable is exploited to measure beam concurrence with $\sim 4\mu \mathrm{m}$ spatial and $\sim 26.7\mathrm{fs}$ temporal accuracy, supported by auxiliary diagnostics. The afterglow interaction fingerprint is highly sensitive and enables ultraversatile femtosecond-micrometer beam metrology.

Figure 1

Experimental setup and afterglow shots: (a) The electron beam propagates through gas and couples with an overlapping laser-generated seed plasma. (b) Example of laser-only plasma afterglow at 587 nm. (c) Example of electron-beam-enhanced plasma afterglow.

Figure 2

PIC-simulation snapshots. (a) The electron beam approaches the emerging laser-generated ${\mathrm{H}}_2/\mathrm{He}$ plasma filament (black dots), reaching the filament center at $\mathrm{\Delta }t=0$. (b),(c) The electron beam crosses the filament and heats plasma electrons via its unipolar transverse field (blue). Plasma surface waves (blue, central slice of simulation) propagate along the filament and transport density perturbations. (d)–(f) Localized plasma electron oscillations visualized by typical electron trajectories (solid lines), color coded by momentary energy.

Figure 3

Simulated evolution of the spectral plasma electron distribution $Q$ ($W_{\mathrm{kin}}>W_{\mathrm{thresh},\mathrm{H}2}\approx 15.4\mathrm{eV}$) outside (red) and inside (gray) the seed plasma region with radius $24\mu \mathrm{m}$. (a)–(e) During the initial interaction, the electron energies quickly assume a broadband, nonrelativistic distribution. (a)–(f) Colored circles represent oscillating electrons shown in Fig. 2 to indicate their changing energies and corresponding momentary cross sections ${\sigma }_{\mathrm{Gas}}$. Electrons inside (transparent) the initial plasma volume cannot collide with neutrals, while electrons outside (solid) ionize ambient neutral gas at impact ionization rates $R={\sigma }_{\mathrm{Gas}}{n}_{\mathrm{Gas}}\beta c$ per electron (black lines).

Figure 4

Spatial alignment scan with 8–10 shots per setting with experimentally observed plasma afterglow enhancement (black) and normalized energy transfer obtained from simulation (red).

Figure 5

Experimental time-of-arrival scan over 256 consecutive shots within $-2.5\mathrm{ps}<\mathrm{\Delta }{t}_{\text{delay}}<4.0\mathrm{ps}$. Black: experimental normalized afterglow enhancement. Blue: sigmoid fit for experimental data. Red: normalized energy transferred into the plasma from PIC simulations. Orange: a FACET-II-class beam yields a substantially steeper transition.