Collimation of Energetic Electrons from a Laser-Target Interaction by a Magnetized Target Back Plasma Preformed by a Long-Pulse Laser

H. B. Zhuo, Z. L. Chen, Z. M. Sheng, M. Chen, T. Yabuuchi, M. Tampo, M. Y. Yu, X. H. Yang, C. T. Zhou, K. A. Tanaka, J. Zhang, and R. Kodama

Phys. Rev. Lett. 112, 215003 – Published 29 May 2014

Abstract

It is demonstrated experimentally and by numerical simulations that the presence of a long-pulse-laser-created back plasma on the target backside can focus the relativistic electrons produced by short-pulse laser interaction with the front of a solid target. Comparing this to that without the back plasma, the number density of the fast electrons is increased by one order of magnitude, and their divergence angle is reduced fivefold. The effect is attributed to the absence of the backside sheath electric field and the collimation effect of the megagauss self-generated baroclinic magnetic field there. Such an acceleration scheme can be useful to applications requiring high-energy and charge-density electron bunches, such as fast ignition in inertial fusion.

Received 22 January 2014

DOI:https://doi.org/10.1103/PhysRevLett.112.215003

Authors & Affiliations

H. B. Zhuo^1,*, Z. L. Chen², Z. M. Sheng^3,4, M. Chen³, T. Yabuuchi⁵, M. Tampo⁵, M. Y. Yu^6,7, X. H. Yang¹, C. T. Zhou^1,8, K. A. Tanaka^2,5, J. Zhang³, and R. Kodama^2,5

¹College of Science, National University of Defense Technology, Changsha 410073, People’s Republic of China
²Graduate School of Engineering, Osaka University 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
³MOE Key Laboratory for Laser Plasmas and Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China
⁴Department of Physics, SUPA, University of Strathclyde, Glasgow G4 0NG, United Kingdom
⁵Institute of Laser Engineering, Osaka University 2-6 Yamada-oka, Suita, Osaka 565-0871, Japan
⁶Institute for Fusion Theory and Simulation and Department of Physics, Zhejiang University, Hangzhou 310027, People’s Republic of China
⁷Institute for Theoretical Physics I, Ruhr University, Bochum D-44780, Germany
⁸Institute of Applied Physics and Computational Mathematics, Beijing 100094, People’s Republic of China

^*hongbin.zhuo@gmail.com

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Vol. 112, Iss. 21 — 30 May 2014

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Images

Figure 1
(a) Schematic representation of the target backside plasma. The electron temperature distribution is drawn basing on the results (at 400 ps) from hydrodynamic simulations of the interaction of a long-pulse laser with the backside of a planar Al target. The contours (thin black closed curves) depict the expected toroidal baroclinic magnetic field, which is peaked around the edge of the laser focal spot and decreases to zero on the axis ( $r = 0$ ) and at large distances from the focal spot. The EEs (blue region) created and accelerated by the target front laser are focused into a narrow bunch by the strong magnetic field. The hydrodynamic simulation results for the back plasma electron density (black) along the $z$ axis, and temperature (blue) along the $r$ at the position of 0.01 cm away from rear surface of the target, are shown in (b) for the Al target with flat backside, and (c) for two plastic targets with different backside curvatures (blue: infinite, and red: 2.5 mm).
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Figure 2
Experimental setup (a). Observed angular distribution of electrons over 0.4 MeV for planar Al target with (b) and without (c) preformed plasma on the backside. The $p$ -polarized main laser pulse defining the angular direction (0°, 0°) is incident obliquely at a 20° angle on the front surface of the target (such that the target normal is in the (20°, 0°) direction. (d) Number of $> 0.4 MeV$ electrons along the gray dashed lines in the panels (b) and (c).
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Figure 3
Experimental angular distributions of energetic electrons over 0.4 MeV for different cylindrical target back curvatures: (a) infinite (or planar), (b) 5.0 mm, and (c) 2.5 mm, for $200 μ m \times (5 \times 5 {mm}^{2})$ plastic targets. The curvature axes in (b) and (c) are perpendicular and parallel, respectively, to the laser polarization. (d) Electron number (obtained from the sandwich detector using photo stimulated luminescence) along the dashed lines in Figs. 2, 2, 3, 3, and 3, denoted by 2, 2, 3, 3, and 3, respectively.
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Figure 4
Results from two-dimensional PIC simulations at $t = 80$ laser periods from the laser impact: magnetic field distribution (left column), distribution of $> 1 MV$ energetic electrons (center column), and electron density at $x = 30 μ m$ (right column), for $B_{0} = 0 MG$ (top row), $B_{0} = 5 MG$ (center row), and $B_{0} = 10 MG$ (bottom row). The $5 μ m$ -thick solid-density ( $20 n_{c}$ , where $n_{c} = 1.1 \times 1 0^{21} / {cm}^{3}$ ) target is at $5 < x [μ m] < 10$ . Note that the magnetic fields associated with the front-laser interaction with the target and the energetic-electron filamentation are much stronger and localized than the baroclinic magnetic fields, shown in (d) and (g) as large blue and red lightly shaded regions, in the model back plasma.
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