# Presentation: 828360

## Suppression of a Parasitic Pump Side-Scattering in

- Backward Raman Amplifiers of Laser Pulses in Plasmas
- A. Solodov, V. M. Malkin, N. J. Fisch

## Abstract

## Conceptual Scheme of Backward Raman Amplifiers (BRA)

**k**p**=k**1**-k**2, *ω*p*=**ω*1*-**ω*2

- The plasma wave forms a 3D Bragg cell grating that scatters power from the pump into the seed

- At the nonlinear amplification stage the amplified seed can completely deplete the pump

## Conceptual Scheme of BRA (Continued)

- Resent studies showed that short pumped pulses of nearly relativistic non-focused intensities are expected: *I~*1017 W/cm2 for *λ*=1 μm [1].
- This is 5 orders of magnitude higher than currently available through chirped pulse amplification [2].
- Additional intensity gain is provided by focusing.
- [1] V. M. Malkin, G. Shvets, and N. J. Fisch, Phys. Rev. Lett. **82**, 4448 (1999).
- [2] G. A. Mourou, C. P. J Barty, and M. D. Perry, Phys. Today **51**, 22 (1998).

## Problem: Pump Can be Prematurely Depleted by Raman Scattering Seeded by Thermal Langmuir Noise, before it Reaches the Seed Pulse

- It is the same efficiency of stimulated Raman scattering, that makes possible the fast compression, which can complicate the pump transporting to the seed.
- The problem is aggravated by the fact that the linear Raman scattering (responsible for the noise amplification) has a larger growth rate than its nonlinear counterpart (responsible for the useful amplification of the seed).

## The Premature Pump Backscattering Can be Suppressed by an External Detuning of the Raman Resonance [3]

*δω**=**δω*plasma-*δω*pump
- It appears to be possible to suppress the unwanted Raman backscattering of the pump by noise, while not suppressing the desirable seed pulse amplification.
- The filtering effect occurs because in the nonlinear regime the pumped pulse duration decreases inversely proportional to the pulse amplitude. The increased frequency bandwidth allows to tolerate larger and larger external frequency detuning.

- [3] V. M. Malkin, G. Shvets, and N. J. Fisch, Phys. Rev. Lett. **84**, 1208 (2000).

## The Goal of the Present Paper is to Analyze How a Parasitic Pump Side-Scattering in BRA Can be Suppressed

- The Raman growth rate maximizes for backscattering:

- (linearly polarized pump with normalized amplitude *a*0*=eA/mc*2, scattering in the plane perpendicular to the pump polarization).
- However, the side-scattered radiation has more time and a longer distance to be amplified by the pump, before it leaves the plasma.
- These two effects practically compensate each other making suppression of pump side-scattering an important task.

## Main Equations

- The linear stage of SRS is described by
- where the vector-potential envelopes of the pump and scattered Stokes waves and Langmuir wave electric field are defined:
- The pump wave frequency, wavenumber, and the unit polarization vector are

## Main Equations (Continued)

## Main Equations (Continued)

## Green’s Function

## Green’s Function (Continued)

## Premature Pump Backscattering and Side-Scattering in Absence of Detuning

## Suppression of a Parasitic Pump Side-Scattering in a

- Backward Raman Amplifier

## Suppression of a Parasitic Pump Side-Scattering in a

- Backward Raman Amplifier (Continued)

## Suppression of a Parasitic Pump Side-Scattering in a

- Backward Raman Amplifier (Continued)

- linear polarization,
*φ**=**π**/*2

- linear polarization,
*φ**=**π**/*6

- circular polarization;
- also
- linear polarization,
*φ**=**π**/*4

## Scattering Cross-Section of the Pump Beam in BRA with Frequency Detuning

## Scattering Cross-Section of the Pump Beam in BRA with Frequency Detuning (Continued)

## Suppression of a Parasitic Pump Scattering in a Raman Amplifier with Arbitrary Angles between Pump and Seed Pulses

## Suppression of a Parasitic Pump Scattering in a Raman Amplifier with Arbitrary Angles between Pump and Seed Pulses

- (Continued)

## Suppression of a Parasitic Pump Scattering in a Raman Amplifier with Arbitrary Angles between Pump and Seed Pulses

- (Continued)

## Suppression of a Parasitic Pump Scattering in a Raman Amplifier with Arbitrary Angles between Pump and Seed Pulses

- (Continued)

## Conclusions

-  Detuning of the Raman resonance by a plasma density gradient and a pump chirp can suppress parasitic pump side-scattering in Backward Raman Amplifiers
-  By selecting appropriate values of the pump chirp and plasma density gradient as well as direction of the density gradient, one can favorably minimize the detuning in the direction of the seed pulse propagation, while strongly suppressing the parasitic pump side-scattering in all the other directions