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Schematic of experimental layout. The 0.7 J, 42 fs laser pulse is reflected from the deformable mirror (DM) and focused into a 2 mm supersonic helium gas jet. Glass wedges (W1 and W2) are used to attenuate the transmitted laser beam, a factor of before lenses (L1 and L2) are used to image the exit mode onto the diagnostics. A Kodak Lanex regular screen placed on the rear side of W1 acts as an electron beam profile screen, which also serves to measure the total charge in the beam for energies .
Images of the laser mode at the exit of the plasma (2 mm from focus) as a function of plasma density for two different quality focal spots ( and ). All the images are on the same colour table.
Energy depleted from the central part of the focal spot (fwhm) of the laser pulse as a function of plasma density for different quality focal spots. Black squares: (330 mJ within the fwhm); Grey circles: (280 mJ within the fwhm); Light grey diamonds: (140 mJ within the fwhm). The total pulse energy was 700 mJ in all three cases. Each point is an average of five shots, error bars are the standard error.
Variation of the transmitted optical spectrum with focal spot quality as a function of plasma density. Diamonds: ; Squares: (black data points: central frequency; red data points: red limit of spectrum; blue data points: blue limit of the spectrum. Limits described in the text). Each point represents an average from at least five shots, the error bar shown represents a typical standard error. The frequency axis is expressed as the percentage difference from the vacuum laser central frequency, i.e., .
Variation of the total accelerated charge with plasma density for different quality focal spots. Black squares: (330 mJ within the fwhm); Grey circles: (280 mJ within the fwhm); Light grey diamonds: (140 mJ within the fwhm). The total pulse energy was 700 mJ in all three cases. Each point is an average of at least five shots, error bars are the standard error.
(a) Radially averaged experimental focal spot intensity distributions (marks) and the Laguerre-Gauss polynomial representation used in the 3D simulations (solid lines). Black squares: ; Light grey diamonds: . (b) Snapshots of the wakefield produced in 3D simulations at after 1 mm propagation for ) and (c) . The greyscale image shows the electron density on which is superimposed acolour image of the laser electric field amplitude. (d) Variation of laser spectrum as the laser propagates in the two simulations (solid lines are a polynomial fit of the data points).
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