Self-compression of stimulated Raman backscattering by a flying focus

The regime of self-compression has been proposed for plasma-based backward Raman amplification upon a flying focus. By using a pumping focus moving with a speed equal to the group velocity of stimulated Raman backscattering (SRBS), only a short part of SRBS which always synchronizes with the flying focus can be amplified. Therefore, instead of a short pulse, plasma noise or a long pulse can seed the BRA amplifiers. Here we demonstrate the regime by 2D particle-in-cell simulations, showing that the pump pulse is compressed from 26 ps to 116 fs, with an output amplitude comparable with the case of a well-synchronized short seed. As only one laser pulse is used in the simulation, the results present a significant path to simplify the Raman amplifiers.

Figure 1

Illustration of the self-compression regime due to SRBS by flying focus. The flying focus with a growth curve is set the same velocity ($c$) as SRBS but counter to the pump velocity ($-c$). (a) The plasma and SRBS is initially created at the entrance by flying focus. (b) The SRBS propagating with flying focus is amplified to a short pulse.

Figure 2

(a) Distribution of flying focus at $t=900T$, where $T=3.3\text{fs}$ is the single cycle of 1 $\mu \mathrm{m}$ laser, $\left|a\right|$ is the absolute value of the pump amplitude. The velocity of flying focus and the moving window is the same (0.99c), so the focus is almost static in the simulation window. (b)Plasma density at $t=900T$, the gas at right boundary of the moving window is kept neutral. (c) 2D distribution of the SRBS amplitude and electron energy obtained by flying focus at $t=4000T$, where $\left|b\right|$ is the absolute value of the SRBS amplitude. (d) Comparison of the electron temperature with static and flying focus at $t=4000T$

Figure 3

(a) Numerical noise of the plasma wave ($E_z$) with different $N$ at $T_e=50$ eV, ${n_e}_0=0.02n_c$. (b) Spectra intensity of the numerical noisy $E_z$ with different $N$ at $T_e=50$ eV, ${n_e}_0=0.02n_c$. (c) On-axial $\left|b\right|$ at $t=900T$ for plasma density of $0.015n_c$ and $0.03n_c$. (d) Spectrum of on-axial $\left|b\right|$ developing from plasma noise at different plasma densities. In the simulation, $\left|b\right|$ is the absolute value of the SRBS amplitude, also normalized by the unit of $m_e{c}^2/e, a_0=0.01$.

Figure 4

The optimal 2D simulation results of the seedless BRA with flying focus, for ${n_e}_0=0.015n_c$. (a) Profile of the normalized pump amplitude at $t=2000T$. (b) Structure of plasma wave ($E_z$) at $t=2000T$. (c) Profiles of the SRBS amplitude ($\left|b\right|$) at different interaction time.

Figure 5

Simulation results of (a) peak intensity and (b) FWHM duration with a 2-ps seed, a 200-fs seed, and plasma noise. (c) On-axial pulse shapes of SRBS at $t=900T$. (d) On-axial pulse shapes of SRBS at $t=4000T$.