Efficient fusion neutron generation from heteronuclear clusters in intense femtosecond laser fields

Abstract

We demonstrate experimentally the efficient fusion neutron generation from Coulomb explosion ͑CE͒ of laser irradiated large-size heteronuclear deuterated methane clusters. A conversion efficiency of 2.1 ϫ 10 6 neutrons/ J of incident laser energy is obtained with a 120 mJ, 70 fs laser pulse. It is 50 times higher than that of homonuclear deuterium clusters of similar size. This enhancement is attributed to the significant increase in the deuteron kinetic energies by fourfold due to energetic boosting and overrun effects during CE of heteronuclear clusters. The yield of 5.5ϫ 10 6 neutrons per pulse is obtained with a 100 TW, 50 fs driving laser pulse at an intensity of 1.5ϫ 10 19 W / cm 2 . This work may facilitate the development of a high-flux The generation of deuterium-deuterium ͑DD͒ fusion neutrons from Coulomb explosion ͑CE͒ of laser-heated cryogenic deuterium clusters ͑D 2 ͒ N was first demonstrated by Ditmire et al. in 1999 ͓1͔ with a high-repetition-rate tabletop laser; an efficiency of about 10 5 fusion neutrons/ J of incident laser energy was achieved, which was close to the efficiency of large-scale laser-driven fusion experiments ͓2,3͔. This kind of short ͑subnanosecond͒ bursts of monoenergetic fusion neutrons could find wide applications in materials science ͓4͔ such as high spatial resolution neutron radiography and time-resolved study of radiation damage which is of particular importance for developing future fusion energy reactor. However, the conversion efficiency of neutron generation should be improved dramatically to be 10 7-8 neutrons/ J of incident laser energy ͓5͔. Extensive researches have been devoted to investigate the fusion dynamics in laser-cluster interactions and the temporal and spatial characterizations of fusion neutron emission, as well as to search for higher neutron yields ͓6-19͔. The effects of the ͑D 2 ͒ N cluster size, the laser energies, and focusing conditions were studied by Zweiback et al. to optimize fusion neutron yields ͓7͔. However, the average kinetic energies ͑KEs͒ of deuterons from explosion of ͑D 2 ͒ N clusters were reported to be in the range of 2.5-7 keV ͓6,7,13-15͔ which are still much lower than the optimal KEs in the range of 40-100 keV for an efficient DD fusion.Last and Jortner proposed a scheme to enhance the deuterons' KEs by using clusters of heteronuclear deuterium containing molecules, e.g., ͑D 2 O͒ N and ͑CD 4 ͒ N ͓9,10,12,16,17͔. For the Coulomb explosion of the heteronuclear clusters, the light deuterons' KEs can be greatly enhanced due to kinematic overrun effect and the energetic boosting caused by the large ionic charge of the heavy ions inside the cluster ͓12,16,17͔. Grillon et al. used deuterated methane clusters ͑CD 4 ͒ N as a novel target in a table-top nuclear fusion experiment, demonstrating a conversion efficiency of about 1 ϫ 10 4 neutrons/ J of incident laser energy ͓11͔. Meanwhile, an independent theoretical work on ͑CD 4 ͒ N made by Last and Jortner predicts that the neutron yields with the heteronuclear clusters are 3.7ϫ 10 5 neutr...

Cooling-induced increase of methane cluster size investigated under a Coulomb explosion scheme

“…The experimental setup had been reported in Ref. [23] and was described briefly in the present study. In the present experiments, the CH 4 clusters were produced using a solenoid valve and a conical nozzle with a throat diameter of 310 µm, a length of 26 mm, and a half opening angle of 4.6…”

“…Despite the limited knowledge on polyatomic cluster formation, particularly at low temperatures, large CH 4 clusters can be produced with high backing pressure and low temperature considering the similarity of the hydrodynamic process of cluster formation process with that of monatomic and diatomic clusters. In previous studies, we increased the CH 4 cluster sizes by increasing the backing pressures at room temperature [22,23,28] . However, the increase in backing pressure is expected to cause serious vacuum loading in the pumping systems, and decrease cluster formation efficiency [28] .…”

“…However, the actual average cluster size should be determined exactly for an accurate comparison. Thus, a cluster size characterization scheme was proposed using a spherical model [23] , in which clusters explode layer by layer [29] with the measured KE spectra. In the proposed scheme, the ions, which are originally located in the radius r in a given single-sized heteronuclear CH 4 cluster with radius R and internal molecular density of ρ = 1.6 × 10 22 cm −3 [19] , are accelerated during a Coulomb explosion to acquire the final average KE as…”

“…We have recently demonstrated an efficient fusion neutron generation by using CD 4 clusters under the irradiation of 120-mJ and 60-fs laser pulses [22] . The key factors for generating fusion neutrons includes the energetic deuterons and the density of deuterons inside and around the plasma channel [19,23] . Larger average cluster sizes are required to further increase the average kinetic energy (KE) of exploded deuterons.…”

An experimental investigation on the performance of conical nozzles for argon cluster formation in supersonic jets

“…The average size of the H 2 clusters and can be determined by fitting the resultant energy spectrum of protons with that formed from Coulomb explosion of H 2 clusters of a lognormal size distribution. 20,21,24,25 …”