The theory of laser self-resonant wake field excitation

Abstract

The three-dimensional evolution of a short laser pulse is considered. The linear stability analysis that takes account of the scattered light convection and predicts a development of pulse modulation at the electron plasma frequency, is carried out. It is demonstrated that a laser pulse with a power above or close to the critical, which is required to provide a pulse relativistic self-focusing, undergoes compression and self-modulation in the course of self-consistent pulse evolution and, as a result, resonant excitation of an extremely strong plasma wave occurs. It is shown that for a properly taken initial pulse and plasma parameters it is possible to obtain a wake field of an extremely high intensity for a fairly long time to provide a new promising outlook for the laser accelerator concept. The influence of plasma inhomogeneity and pulse initial focusing on the considered self-resonant plasma wave excitation is studied.

Excitation and damping of a self-modulated laser wakefield

“…͑This small spot contains 60% of the pulse energy, and the rest ͑40%͒ is in a large spot of Ͼ100 m in diameter.͒ A co-propagating second-harmonic probe pulse was focused by the same parabolic mirror to a spot of 7 m FWHM, overlapping with the pump-beam focus. 17 When a flat-topped gas jet ͑1000 m in length͒ with a sharp gradient ͑250 m͒ was used, relativistic-ponderomotive self-channeling of the pump pulse was observed when the peak laser power was у1.5 TW for 3.7ϫ10 19 cm Ϫ3 plasma density. 24,25 Under such conditions, the probe pulse was observed to be guided by the plasma-density depression created by the selfchanneled pump pulse.…”

“…Figure 2 shows the Raman spectra in the 15°channel for various plasma densities at 2.5 TW pump power, and Fig. 3 shows the results for various laser powers at 3.7ϫ10 19 cm Ϫ3 plasma density. The dependence of the anti-Stokes frequency shift on plasma density is consistent with the theoretical prediction FIG.…”

“…Strong spectral cascading was observed ͑up to the fourth anti-Stokes satellite͒, indicating strong coupling to a plasma wave generated by Raman forward scattering and selfmodulation instabilities. [19][20][21][22][23] The amplitude of the plasma wave, which is proportional to the square root of the relative strength between the anti-Stokes signal and the fundamental, increases with increase of laser power and plasma density, as expected from the temporal growth rate for Raman forward scattering 11…”

“…Side images of the first Stokes-Thomson satellite at various probe delays for 2.5 TW laser power and 3.7ϫ10 19 cm Ϫ3 plasma density: ͑a͒ Ϫ1 ps, ͑b͒ ϩ0.2 ps, ͑c͒ ϩ0.6 ps, and ͑d͒ ϩ1.6 ps. The arrow indicates the direction of laser propagation.…”

“…Observation of anti-Stokes satellites indicates strong coupling to a plasma wave driven by a Raman-type instability, either Raman forward scattering or self-modulation instability. [19][20][21][22][23] In addition, relativistic electrons generated by the electron plasma wave were observed. These results have been reported previously in Ref.…”