ИСТИНА

We demonstrated efficient electron acceleration in the plasma channel with injection through the breaking of plasma waves generated by parametric instabilities. It was shown experimentally that in the case of optimal preplasma parameters femtosecond laser pulse with an intensity of 5×1018 W/cm2 and an energy of 50 mJ generates a collimated electron bunch having divergence of 50 mrad, exponential spectrum with the slope of ~2 MeV and charge of tens of pC. The charge was confirmed measuring neutron yield from Be(g,n) photonuclear reaction with threshold of 1.7 MeV. This bunch was produced using the specific plasma profile containing arbitrary sharp gradient at the vicinity of 0.1-0.5 critical density and a long tail of teneous preplasma. We successfully formed such a gradient by an additional nanosecond laser pulse with intensity of 5×1012 W/cm2. Its intensity must be enough for the plasma gradient steepening down to L ~ 0.5λ by the ponderomotive force. This steepening happens near the critical density and provides for the efficient reflection of the subsequent femtosecond laser pulse of relativistic intensity. This reflected pulse creates plasma channel that serves for the DLA of electrons. Finally, well collimated bunch of high energy electrons emerges with mean electron energy well above the ponderomotive energy of the femtosecond pulse. The steepening is much weaker at lower intensities of the nanosecond laser pulse, and relativistic femtosecond laser pulse experiences strong absorption and scattering. The pulse, reflected under these conditions, does not create a channel in plasma and the VLA of electrons occurs, resulted in the larger divergence and lower energies and charge of the electron bunch. Our calculations shown that at the higher steepening of L~0.25λ electron beam charge decreased by a factor of ~ 2. This is due to the smaller volume of plasma from which electrons enter the channel and changing of electron injection source from parametric plasma instabilities to surface plasmons. Additional calculations showed that L~0.5λ is the optimal scale length for the considered mechanism of electron acceleration. Simulations of a test electron’s motion in the complex electromagnetic field consisting of the laser pulse and static azimuthal electric and magnetic fields showed that an electron acquires maximum energy at the channel exit if its initial energy amounts to several hundred keV and it is injected at the instant of the maximal field of the laser pulse. Simple estimates showed that plasma waves of the hybrid SRS-TPD instability are capable of injecting electrons with required energies in the channel. This instability generates two groups of waves: one moves along the plasma surface and the other - approximately in the perpendicular direction toward lower densities. Analysis of electron’s trajectories obtained from the PIC-modeling showed that the first group of waves accelerates electrons, while the field of the second group pushes electrons into the plasma channel. These waves were evidenced experimentally and numerically from the scattered 3/2w0 harmonic. The briefed mechanism of injection requires deeper insight, while the acceleration regime described could be further optimized. In particular, it is of special interest to check its applicability at higher intensities to path a way to the table top production of well collimated electron bunches with high, tens of nC, charge and mean energy (or slope) above 10 MeV.