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

Abstract

In this letter, we discuss the increase in the average cluster size by lowering the stagnation temperature of the methane (CH4) gas. The Coulomb explosion experiments are conducted to estimate the cluster size and the size distribution. The average CH4 cluster sizes Nav of 6 230 and 6 580 are acquired with the source conditions of 30 bars at 240 K and 60 bars at 296 K, respectively. Empirical estimation suggests a five-fold increase in the average size of the CH4 clusters at 240 K compared with that at room temperature under a backing pressure of 30 bars. A strong nonlinear Hagena parameter relation (Γ * ∝ T −3.3 0 ) for the CH4 clusters is revealed. The results may be favorable for the production of large-sized clusters by using gases at low temperature and high back pressures.OCIS codes: 020.2649, 350.5400. doi: 10.3788/COL201311.S20201.Clusters, with van der Waals bonded agglomerations of up to 10 7 atoms, have received considerable attention, and have been investigated because of their density characteristics in internal solid and general gas. Such clusters were first discovered by Becker et al.[1] in 1956 and were investigated systematically by Hagena [2] . Moreover, such clusters have been used for various applications, such as X-ray generation [3] , energetic ion production [4] , and nuclear fusion [5] , after the invention of intense femtosecond laser systems. In 1999, Ditmire et al. demonstrated the deuterium-deuterium (D-D) nuclear fusion in deuterium cluster jets under the irradiation of intense femtosecond laser pulses [5] . Thereafter, intensive studies on laser-cluster interaction have been conducted to generate higher energetic ions and consequently increase fusion neutron yields [6−14] . Theoretical results have revealed the essence of laser-cluster interaction and have proposed the use of heteronuclear clusters, such as deuterated methane (CD 4 ) and heavy water (D 2 O), as targets for generating more energetic light ions compared with homonuclear clusters such as D 2 with the same sizes because of the energetic and kinematic effects [15−18] . The proposal paves the way for future research on the tabletop laser-driven fusion [13,19] , which can provide highflux sources of clean fusion neutrons and short pulse durations. A petawatt-laser-produced [20] fusion neutron source may be of considerable importance to studies on radiation-induced damage on materials, as well as benefit fast neutron radiography, in which the small source size could lead to a high spatial resolution [21] . 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.Gaseous clusters are typically produced through the supersonic expansion of a high backing pressure...

Efficient generation of fusion neutrons from cryogenically cooled heteronuclear clusters irradiated by intense femtosecond lasers

“…Therefore, comparing the cluster size of 296 K with that of 250 K, we obtain ¡ = ¹3.5 and À Ã / T À3:5 0 , which is consistent with a nonlinear relationship of À Ã / T À3:3 0 for methane (CH 4 ) clusters reported in Ref. 23. This nonlinear relationship between the cluster size and the stagnation temperature for CD 4 clusters is much stronger than that for monoatomic or diatomic clusters.…”

“…From the measured energy spectrum of deuterons, the CD 4 cluster size can be estimated through a fitting procedure, in which the cluster jet is described by a logarithmic normal size distribution, and the overrun effects in the explosion of heteronuclear CD 4 clusters as well as the effect of carbon ions over the energy spectrum of deuterons are considered. 19,20,23) Figure 2 shows the corresponding average cluster sizes as a function of time delay in the case of 50 bars and stagnation temperatures of 296, 250, and 200 K. It is obvious that variations in the average cluster sizes and the average KEs of deuterons show similar features, implying temporal evolutions of the cluster formation. As the temperature drops from 296 and 250 to 200 K, the average cluster radius at the optimal time delay increases from 4.0 and 6.4 to 9.2 nm with respect to the average cluster molecule numbers of 4280, 17550, and 52150, respectively.…”