Figure 1

Schematic diagram of the experimental layout.Reuse & Permissions

Figure 2

Total electron beam charge $Q$ measured on L1 as a function of delay $t_d$ for $P=80$, 110, 200 mbar. The solid lines are a moving average using a Gaussian function, defined by $e^{-t_d^2/{\tau }^2}$, where $\tau =5\mathrm{ns}$. Inset: Delay $t_d$ at which electron beams were first observed, and delay at which the highest bunch charge $Q$ was observed, as a function of $P$.Reuse & Permissions

Figure 3

Raw electron energy spectra (a)–(d) showing the fluorescence from L2, and spectra in units of charge per relative energy spread (e)–(h) for: (a),(e) $P=50\mathrm{mbar}$, $t_d=147\mathrm{ns}$; (b),(f) $P=80\mathrm{mbar}$, $t_d=189\mathrm{ns}$; (c),(g) $P=110\mathrm{mbar}$, $t_d=181\mathrm{ns}$; (d),(h) $P=200\mathrm{mbar}$, $t_d=150\mathrm{ns}$.Reuse & Permissions

Figure 4

Total beam charge $Q$ measured on L1 as a function of input laser energy with and without the aperture in position, for $P=95\mathrm{mbar}$. Inset: Images of the focal spot with and without the aperture, on a logarithmic scale to emphasize features in the wings of the pulse.Reuse & Permissions

Figure 5

Calculated values, as a function of propagation distance $z$, of: (a) peak $a_0$, (b) mean spot radius ($\frac{1}{e^2}$) (solid) and FWHM duration (dashed) of laser pulses propagating through a gas cell (black) and a plasma channel (red/blue). For these calculations the channel was parabolic, with $n_e(0)=1.8\times {10}^{18}{\mathrm{cm}}^{-3}$ and lowest-order mode of $41\mu \mathrm{m}$ FWHM, corresponding to $P=80\mathrm{mbar}$; the gas in the cell was fully ionized, with an electron density of $n_e(0)$. The temporal and transverse spatial profiles of the input laser pulses were Gaussian, with initial FWHM duration of 80 fs and FWHM focal spots of $35\mu \mathrm{m}$ (black/red) and $22\mu \mathrm{m}$ (blue). The input laser energies were adjusted to give the measured input fluences when the total on-target laser energy was 2.5 J, i.e., 2 and 1.2 J with and without the aperture, respectively.Reuse & Permissions