Four-photon amplification of powerful laser pulses in plasma*

A new scheme for amplification of laser pulses in plasma is proposed, which may overcome limitations of the backward Raman amplification (BRA) scheme. Those limitations are associated primarily with the fragility of the short-wavelength Langmuir waves mediating the resonant energy transfer, which limits the allowed laser-to-plasma frequency ratios. The largest frequency ratio allowed decreases with the laser wavelengths and completely disappears at for hard x-rays. The BRA limitation on laser-to-plasma frequency ratio can be overcome by beating two auxiliary laser pulses, rather than a Langmuir wave, for mediating the energy transfer. This leads to a 4-photon amplification scheme, which may be used, for example, to amplify optical laser pulses in very rarefied plasmas, or to amplify x-ray pulses in dense plasmas. In the 4-photon scheme, a pump and an auxiliary laser pulse-1 are scattered into an amplified pulse and an auxiliary laser pulse-2. The energy can keep flowing from the pump to the amplified pulse even for amplified pulse intensity much exceeding the pump intensity. The opposite process can be excluded by absence of the auxiliary laser pulse-2 in freshly encountered plasma layers.