Abstract
The double laser pulse approach to relativistic electron beam (REB) collimation in solid targets has been investigated at the LULI-ELFIE facility. In this scheme two collinear laser pulses are focused onto a solid target with a given intensity ratio and time delay to generate REBs. The magnetic field generated by the first laser-driven REB is used to guide the REB generated by a second delayed laser pulse. We show how electron beam collimation can be controlled by properly adjusting the ratio of focus size and the delay time between the two pulses. We found that the maximum of electron beam collimation is clearly dependent on the laser focal spot size ratio and related to the magnetic field dynamics. Cu-Kα and CTR imaging diagnostics were implemented to evaluate the collimation effects on the respectively low energy (≤100 keV) and high energy (≥MeV) components of the REB.
Highlights
This scheme was experimentally investigated by Scott et al.[38] who have shown the existence of an optimum delay between the laser pulses of the order of the laser pulse duration (Δt ∼ τ), at which a maximum electron beam collimation is reached
Following such interpretation we have performed an experimental campaign in which we used two independent focusing parabolic mirrors, allowing to vary the ratio φ1/φ2 between the two laser focal spots, controlling the ratio between the radius of the azimuthal magnetic field created by the first beam R1 and the radius of the second electron beam R2
The maximum compression corresponds to the maximum value of Cu-Kα emission at the delay time of 3 ps where the electron beam area is decreased by a factor of 0.5 and the Cu-Kα intensity is increased by a factor of 1.37 [Fig. 4]
Summary
The study of the transport of relativistic laser-driven electrons is a subject of interest for many applications including proton-ion acceleration[1,2,3,4,5], fast ignition approach to inertial confinement fusion (ICF)[6,7,8,9], astrophysics applications[10], isohoric heating of matter[11,12,13,14] as well as high brilliance and compact laser-based x-ray sources[15,16]. The electron beam produced by the first, less intense, laser pulse generates a resistive azimuthal magnetic field (seed magnetic field) which is used to guide the main electron population generated by the second beam This scheme was experimentally investigated by Scott et al.[38] who have shown the existence of an optimum delay between the laser pulses of the order of the laser pulse duration (Δt ∼ τ), at which a maximum electron beam collimation is reached. From this study a clearer picture of the physical mechanism is obtained suggesting the relevant role of the ratio between the REB sizes Following such interpretation we have performed an experimental campaign in which we used two independent focusing parabolic mirrors, allowing to vary the ratio φ1/φ2 between the two laser focal spots, controlling the ratio between the radius of the azimuthal magnetic field created by the first beam R1 and the radius of the second electron beam R2. The performed experimental study with various laser parameters allowed us to make a detailed characterization of the collimation efficiency
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