Interaction of strong electromagnetic (EM) waves with an underdense plasma now casts challenging issues in a variety of fields of plasma science[1]. In particular, stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) have been intensively investigated, since these instabilities can reflect laser energy and preheat a target in inertial fusion experiments. We found a new type of electronic instability by means of one-dimensional relativistic electromagnetic particles-in-cell simulations. When a relativistic EM (laser) pulse, stronger than a critical intensity, is injected into a uniform plasma at a sub-critical density range (n_c/4<n₀/γ<n_c), strong reflection is observed. The frequency of the back-scattered wave is near the effective electron plasma frequency which is well below its unperturbed value. This novel stimulated scattering instability[2] is recognized as a three-wave parametric resonant decay of the incident wave into an electron-acoustic wave (ω<<ω_p) and a scattered EM Stokes sideband. The slow Stokes lightwave gradually builds up to eventually propagate through the plasma-vacuum interface in a form of short superintense reflectivity bursts of coherent low-frequency EM radiation. The plasma electrons which are rapidly heated to relativistic energies further create a space-charge effect for an efficient ion acceleration to MeV energies.

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