Guoquan Ni

Two overrun phenomena and their effects on fusion yield in the Coulomb explosion of heteronuclear clusters

Two overrun effects in the Coulomb explosion dynamics of heteronuclear clusters have been investigated theoretically by the use of a simplified electrostatic model. When the charge-to-mass ratio of light ions is higher than that of heavy ions, the light ions can overtake the heavy ions inside the cluster and acquire a higher kinetic energy. Further, if the charge density of the heavy ions is twice as high as that of the light ions, i.e. a proposed competitive parameter xi = rho BqB/rho AqA > 2, the inner light ions can overtake those light ions on the surface of the cluster and form a shock shell during the explosion, which might drive the intracluster collision and fusion of the light ions. Different regimes of nuclear fusion are discussed and the corresponding neutron yields are estimated. Our analysis indicates that the probability of intracluster fusion is quite low even if deuterated heteronuclear clusters such as (DI)(n) with large size and high competitive parameter are employed. However, heteronuclear clusters are still a better candidate compared with homonuclear clusters for enhancing the total intercluster fusion yield because both a higher energy region and a higher proportion of deuterons distributing in the energy region can be created in the deuterated heteronuclear clusters.

Temporal optimization of neutron generation from the exploding deuterated methane jet of clusters subjected to an intense laser pulse

An experimental investigation on the interaction of an ultraintense femtosecond laser pulse at the intensity of 2 × 10 17 W / cm 2 (60 fs, 120 mJ at 800 nm) with clusters in a supersonic jet of deuterated methane gas has shown the generation of energetic deuterons and nuclear fusion events. The deuteron density and the average size of the clusters in the gas jet, as well as the fusionneutron yields under different backing pressures were measured simultaneously as a function of the time delays of the laser pulses with respect to the puffing of the gas jet. The results demonstrate that during the development of the gas jet expanding through a conical nozzle, the clusters grew up with time, and the average size of the clusters reached the maximum when the molecular density in the jet started to drop. The fusionneutron yields were found to increase with the larger average cluster size and the higher deuteron density, in accordance with the theoretical prediction. Experimental data indicate the existence of a ∼ 1 ms steady region in which the fusionneutron yields have reached the maximum of 2.0 × 10 5 per shot at the backing pressure of 74 bars. Consequently, an efficiency of 1.6 × 10 6 neutrons per joule of incident laser energy was realized.

Effects on the proton production from explosions of hydrogen clusters subjected to intense femtosecond laser fields

The dependence of the maximum and average energies of protons, which were produced in the interaction of an intense laser pulse (similar to 1 x 10(16) W cm(-2), 65 fs) with hydrogen clusters in a gas jet backed up to 80 bar at liquid nitrogen temperature (similar to 80 K), on the backing pressure has been studied. The general trend of the proton energy dependence on the square of the average cluster radius, which is determined by a calibrated Rayleigh scattering measurement, is similar to that described by theory under the single size approximation. Calculations are made to fit the experimental results under a simplified model by taking into account both a log-normal cluster size distribution and the laser intensity attenuation in the interaction volume. A very good agreement between the experimental proton energy spectra and the calculations is obtained in the high- energy part of the proton energy distributions, but a discrepancy of the fits is revealed in the low-energy part at higher backing pressures which are associated with denser flows. A possible mechanism which would be responsible for this discrepancy is discussed. Finally, from the fits, a variation of the cluster size distributions was revealed to be dependent on the gas backing pressure as well as on the evolving time of the gas flow of clusters.

Pure Coulomb explosions of highly charged methane clusters investigated by a simple electrostatic model

The pure Coulomb explosions of the methane clusters (CA(4))(n), (light atom A = H or D) have been investigated by a simplified electrostatic model for both a single cluster and an ensemble of clusters with a given cluster size distribution. The dependence of the energy of ions produced from the explosions on cluster size and the charge state of the carbon ions has been analysed. It is found that, unlike the average proton energy which increases with the charge q of the carbon ions, the average deuteron energy tends to saturate as q becomes larger than 4. This implies that when the laser intensity is sufficiently high for the (CD4)(n) to be ionized to a charge state of (C4+D4+)(n), the neutron yield from a table-top laser-driven Coulomb explosion of deuterated methane clusters (CD4)(n) could be increased significantly by increasing the interaction volume rather than by increasing the laser intensity to produce the higher charge state (C6+D4+)(n). The flight-time spectra of the carbon ions and the light ions have also been studied.

Propagation effects on fusion neutron generation in the Coulomb explosion of deuterated methane clusters

In this paper, fusion neutron yields from the Coulomb explosion of (CD4)N clusters under the irradiation of an intense femtosecond laser pulse is optimized by controlling the propagation of the laser pulse in a cluster jet. A correlated study of fusion neutron yields, kinetic energies of deuterons, together with the plasma channels diagnosed by a pump–probe interferometer, is performed. It has been found that by controlling the focal position related to the cluster jet, the plasma defocusing effect can balance with the tight focusing effect of the laser pulse induced by an off-axis parabolic mirror, and results in a cylindrical-shaped and relatively narrow plasma channel crossing the gas jet. The most energetic deuterons and the maximum yields of fusion neutrons are produced at the same time. For better understanding of the experimental results, numerical simulations for the nonlinear propagation of the femtosecond laser pulse in (CD4)N gas-cluster jets are performed. Simulated results show that plasma defocusing and laser attenuation play dominant roles in this case of optimization.

Coulomb explosion of hydrogen clusters irradiated by an ultrashort intense laser pulse

The explosion dynamics of hydrogen clusters driven by an ultrashort intense laser pulse has been analyzed analytically and numerically by employing a simplified Coulomb explosion model. The dependence of average and maximum proton kinetic energy on cluster size, pulse duration, and laser intensity has been investigated respectively. The existence of an optimum cluster size allows the proton energy to reach the maximum when the cluster size matches with the intensity and the duration of the laser pulse. In order to explain our experimental results such as the measured proton energy spectrum and the saturation effect of proton energy, the effects of cluster size distribution as well as the laser intensity distribution on the focus spot should be considered. A good agreement between them is obtained.

Nuclear Fusion Driven by Coulomb Explosion of Deuterated Methane Clusters in an Intense Femtosecond Laser Field

We have made experimental studies on the generation of deuterium-deuterium fusion neutrons from intense Coulomb explosions (CE) of large-size (CD4)N cluster jets under the irradiation of intense femtosecond laser pulses. By optimizing the propagation of a laser pulse in the cluster gas and the time delay between the laser pulse and the gas flow, the maximum neutron yield of 2:5×10^5, which corresponds to a conversion efficiency of 2:1×10^6 fusion neutrons per joule of incident laser energy, has been obtained with a 120-mJ, 60-fs laser pulse and cluster jets with an average molecular density of 3:6×10^(18) cm^(-3) and cluster radius of 7 nm. We have demonstrated that the neutron yields can be dramatically increased by using heteronuclear (CD4)N clusters as compared with the similar sized homonuclear (D2)N clusters. This enhancement is attributed to the significant increase in the deuteron kinetic energies due to energetic boosting and overrun effects during CE of heteronuclear clusters.

Cluster Formation Processes Investigated by Linearly Chirped Spectral Scattering

By employing linearly chirped spectral scattering technique, the time-resolved formation processes of argon clusters have been investigated. The redshifts of scattering spectra as a function of backing pressure as well as the time delay between the laser and the firing switch of the cluster flow have been measured. It has been found that very large-size cluster sources with very low gas density can be produced by adiabatic expansion of gases at low pressures through a conical nozzle into vacuum. It can be used as clean and important cluster target.

Average kinetic energy of ions ejected from pure Coulomb explosions of methane clusters

We use an electrostatic model to study the average kinetic energy of ions ejected from the pure Coulomb explosions of methane clusters (CA4)n (light atom A=H and D). It is found that the ratio of the average kinetic energy of the ions to their initial average electrostatic potential energy is irrelevant to the cluster size. This finding implies that as long as the ratio is given, the average kinetic energies of the ions can be simply estimated from their initial average electrostatic potential energies, rather than from the time-consuming simulations. The ratios for the different charge states of carbon ions are presented.

Scaling of ion energies with size of atomic clusters irradiated by high-intensity femtosecond laser pulses

Energies of ions emitted from large rare gas clusters irradiated by high intensity femtosecond laser pulses have been measured and the dependence of the average energy of ions on cluster sizes have been studied experimentally.