(Color online) Experimental setup used for laser wake-field acceleration.
(Color online) Focal spot of the laser beam (45 fs duration) when focused with f/6 off-axis parabolic mirror. (a) 2D image of the focal spot, and (b) intensity profile of the focus along a horizontal line at Y = 0.
(Color online) Pre-pulse due to amplified spontaneous emission (ASE) measured with a fast photo-diode with 0.8 ns rise time. The dash-dot (∼1 ns) and the dashed (∼3 ns) traces correspond to different switching times of the pulse cleaning Pockels cell.
Shadowgram of the pre-formed plasma channel produced by 1 ns long section of ASE pre-pulse of intensity 3 × 1012 W/cm2. The main laser beam was focused near the entrance edge of the gas jet, which is indicated by the vertical dotted line and the direction of laser propagation is indicated by a horizontal arrow on the left. The density of helium gas was nHe ∼ 3.2 × 1019 cm−3.
Guiding of 45 fs duration, 4 TW laser pulse focused to intensity 3 × 1018 W/cm2. The images of guiding recorded by (a) top-view CCD, and (b) side-view CCD. The images were recorded at helium gas density ∼3.2 × 1019 cm−3 under the conditions of ASE pre-pulse duration ∼1ns and its intensity ∼3 × 1012 W/cm2.
Side-view image of 45 fs laser pulse guiding in plasma with channel splitting (as indicated by two arrows) observed in some of the shots. The arrow and a vertical line on the left of the channel image represent direction of laser propagation and the front edge of the gas jet. The images were recorded at helium gas density ∼3.2 × 1019 cm−3 under the conditions of ASE pre-pulse duration ∼1ns and its intensity ∼3 × 1012 W/cm2.
The side-view image of plasma due to non-linear Thomson scattering of main 45 fs duration laser pulse at helium gas density 3.2 × 1019 cm−3 and with ASE pre-pulse duration of ∼2 ns and intensity 3 × 1012 W/cm2.
(Color online) Results of the electron beam profile measurement. (a) Image of the low divergence electron beam observed when ASE pre-pulse duration is 1 ns. The concentric circles represent 5° and 10° full cone angles and the common centre of the circles represents the laser axis in vacuum and is shown with white colored dot. (b) Low intensity and large divergence images of electrons observed occasionally when ASE pre-pulse duration is 2 ns.
(Color online) Measurement of pointing variation of the electron beam produced in a series of laser shots. (a) Images of the low divergence electron beam. The concentric circles in each image represent 5°, 10°, and 15° full cone angles and the common centre of the circles represents the laser axis in vacuum. (b) Plot of pointing variation of well collimated electron beam. The dotted circle represents laser divergence (in vacuum) cone angle with its center at laser axis. The rectangle at the centre represents the acceptance angle of the electron energy spectrometer. The average pointing angle of the electron beam is shown with symbol ‘⊗’.
(Color online) Result of energy measurement of the accelerated electron beam. (a) Image of the energy dispersed electron beam showing low divergence and quasi-mono-energetic feature, and (b) energy spectrum with quasi-mono-energetic distribution with peak at 50 MeV.
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