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Boron-Proton Nuclear-Fusion Enhancement Induced in Boron-Doped Silicon Targets by Low-Contrast Pulsed Laser

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Boron-Proton Nuclear-Fusion Enhancement Induced in Boron-Doped Silicon Targets by Low-Contrast Pulsed Laser PHYSICAL REVIEW X 4, 031030 (2014) Boron-Proton Nuclear-Fusion Enhancement Induced in Boron-Doped Silicon Targets by Low-Contrast Pulsed Laser † A. Picciotto,1,* D. Margarone,2, A. Velyhan,2 P. Bellutti,1 J. Krasa,2 A. Szydlowsky,3,4 G. Bertuccio,5 Y. Shi,5 A. Mangione,6 J. Prokupek,2,7 A. Malinowska,4 E. Krousky,8 J. Ullschmied,8 L. Laska,2 M. Kucharik,7 and G. Korn2 1Micro-Nano Facility, Fondazione Bruno Kessler, 38123 Trento, Italy 2Institute of Physics ASCR, v.v.i. (FZU), ELI-Beamlines Project, 182 21 Prague, Czech Republic 3Institute of Plasma Physics and Laser Microfusion, 01-497 Warsaw, Poland 4National Centre for Nuclear Research, 05-400 Otwock, Poland 5Politecnico di Milano, Department of Electronics Information and Bioengineering, 22100 Como, Italy 6Institute of Advanced Technologies, 91100 Trapani, Italy 7Czech Technical University in Prague, FNSPE, 115 19 Prague, Czech Republic 8Institute of Plasma Physics of the ASCR, PALS Laboratory, 182 00 Prague, Czech Republic (Received 12 January 2014; revised manuscript received 1 April 2014; published 19 August 2014) We show that a spatially well-defined layer of boron dopants in a hydrogen-enriched silicon target allows the production of a high yield of alpha particles of around 109 per steradian using a nanosecond, low-contrast laser pulse with a nominal intensity of approximately 3 × 1016 Wcm−2. This result can be ascribed to the nature of the long laser-pulse interaction with the target and with the expanding plasma, as well as to the optimal target geometry and composition. The possibility of an impact on future applications such as nuclear fusion without production of neutron-induced radioactivity and compact ion accelerators is anticipated. DOI: 10.1103/PhysRevX.4.031030 Subject Areas: Nuclear Physics, Plasma Physics I. INTRODUCTION It is well known both from theoretical calculations and The 11Bðp; αÞ2α nuclear-fusion reaction was first stud- experimental measurements (performed with standard ied in the 1930s by Oliphant and Rutherford [1]. Interest accelerators) that the reaction has a main channel with a from the scientific community in this reaction is now maximum cross section for protons with energies of increasing due to the possibility of producing energetic 600–700 keV [7–9]. Typically, this channel generates alpha alpha particles without neutron generation, which may particles with an energy distribution ranging from 2.5 to allow the building of an “ultraclean” nuclear-fusion reactor 5.5 MeV and a maximum at around 4.3 MeV (α1). [2–4]. The first experimental demonstration of a laser- A secondary channel generating alpha particles in the 11 driven Bðp; αÞ2α reaction was performed using the energy range 6–10 MeV can also be present (α2) [5,10,11]. interaction of a high-intensity (around 2 × 1018 Wcm−2) picosecond laser pulse with a boron-enriched polymeric II. METHODS target. This experiment gave an alpha-particle yield per pulse of around 103 per steradian [5]. Recently, a much The experiment is performed at the Prague Asterix Laser higher particle yield (around 107 per steradian) has been System (PALS) facility in Prague [12], where a kJ-class reported by using a more sophisticated experimental setup (around 500 J in our experiment), subnanosecond (0.3-ns and similar laser intensity [6]. full width at half maximum, FWHM), linearly polarized Our experiment aims to produce a high yield of alpha laser is used to irradiate massive silicon targets. The laser 11 particles by triggering the proton (p)-boron ( B) nuclear pulse is focused in an 80-μm focal spot by an f=2 spherical reaction: lens to reach a nominal intensity of 3 × 1016 Wcm−2 on the 6 −5 11B þ p → 3α þ 8.7 MeV: ð1Þ target surface in vacuum conditions of 10 –10 mbar. The incidence angle between the laser-beam axis and the target normal is 30°. The nominal laser intensity is low compared *Corresponding author. with the high intensities achievable with picosecond and [email protected] femtosecond laser systems. Nevertheless, recent experi- † Corresponding author. ments on laser-driven ion beams performed at PALS have [email protected] shown maximum proton energies of a few MeV [13] which are typical for laser intensities of 5 × 1018–1019 Wcm−2 Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distri- when shorter pulses are used [14]. bution of this work must maintain attribution to the author(s) and Most of these targets were previously enriched with the published article’s title, journal citation, and DOI. hydrogen and boron atoms through thermal annealing and 2160-3308=14=4(3)=031030(8) 031030-1 Published by the American Physical Society A. PICCIOTTO et al. PHYS. REV. X 4, 031030 (2014) 50-keV ion-implantation doping processes performed at the (a) Microtechnologies Laboratory (MTLab) at Fondazione Bruno Kessler in Trento [15,16]. Thus, well-known chemical-physical processes used in microelectronics help us to develop the idea of using boron-hydrogen-silicon targets for triggering a specific fusion reaction by precisely control- ling the concentration and implantation or diffusion depth of the doping element. Here, we report mainly the results obtained by laser irradiation of massive silicon samples (0.5 mm thick) that are implanted with boron atoms (amu ¼ 11) located at a depth of 190 nm and having a 1022 −3 concentration of about cm . (The boron layer is (b) approximately 100 nm thick.) The concentration of hydrogen 100 in the samples is increased through a thermal process 10-1 where hydrogen atoms diffusing into the silicon matrix -2 can link to it by forming Si-H bonds, which neutralize 10 0 I II III the silicon dangling bonds [17–20], thus reaching a concen- 10-3 20 −3 tration higher than 10 cm . Silicon samples where boron -4 [a.u.] 10 L I atoms are homogenously diffused into the substrate via the -5 thermal annealing process (1 h at 1000 °C) are also fabricated 10 -6 E (0 ns,1 ns) = 1.68 mJ and irradiated. In the latter case, the B atoms are distributed in 10 L -7 E (1 ns,1.85 ns) = 56 J a much thicker layer, starting from the sample surface up to a 10 L E (1.85 ns, 2.15 ns) = 488 J L certain depth (>10 μm), resulting in lower local boron -8 20 −3 10 concentration (approximately 10 cm ). These targets -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 τ [ns] are irradiated in a series of measurements at the beginning L of the experiment to be later compared with the results obtained with the implanted samples. Furthermore, silicon FIG. 1. (a) Experimental setup. T is the target, p is the proton “pure” samples (not artificially hydrogenated and without beam, α is the alpha-particle beam, PM-355 is the nuclear-track boron doping) are also irradiated for comparison. detector, TP is the Thomson-parabola spectrometer, MCP is the Ion analyzers, such as a Thomson-parabola (TP) spec- microchannel plate, and SiC is the silicon-carbide detector. The angles are measured with respect to the target normal. (b) Laser trometer, time-of-flight (TOF) silicon-carbide (SiC) I τ detectors [21–23], and solid-state nuclear-track detectors intensity ( L) against time ( L) in arbitrary units. The laser-energy content in different time windows is reported in the legend. of PM-355 type [24,25], are used to detect and identify the produced proton and alpha-particle beams and measure their energy distribution in various directions with respect laser-absorption modeling. The simulations shown in this to the target normal. A detailed description of the exper- paper are performed in the Lagrangian regime; thus, the imental setup is sketched in Fig. 1(a). layers of the targets remain separated throughout the whole The ns-long laser-pulse temporal shape is reported in simulation. The initial computational mesh is adjusted to Fig. 1(b). Although approximately 500 J are contained in the layered targets using a finer mesh resolution at the target the main pulse (0.3 ns at FWHM), the residual laser energy surface. The simulations are stopped after 1.85 ns [see (about 100 J) is stored in the nanosecond pedestal. (Around Fig. 1(b) as a reference]. The simulations are performed by 50 J of the pulse energy comes before the maximum- considering different target geometries: (a) a B-implanted intensity peak is reached.) Figure 1(b) shows the presence layer in a Si substrate with the above-described character- of three intensity regions: 0-I represents the low-intensity istics, (b) a B-diffused layer in a Si substrate with the 10 −2 (3 × 10 Wcm ) laser precursor, I-II is the high- above-described characteristics, and (c) a B layer deposited 15 −2 intensity (10 Wcm ) prepulse, and II-III is the high- on a Si-substrate surface with the same atomic concen- 16 −2 intensity (nominally 3 × 10 Wcm ) main pulse. tration as in (a). The comparison of the hydrodynamic A series of hydrodynamic simulations is performed by simulations corresponding to these three different geom- using a 2D Prague Arbitrary Lagrangian-Eulerian (PALE) etries is shown in Fig. 2, where z is the plasma-expansion code [26]. This code employs the compatible staggered longitudinal direction (parallel to the target normal) and r is Lagrangian numerical scheme [27] and a rezoning or the plasma-expansion transverse direction (parallel to the remapping mechanism improving the robustness of the target surface). The simulations take into account the time simulations. Thermal conductivity is treated by a mimetic evolution of the laser pulse in the range 1–1.85 ns.
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