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Scientists produced ultrahigh-charge relativistic electron beams in laser-solid interaction

Date:10-07-2018 Print

Laser-plasma based accelerators have undergone rapid development in the last three decades. Owing to the characteristics of plasma, the accelerating gradient of laser-plasma accelerators is thousands of times higher than that of the traditional radio frequency accelerators. This shrinks the kilometer scale large facilities to tabletop, stimulating the study of laser-plasma accelerators worldwide due to the compactness and low cost.

However, currently the laser-plasma accelerators meet a serious bottleneck. It cannot produce electron beams with high charge and small divergence angle simultaneously. In laser-gases interaction, the divergence angle can be very small while the beam charge is limited to only tens of picocoulombs. In laser-soild interaction, the beam charge could reach a few nanocoulombs, but with very large divergence angles, unfortunately.

Recently, MA Yong, ZHAO Jiarui, Li Yifei and Li Dazhang in Prof. CHEN Liming and Prof. ZHANG jie’s group from Institute of Physics, Chinese Academy of Sciences have produced relativistic electron beams with extremely high beam charge and small divergence angle. In their experiment, the super-intense (200 TW) ultra-short (1 ps) Titan laser at Lawrence Livermore National Laboratory was irradiated at copper target, producing relativistic electron beams with an extremely high charge of 100 nanocoulombs level and divergence angle smaller than 3 degrees. The researchers found out that the generation of such electron beams can be well controlled by adjusting the laser contrast and the laser energy. They also revealed a new electron acceleration mechanism in laser-solid interaction by performing computational numerical simulations.

Owing to the sub-picoseconds pulse duration, the peak current of such electron beams reaches over 100 kA. Moreover, the brightness of the beam can be as high as 1016 A/m2, which is comparable to the highest of traditional accelerators around the world. If deposit all the energy into high-Z materials, for example Au, the resulting energy density can be as high as 1012 J/m3. Therefore, this kind of electron beam source could be an ideal tool to drive warm and even hot dense matters. It might also find wide applications in seeding high-flux γ-ray source, single-shot electron radiography and even serving as an ignitor in fast ignition for inertial confinement fusion.

This study entitled “Ultrahigh-charge electron beams from laser-irradiated solid surface” was published on Proceedings of the National Academy of Sciences of the United States of America (PNAS).

The study was supported by the National Science Foundation, the Ministry of Science and Technology of China and the Chinese Academy of Sciences.

Fig. 1 Experimental setup. (Image by Institute of Physics)
Fig.2 Angular distribution of the electron beams. (Image by Institute of Physics)
Fig. 3 Depends of electron beam charge and divergence angle on laser contrast. (Image by Institute of Physics)
Fig. 4 Numerical simulations. (Image by Institute of Physics)

Institute of Physics
CHEN Liming

Key word:
laser–plasma interaction, direct laser acceleration, ultrahigh-charge beam, high energy density, near–critical-density plasma

Compact acceleration of a tightly collimated relativistic electron beam with high charge from a laser-plasma interaction has many unique applications. However, currently the well-known schemes, including laser wakefield acceleration from gases and vacuum laser acceleration from solids, often produce electron beams either with low charge or with large divergence angles. In this work, we report the generation of highly collimated electron beams with a divergence angle of a few degrees, nonthermal spectra peaked at the megaelectronvolt level, and extremely high charge (~100 nC) via a powerful subpicosecond laser pulse inter- acting with a solid target in grazing incidence. Particle-in-cell simulations illustrate a direct laser acceleration scenario, in which the self-filamentation is triggered in a large-scale near–critical- density plasma and electron bunches are accelerated periodically and collimated by the ultraintense electromagnetic field. The energy density of such electron beams in high-Z materials reaches to ~1012 J/m3, making it a promising tool to drive warm or even hot dense matter states.