Improving a high-power laser based relativistic electron source: the role of laser pulse contrast and gas jet density profile

A relativistic electron source based on high power laser interaction with gas jet targets has been developed at the Institute of Plasma Physics & Lasers of the Hellenic Mediterranean University. Initial measurements were conducted using the “Zeus” 45 TW laser with peak intensities in the range of 1018-1019 W/cm2 interacting with a He pulsed gas jet having a 0.8 mm diameter nozzle. A significant improvement of the electron signal was measured after using an absorber to improve the laser pulse contrast from 10-10 to 10-11. A high stability quasi-monoenergetic electron beam of about 50 MeV was achieved and measured using a magnetic spectrometer for pulsed gas jet backing pressure of 12 bar. Supplementary studies using a 3 mm diameter nozzle for backing pressures in the range of 35 to 40 bar showed electron beam production with energies spread in the range from 50 to 150 MeV. The pulsed jet density profile was determined using interferometric techniques. Particle-in-cell (PIC) simulations, at the above experimentally determined conditions, support our experimental findings.

Ion-Focused Propagation of Relativistic Electron Beam in the Self-Generated Plasma in the Atmosphere

It is known that ion-focused regime can effectively suppress the expansion of relativistic electron beam (REB). By using particle in cell-Monte Carlo collision (PIC-MCC) method, the propagation of REBs in neutral gas is numerically investigated. The numerical results demonstrate that the beam body is charge neutralization and a stable IFR can be established. As a result, the beam transverse dimensions and longitudinal velocities keep close to the initial parameters. We also calculated the charge and current neutralization factors of REBs. Combined with envelope equations, we obtained the variations of beam envelopes, which agree well with the PIC simulations. However, both the energy loss and instabilities of REBs may lead to a low transport efficiency during long-range propagation. It has been proved that decreasing the initial pulse length of REBs can avoid the influence of electron avalanche. Using parts of REB pulses to build a long-distance IFR in advance can improve the beam quality of subsequent pulses. Further, a long-distance IFR may contribute to the implementation of long-range propagation of REBs in the space environment.

Multi-layer structure formation of relativistic electron beams in plasmas

A two-dimensional(2D) electromagnetic particle-in-cell(PIC) simulation model is proposed to study the density evolution and collective stopping of electron beams in background plasmas. We show here the formation of the multi-layer structure of the relativistic electron beam in the plasma due to the different betatron frequency from the beam front to the beam tail. Meanwhile, the nonuniformity of the longitudinal wakefield is the essential reason for the multilayer structure formation in beam phase space. The influences of beam parameters (beam radius and transverse density profile) on the formation of the multi-layer structure and collective stopping in background plasmas are also considered.

Identify spin property of relativistic electrons in fully relativistic laser fields

A semiclassical method is developed to study the spin evolution of a relativistic electron in an fully relativistic laser pulse. Different from the previous classical method which is based on the direct generalization of nonrelativistic spin precession equation, we perform first-principle calculations on the mean values of various spin operators with respect to a relativistic electron wavepacket. It is demonstrated, via theoretical derivation and numerical simulation, that although the Foldy-Wouthuysen operator merits the single-particle interpretation, its mean value obviously deviates from the result of the classical method, which sheds light on not only the understanding of relativistic spin itself but also broad related applications. To achieve a direct observation of such effect, a feasible experimental setup utilizing the asymmetric field of a single-cycle laser is proposed. In such geometry, the deviation is evidenced in the total change of spin which can be easily measured after the interaction.