Temporal Characterization of Femtosecond Laser-Plasma-Accelerated Electron Bunches Using Terahertz Radiation

The temporal profile of relativistic laser-plasma-accelerated electron bunches has been characterized. Coherent transition radiation at THz frequencies, emitted at the plasma-vacuum boundary, was measured through electro-optic sampling. Frequencies up to the crystal detection limit of 4 THz were observed. Comparison between data and theory indicates that THz radiation from bunches with structure shorter than $\simeq 50\mathrm{fs}$ (root-mean-square) is emitted. The measurement demonstrates both shot-to-shot stability of the laser-plasma accelerator and femtosecond synchronization between bunch and probe beam.

Figure 1

Schematic representation of the LWFA, the THz (CTR) beam path, and the EO detection setup.

Figure 2

The main plot shows the modeled EO signal $S_{\mathrm{EO}}(\tau )$ using a ZnTe crystal (solid curve) and using a dispersion-, absorption-, and mismatch-free crystal (dashed curve). In this model, the THz radiation is CTR from a 50 fs (rms) Gaussian electron bunch. The inset shows the amplitude of the CTR spectrum $|E_{\mathrm{THz}}(\nu )|$ (dashed curve), as well as the ZnTe transfer function $|T_{\mathrm{ZnTe}}(\nu )|$ (solid curve), and spectrum of the envelope of the probe beam (dotted curve).

Figure 3

The main plot shows the measured EO signal $S_{\mathrm{EO}}(\tau )$ (gray dots), with the black curves representing the modeled EO signal based on CTR from a single-bunch charge profile (dashed curve) and double-bunch profile (solid curve, with 30% of charge in leading bunch at 230 fs temporal separation). The length of each bunch in the model is set at 50 fs (rms). The Fourier transformation of the data (gray dots) and modeled waveforms are shown in the inset (solid curve for double-bunch profile, dashed curve for single-bunch profile).