Fast Neutron Emission from a High-Energy Ion Beam Produced by a High-Intensity Subpicosecond Laser Pulse

Abstract

Neutron emission as high as 10 7 is observed when a high intensity (a few 10 19 W͞cm 2 ) subpicosecond laser pulse at 529 nm wavelength is focused on a deuterated polyethylene target. Neutron emission is also measured in different directions. The emission of neutrons along the laser axis is higher than in the transverse direction. Nonisotropic emission is consistent with neutrons generated by D͑d, n͒-3 He reaction for 0.3 -1 MeV deuterons accelerated in the direction of the laser beam. The energy transferred to the ions is roughly estimated and compared with the energy carried out by the electrons. [S0031-9007(98)08299-4] PACS numbers: 52.40.Nk, 52.50.Jm, 52.70.Nc The development of chirped pulse amplification has made possible the generation of energetic subpicosecond laser pulses [1]. The interaction of the laser pulse with a target generates energetic particles, like MeV electrons and ions [2]. The fast ignitor concept [3], relevant to the inertial confinement fusion (ICF), enhances the interest in this process: Hot particles could heat to thermonuclear temperature an already compressed deuterium-tritium fuel.High-intensity subpicosecond laser pulses produce fast neutrons when they interact with a deuterated target [4]. Hot deuterium ions create neutrons from D͑d, n͒-3 He reaction. Measurements of this neutron emission is a useful method to diagnose fast ions (in the keV to MeV range) generated by the interaction of the laser with the target. Particle-in-cell (PIC) calculations show that highenergy ions are accelerated by a shock wave propagating inside the target [5][6][7]. It knocks the ions along the direction of the laser propagation [8], but collisions stop the ions in the thickness of the target. Neutron emission can identify these ions which cannot be directly measured.Here, we report neutron emission from a deuterated polyethylene target irradiated with a subpicosecond 529 nm laser. The focused intensity is a few 10 19 W͞cm 2 . The experimental conditions are similar to other experiments [1,9]. The laser system provides a chirped pulse with energies up to 30 J at the fundamental wavelength of 1.058 mm. After compression by a pair of diffraction gratings, the pulse duration, measured by an autocorrelation method, is routinely 400 fs. A KH 2 PO 4 (KDP) crystal is used to convert the laser beam to 529 nm with an efficiency of 70%. The pulse duration is less than 300 fs. An f͞3 off-axis parabolic mirror focuses the laser pulse to a 5-mm-diameter spot containing about 30% of the total energy. The highest intensity is 3.5 3 10 19 W͞cm 2 for a laser energy of 7 J at 529 nm.At 1.058 mm, the contrast ratio measured at a few tens of picosecond before the main pulse by a third order cross correlator is 10 8 . Three dichroic mirrors located after the KDP crystal increase the contrast ratio to 10 12 at 529 nm. The optical intensity before the pulse is insufficient to ionize the target, so the pulse interacts directly with the solid target and not with a plasma.The target is made from deuterated polyethylene powd...

Observation of neutrons in the interaction of high intensity laser pulses with solid targets

“…The other source of neutrons e.g. electro disintegration of D atoms or photo dissociation of D or C atoms as well as other beam target reaction are expected to have negligible contribution in our relatively low intensity laser irradiation condition [11,12]. Here it is also important to note that for the low contrast laser, as in our case, deuterium ions are essentially accelerated in radial direction and it is observed that neutrons are emitted isotropically (or with weak anisotropy) [7].…”

“…Here it is also important to note that for the low contrast laser, as in our case, deuterium ions are essentially accelerated in radial direction and it is observed that neutrons are emitted isotropically (or with weak anisotropy) [7]. In contrast, for a high contrast laser pulses, the deuterons are mainly accelerated in forward direction and in this case the neutron anisotropy will be large [11,12].…”

High repetition-rate neutron generation by several-mJ, 35 fs pulses interacting with free-flowing D2O

“…Conventional fast neutron sources include deuterium−deuterium (D−D) and deuteriumtritium (D−T) fusion generators, as well as light-ion, photoneutron and spallation sources. Laser plasma interactions in the relativistic regime can also generate charged particles and subsequently accelerate them to energies high enough to trigger nuclear fusion reactions, resulting in neutron production [4][5][6][7][8][9][10][11][12][13][14][15][16] . Recent advances in ultra-high power laser technology now enable tabletop scale systems, which may be further reduced in size for use as drivers for portable neutron generators in the future.…”

Time evolution of solid-density plasma during and after irradiation by a short, intense laser pulse

“…Such ultrashort, ultraintense (USUI) laser pulses make possible exploration of intriguing phenomena related to laser interaction with solid-density plasmas, such as the acceleration of electrons and ions to high energies (Zhuo et al, 2010;Yu et al, 2010;Macchi et al, 2005;Hatchett et al, 2000;Koyama et al, 2006;Limpouch et al, 2008;Liu et al, 2009;Malka & Fritzler, 2004;Mckenna et al, 2008;Nickles et al, 2007;Schwoerer et al, 2006;Snavely et al, 2000;Yu et al, 2005), generation of relativistic positrons (Chen et al, 2009), production of intense quasi-static self-magnetic fields (Humphries & Petillo, 2000;Obenschain & Luhmann, 1979;Cai et al, 2011), etc. These phenomena are not only important to basic physics but also pivotal to many novel applications such as laser-driven particle accelerators, ion lithography schemes (Alves et al, 2006), hadron therapy (Bulanov & Khoroshkov, 2002;Salamin et al, 2008), X-ray lasers (Tabak et al, 1994), compact neutron sources (Disdier et al, 1999), etc.…”