Suppressing Parametric Instabilities with Laser-Frequency Detuning and Bandwidth

High-frequency laser–plasma instabilities, such as stimulated Raman scattering (SRS) and two-plasmon decay (TPD), present a major challenge for laser-driven inertial confinement fusion (ICF). The plasma waves driven in these instabilities inhibit the compression of the fusion capsule, and ultimately the gain, by scattering light and accelerating hot electrons that preheat the fuel. Attempts at mitigation have primarily focused on modifying the implosion design to limit the instability growth rate.[1] This approach severely restricts the ICF design space. The introduction of temporal incoherence in the drive lasers offers a path toward mitigation without sacrificing hydrodynamic efficiency in the implosion design. Through the use of 3-D laser–plasma interaction simulations, we show that two forms of temporal incoherence—frequency detuning and laser bandwidth—can suppress SRS and TPD. The simulations were conducted with LPSE (laser‑plasma simulation environment), which provides a unique capability to model the dynamic evolution of parametric instabilities at the long scale lengths and with the complex beam geometries encountered in ignition plasmas. The viability of each form of temporal incoherence, both in terms of instability suppression and practical implementation, is considered. For instance, a three-wavelength scheme, with the detuning available on current laser architectures, has been found to nearly eliminate TPD and the associated hot electrons.[2] A next-generation ICF driver capable of suppressing TPD and SRS with temporal incoherence will allow for higher laser intensities and ablation pressures, greatly expanding the ICF design space.

[1] R. K. Follett et al., Phys. Rev. Lett. 116, 155002 (2016).

[2] R. K. Follett et al., Phys. Rev. Lett. 120, 135005 (2018).