Reducing parametric backscattering by polarization rotation

Abstract

When a laser passes through underdense plasmas, Raman and Brillouin Backscattering can reflect a substantial portion of the incident laser energy. This is a major loss mechanism, for example, in employing lasers in inertial confinement fusion. However, by slow rotation of the incident linear polarization, the overall reflectivity can be reduced significantly. Particle in cell simulations show that, for parameters similar to those of indirect drive fusion experiments, polarization rotation reduces the reflectivity by a factor of 5. A general, fluid-model based, analytical estimation for the reflectivity reduction agrees with simulations. However, in identifying the source of the backscatter reduction, it is difficult to disentangle the rotating polarization from the frequency separation based approach used to engineer the beam's polarization. Although the backscatter reduction arises similarly to other approaches that employ frequency separation, in the case here, the intensity remains constant in time.

Multifrequency Raman amplifiers

“…Also, although we consider here linear polarizations, similar results are predicted for circularly polarized waves or linearly polarized waves, but with perpendicular polarization. These types of waves have the property of reduced parasitic backscattering [25], but since they do not have a beat-wave waveform, no improvement in the maximum intensity is expected. These results should also carry over to using a multifrequency plasma seed instead of a seed laser [41] or to pulses with nonzero orbital angular momentum [42].…”

“…An alternative method to suppress backscattering from noise envisions splitting the pump energy over a few frequencies [24], where, in order to preserve the amplification efficiency, the allowed frequency spacing is limited by the spectral width of the singlefrequency amplified seed. Noise suppression by multifrequency pulses, has also been suggested for inertial confinement fusion systems, where the extent of penetration without backscattering depends on the frequency spacing [25,26]. Incoherent pump lasers, with not too small * ido.barth@mail.huji.ac.il a correlation time, can amplify coherent seeds similarly to coherent pump lasers, but with the advantage of less backscattering due to noise [27].…”

Space-time resonances and high-frequency Raman instabilities in the two-fluid Euler-Maxwell system

“…At frequencies such that Φ jj ′ = 0, the spectrum stays purely imaginary if and only if b + jj ′ b − j ′ j < 0. If b + jj ′ b − j ′ j > 0 at a resonant frequency, then the operator (1.11) is elliptic and instabilities may arise 2 . A more refined analysis shows however that the relevant spectral computation is not (1.11): due to the presence of fast transverse variations, it is space-time resonances that matter.…”

“…According to the physics literature, high-frequency instabilities play an important role in the current failure of large-scale inertial confinement fusion experiments to deliver significant amounts of energy. This is described for instance in [2], and in the 2019 White paper on opportunities in plasma physics, which we quote here: "Laser-plasma instabilities inhibit the deposition of energy ... [New broad-bandwith lasers could potentially] suppress high-frequency instabilities like [...] stimulated Raman scattering" ( [28], pages 2-3).…”

“…Then, (E, n) solves a Zakharov system of the form 2 , where c(k) is a group velocity, different from 0 if k = 0. (For legibility, we did not include physical parameters in (Z) other than the group velocity c(k).…”