Experimental Measurements of Electron-Bunch Trains in a Laser-Plasma Accelerator

Spectral measurements of visible coherent transition radiation produced by a laser-plasma-accelerated electron beam are reported. The significant periodic modulations that are observed in the spectrum result from the interference of transition radiation produced by multiple bunches of electrons. A Fourier analysis of the spectral interference fringes reveals that electrons are injected and accelerated in multiple plasma wave periods, up to at least 10 periods behind the laser pulse. The bunch separation scales with the plasma wavelength when the plasma density is changed over a wide range. An analysis of the spectral fringe visibility indicates that the first bunch contains most of the charge.

Figure 1

Selected and sorted examples of sinusoidal CTR modulations, obtained using CPI for $n_e=10\times {10}^{18}{\mathrm{cm}}^{-3}$ and ${\lambda }_p=10.6\mu \mathrm{m}$. The three panels in each row represent one measurement. [(a)–(g)] CTR spectra. [(h)–(n)] Square amplitude of the Fourier transform of the CTR spectra. [(o)–(u)] Corresponding electron spectra.

Figure 2

Selected examples of complex CTR modulations, obtained using CPI for $n_e=10\times {10}^{18}{\mathrm{cm}}^{-3}$ and ${\lambda }_p=10.6\mu \mathrm{m}$. The three panels in each row represent one measurement. [(a)–(d)] CTR spectra. [(e)–(h)] Fourier transform of the CTR spectra. [(i)–(l)] Corresponding electron spectra.

Figure 3

Fringe visibility for sinusoidal oscillation that contained only one significant peak in the Fourier transform, as a function of the position of that peak. The charge ratio $\xi =q_n/q_o$ is calculated from the visibility using Eq. (2) in the limit where both bunches have similar shapes and energy distributions. The measurements were performed using CPI for $n_e=10\times {10}^{18}{\mathrm{cm}}^{-3}$.

Figure 4

Average positions of the Fourier peaks for different plasma densities (o) are compared to multiples of the plasma wave period (lines). Horizontal error bars denote the standard deviation over the complete data set. Vertical error bars denote estimates of the uncertainty in plasma density.