Zeev Toroker

Backward Raman amplification in the Langmuir wavebreaking regime

In plasma-based backward Raman amplifiers, the output pulse intensity increases with the input pump pulse intensity, as long as the Langmuir wave mediating energy transfer from the pump to the seed pulse remains intact. However, at high pump intensity, the Langmuir wave breaks, at which point the amplification efficiency may no longer increase with the pump intensity. Numerical simulations presented here, employing a one-dimensional Vlasov-Maxwell code, show that, although the amplification efficiency remains high when the pump only mildly exceeds the wavebreaking threshold, the efficiency drops precipitously at larger pump intensities.

The efficiency of Raman amplification in the wavebreaking regime

We compare previous analytic predictions, Vlasov-Maxwell simulations, and particle-in-cell results with a new set of comprehensive one and two dimensional particle-in-cell simulations in an effort to clarify apparent discrepancies between the predictions of different models for the efficiency of Raman amplification in the wavebreaking regime. We find reasonable agreement between our particle-in-cell simulations and previous results from Vlasov-Maxwell simulations and analytic work, suggesting a monotonic decrease in conversion efficiency for increased pump intensities past the wavebreaking threshold.

Laser duration and intensity limits in plasma backward Raman amplifiers

The shortest duration and the largest non-focused intensity of laser pulses produced by means of backward Raman amplification (BRA) in plasmas are calculated. These limits occur in moderately undercritical plasmas and are imposed by combined effects of moderately small group velocity dispersion and relativistic electron nonlinearity of the amplified pulses. The efficient BRA range covered by this theory is broader than one known previously. This can be useful for BRA of x-ray pulses in regular or compressed solids and ultra-powerful optical pulses in the lowest density solids.

Optimized split-step method for modeling nonlinear pulse propagation in fiber Bragg gratings

We present an optimized split-step method for solving nonlinear coupled-mode equations that model wave propagation in nonlinear fiber Bragg gratings. By separately controlling the spatial and the temporal step size of the solution, we could significantly decrease the run time duration without significantly affecting the result accuracy. The accuracy of the method and the dependence of the error on the algorithm parameters are studied in several examples. Physical considerations are given to determine the required resolution.

Saturation of the leading spike growth in backward Raman amplifiers

Backward Raman amplification of laser pulses in plasmas can produce nearly relativistic unfocused output intensities and multi-exawatt powers in compact devices. The largest achievable intensity depends on which of major competitive processes sets this limit. It is shown here that the relativistic electron nonlinearity can cause saturation of the leading amplified spike intensity before filamentation instabilities develop. A simple analytical model for the saturation, which supports numerical simulations, is suggested. The upper limit for the leading output spike unfocused intensity is calculated.

Geometrical constraints on plasma couplers for Raman compression

Backward Raman compression in plasma is based on a 3-wave resonant interaction, which includes two counter-propagating laser pulses (pump and seed pulses) and an electron plasma wave (Langmuir wave). The resonant interaction can be ensured in nearly homogeneous plasmas. However, for high-power, large-aperture experiments, the homogeneous region becomes pancake-shaped and would likely be surrounded by thicker regions of inhomogeneous plasma. When these inhomogeneous plasma regions are extensive, significant inverse bremsstrahlung and seed dispersion may impede the compression effect. These deleterious effects may, however, be mitigated by chirping the seed and pump pulses.

Exceeding the leading spike intensity and fluence limits in backward Raman amplifiers.

The leading amplified spike in backward Raman amplifiers can reach nearly relativistic intensities before the saturation by the relativistic electron nonlinearity. The saturation sets an upper limit to the largest achievable leading spike intensity. It is shown here that this limit can be substantially exceeded by the initially subdominant spikes, which surprisingly outgrow the leading spike after its nonlinear saturation. Furthermore, an initially negligible group velocity dispersion of the amplified pulse in strongly undercritical plasma appears to be capable of delaying the longitudinal filamentation instability in the nonlinear saturation regime. This enables further amplification of the pulse to even larger output fluences.

Beyond nonlinear saturation of backward Raman amplifiers

Backward Raman amplification is limited by relativistic nonlinear dephasing resulting in saturation of the leading spike of the amplified pulse. Pump detuning is employed to mitigate the relativistic phase mismatch and to overcome the associated saturation. The amplified pulse can then be reshaped into a monospike pulse with little precursory power ahead of it, with the maximum intensity increasing by a factor of two. This detuning can be employed advantageously both in regimes where the group velocity dispersion is unimportant and where the dispersion is important but small.

Two-beam accelerator with active medium as the energy source

While a Cherenkov wake propagates in a perfectly reflecting waveguide loaded with active medium, a part of its spectrum is amplified. Due to gain enhancement the amplified frequency components become dominant making the wake an effectively monochromatic wave. As in a two-beam accelerator, this intense wake may accelerate a different (trailing) train of microbunches.

High intensity regimes for resonant Raman compression

In order to achieve the largest laser intensities, a plasma might be used as the amplification medium, thereby avoiding the material limits of conventional materials. The technique considered is resonant backward Raman amplification in plasma, wherein a short counter-propagating seed pulse, with frequency downshifted from a long pump pulse by the plasma frequency, absorbs the pump energy through a resonant decay interaction of the two counter-propagating light waves and a plasma wave. In the pump-depletion regime, the counter-propagating seed pulse assumes a self-contracting self-similar form, capturing the pump energy in a pulse of far shorter duration. This technique encounters limitations both at high laser seed output intensities and high pump laser intensities. At high seed output intensities, there are modulation instabilities that break up the output seed. At high pump intensities, the resonant interaction is interrupted by wavebreaking of the plasma wave. These limitations, while limiting, may not be as limiting as might be at first thought.