A schematic of the experimental setup. The three primary diagnostics are shown: forward electron spectrometer utilizing the 0.5 T permanent magnet, Michelson interferometry, and a forward image of the spot at the exit plane.
Measured plasma density profiles from the 3, 5, and 8 mm gas jets and from the 14-mm-long gas cell. Direct measurements of the density of the gas cell were limited to the window region. The dashed line shows the extrapolated region.
Images of the raw images plates showing electron charge with the direction of energy gain indicated are shown in (a). The charge in pC/MeV is plotted in (b) for the results shown in (a). A plot of the energy of the quasimonoenergic feature of the accelerated electrons as a function of density is plotted in (c) as the black dots. The curve is a plot of Eq. (2) representing the maximum (dephasing limited) energy of trapped electrons at these densities.
Data showing a decrease in self-trapped charge with decreased density in pure helium plasma. A threshold is shown at approximately below which no trapped charge is observed.
An image of the laser pulse at the exit of the 14 mm gas cell is shown for the case of vacuum (a) and with pure helium plasma with a density of (b). The white curve in shows a sum out over the entire chip along the vertical direction, while the black curve shows a lineout through the peak. The color tables represent energy/pixel (proportional to CCD counts) in the ranges of (a) 0–6100 and (b) 0–8800.
2D particle-in-cell simulations at a density of comparing the case of no oxygen (a) in which no trapped charge is present vs 3% oxygen (b) which shows trapped charge with the same plasma and laser parameters.
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