The dependence of spatial autoresonance in SRS onkLλD

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Spatial autoresonance is investigated as a mechanism for the enhancement of stimulated Raman scattering (SRS) in the kinetic regime (kLD > 0.29). Autoresonance in 3-wave simulations was demonstrated in a previous study (Chapman et al. ,P hys. Plasmas17, 122317 (2010)). These results are applied to particle-in-cell (PIC) simulations. Good agreement is found between PIC simulations and a 3-wave model using a nonlinear frequency shift beyond the regime usually referred to as weakly kinetic. Autoresonance is studied for a range of values of kLD. A nonlinear oscillator driven from rest at a fixed frequency will quickly dephase from its driver, ending the efficient exchange of energy. However, if the driver is instead swept in frequency, passing slowly through resonance with the oscillator, the oscillator may undergo autoresonance, phase-locking to the driver and remaining in resonance as its amplitude grows. During autoresonance, the oscillator self- adjusts its amplitude, and therefore frequency, in order to maintain resonance. This process may occur without feedback, and thus is applied in a range of contexts where the constraints on the speed and precision of driver modulation required to maintain resonance artificially would be unattainable: recent examples in plasmas include the control of the diocotron mode in a Malmberg-Penning trap (1), and the manipulation of anti-protons and positrons to form anti-hydrogen (2). Stimulated Raman scattering is the 3-wave parametric instability in which laser light (the pump wave) scatters off of a density fluctuation in a plasma, driving a Langmuir wave. The scattered light (the seed wave) beats with the laser light, further enhancing the ponderomotive force that drives the Langmuir wave, leading to unstable growth. A range of processes may detune the 3-wave resonance necessary for the efficient scattering of the laser light. This article considers the interplay between two detuning mechanisms: the inhomogeneity of the plasma density profile, and kinetic effects. The linear dispersion relations of the three waves are functions of the local electron plasma frequency, pe. Consequently, as a given excited mode (electromagnetic or electrostatic) propagates, it undergoes a wave number shift. Three waves initially resonant at density n0, with pump, seed and Langmuir wave frequencies 0,1,L and wave numbers k0,1,L, respectively, will dephase from resonance due to this wave number shift as they propagate through the changing density. At n0, the resonance conditions 0 = 1 + L and k0 = k1 + kL are satisfied. The wave number shift � ≡ x(k0 − k1 − kL) is dominated by the Langmuir wave, thus in a linear density profile, � ≈− xkL = 2 /6Lv 2