Effect of chirp on self-modulation and laser wakefield electron acceleration in the regime of quasimonoenergetic electron beam generation

Strongly self-modulated regime of laser wakefield electron acceleration has been experimentally studied using positively/negatively chirped Ti:sapphire laser pulses with duration $\ge 45\mathrm{fs}$, and intensity $\le 1.2\times {10}^{18}\mathrm{W}/{\mathrm{cm}}^2$ interacting with a plasma having density $>5\times {10}^{19}{\mathrm{cm}}^{-3}$. The total charge of the high energy electrons produced from self-injection and acceleration in the wakefield was found to depend strongly and asymmetrically on the magnitude and sign of the chirp, which was also corroborated by the forward Raman scattering signals. Generation of low divergence ($\le 10\mathrm{mrad}$), quasimonoenergetic electron beam was observed with a maximum energy of $\sim 28\mathrm{MeV}$ for a positively chirped laser pulse of duration $\sim 70\mathrm{fs}$. The observations are explained considering faster rise time associated with a positive chirped laser pulse which generates intense seed for stronger modulation of the laser pulse and consequently stronger wakefield excitation leading to higher electron beam charge and energy.

Figure 1

A schematic diagram of the experimental setup for electron acceleration.

Figure 2

Variation in the integrated electron beam charge at two different plasma densities (solid squares: $8.5\times {10}^{19}{\mathrm{cm}}^{-3}$; and hollow squares: $6.5\times {10}^{19}{\mathrm{cm}}^{-3}$) with compressor grating pair separation (chirp) measured with respect to the “zero” setting (separation corresponding to minimum pulse duration). The total charge for zero setting of the compressor is 2 and 8 nC, respectively, for plasma density $6.5\times {10}^{19}{\mathrm{cm}}^{-3}$ and $8.5\times {10}^{19}{\mathrm{cm}}^{-3}$. The variation of the laser pulse width (circles) with grating separation is also shown.

Figure 3

The transmitted laser spectra at two different plasma densities: (a) without interaction with plasma, (b) to (f) with interaction at $n_e\simeq 6.5\pm 0.5\times {10}^{19}{\mathrm{cm}}^{-3}$, for grating separation of 0 (45 fs), $-320\mu \mathrm{m}$ (70 fs), $-600\mu \mathrm{m}$ (125 fs), $-800\mu \mathrm{m}$ (170 fs), and $+200\mu \mathrm{m}$ (52 fs), respectively, (g) to (i) at $n_e\simeq 8.5\pm 0.5\times {10}^{19}{\mathrm{cm}}^{-3}$ for grating separation of 0 (45 fs), $-400\mu \mathrm{m}$ (85 fs), and $+200\mu \mathrm{m}$ (52 fs), respectively. No distinct Raman peak could be resolved from the transmitted spectra due to large broadening of laser spectrum [as shown in (e)] for grating separation $\gtrsim -800\mu \mathrm{m}$ for a density of $6.5\pm 0.5\times {10}^{19}{\mathrm{cm}}^{-3}$. Similar observation was made for $n_e\simeq 8.5\pm 0.5\times {10}^{19}{\mathrm{cm}}^{-3}$ at grating separation of $>-400\mu \mathrm{m}$ (spectra not shown here). Raman peaks disappear for grating separation $\gtrsim +200\mu \mathrm{m}$ (-ve chirp).

Figure 4

(a) The electron beam profile at $n_e\simeq 8.5\pm 0.5\times {10}^{19}{\mathrm{cm}}^{-3}$, for a grating separation of $0\mu \mathrm{m}$ (45 fs). (b) Image of multiple low divergence ($\lesssim 10\mathrm{mrad}$) and high energy ($\gtrsim 2.5\mathrm{MeV}$) electron beams at $n_e\simeq 6.5\pm 0.5\times {10}^{19}{\mathrm{cm}}^{-3}$, for a grating separation of $-400\mu \mathrm{m}$ (85 fs).

Figure 5

Images of quasimonoenergetic electron spectra recorded at $n_e\simeq 8.5\pm 0.5\times {10}^{19}{\mathrm{cm}}^{-3}$, for different separations of the laser pulse compressor gratings: (a) $0\mu \mathrm{m}$ (45 fs), (b) $-320\mu \mathrm{m}$ (70 fs), and (c) $-640\mu \mathrm{m}$ (135 fs).