High-Current Beams of Relativistic Electrons at Direct Laser Acceleration in Near-Critical Density Plasma

The results of particle-in-cell (PIC) simulation of the effect of intense laser pulses (peak intensities of 10¹⁹ to 10²¹ W/cm²) on near-critical density (NCD) plasma have been analyzed. As they propagate through NCD plasma, these laser pulses create an ion channel with lower electron density and strong quasi-stationary fields. Radial inhomogeneity of the electron density creates a radial electrostatic field due to the ponderomotive expulsion of background plasma electrons from the channel. At the same time, the current of accelerated electrons generates an azimuthal magnetic field. In direct laser acceleration (DLA), relativistic electrons trapped in the channel by focusing quasi-stationary fields experience transverse betatron oscillations and gain energy efficiently from the laser pulse when the frequency of the betatron oscillations becomes resonant with the Doppler shifted laser frequency. The potential for producing electron bunches with energies of tens of megaelectronvolts and charges of hundreds of nanocoulombs using DLA in NCD-plasma has been demonstrated. It is shown that the energy spectra of accelerated electrons can be approximated by Maxwell distributions. A scaling is proposed for the dependence of the effective temperature of “superponderomotive” electrons on the laser pulse and plasma parameters.