Long-Range Persistence of Femtosecond Modulations on Laser-Plasma-Accelerated Electron Beams

Laser plasma accelerators have produced femtosecond electron bunches with a relative energy spread ranging from 100% to a few percent. Simulations indicate that the measured energy spread can be dominated by a correlated spread, with the slice spread significantly lower. Measurements of coherent optical transition radiation are presented for broad-energy-spread beams with laser-induced density and momentum modulations. The long-range (meter-scale) observation of coherent optical transition radiation indicates that the slice energy spread is below the percent level to preserve the modulations.

Figure 1

Schematic of the experimental setup for detection of OTR from the $e$ beam propagating through two foils (OTR1 and OTR2), positioned several meters from the LPA source. The inset figures on the top illustrate how an example optical modulation on the longitudinal $e$-beam velocity distribution $v_z(z)$ can develop into a density modulation $n(z)$.

Figure 2

(a) The coherent enhancement at OTR2 as a function of charge with different targets. The experimental parameters are shown in the legend, with labels I–V. The error bars show the standard deviation. The gray area indicates the range where signals are considered to be incoherent. Typical OTR2 and phosphor images for each target are listed on the right of the plot, with decreasing coherent enhancement from shots (b)–(f).

Figure 3

(a) Modeled coherent enhancement as a function of electron energy and slice energy spread (see the text for an explanation of the parameters used). The experimental observations ($\times 1000$ enhancement) suggest that the coherence is attributed to high-energy electrons ($\gamma \gtrsim 300$) with a slice energy spread $\lesssim 1\%$. Inset (b) shows the 800-nm spectral component of the on-axis longitudinal density modulation $|\int dz\rho (0,z)e^{i{k}_{m}z}|$ as a function of ${\delta }_L$ (by using $L_0=3.8\mathrm{m}$, $\gamma =500$, and $\Delta \gamma /\gamma =0.5\%$).

Figure 4

(a)–(d) Comparison of the simultaneously measured single-shot OTR1 and OTR2 images in the 0.9 mm gas jet experiment III. The color scale represents the calibrated photon counts. Note that in images (c) and (d) the OTR2 foil, while farther from the source, emits more COTR than the OTR1 foil. (e) One of several shots where the optical spectrum was measured (few-nanometer spectral resolution).