The experimental results of low pressure supersonic molecular beam injection (SMBI) fuelling on the HL-2A closed divertor indicate that during the period of pulsed SMBI the power density convected at the target plate surfaces was 0.4 times of that before or after the beam injection. An empirical scaling law used for the SMBI penetration depth for the HL-2A plasma was obtained. The cluster jet injection (CJI) is a new fuelling method which is based on and developed from the experiments of SMBI in the HL-1M tokamak. The hydrogen clusters are produced at liquid nitrogen temperature in a supersonic adiabatic expansion of moderate backing pressure gases into vacuum through a Laval nozzle and are measured by Rayleigh scattering. The measurement results have shown that the averaged cluster size of as large as hundreds of atoms was found at the backing pressures of more than 0.1 MPa. Multifold diagnostics gave coincidental evidence that when there was hydrogen CJI in the HL-2A plasma, a great deal of particles from the jet were deposited at a terminal area rather than uniformly ablated along the injecting path. SMB with clusters, which are like micro-pellets, will be of benefit for deeper fuelling, and its injection behaviour was somewhat similar to that of pellet injection. Both the particle penetration depth and the fuelling efficiency of the CJI were distinctly better than that of the normal SMBI under similar discharge operation. During hydrogen CJI or high-pressure SMBI, a combination of collision and radiative stopping forced the runaway electrons to cool down to thermal velocity due to such a massive fuelling.

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...

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.

We study the carbon-dope aluminum clusters by using time-of-flight mass spectrum experiments and ab initio calculations. Mass abundance distributions are obtained for anionic aluminum and aluminum-carbon mixed clusters. Besides the well-known magic aluminum clusters such as Al In recent years, clusters and cluster-based materials have been a field of intensive research due to both fundamental and technological importance 1-7 . The structural, electronic, magnetic, and optical properties of the clusters are different from those of constitute atoms or bulk phase and depend sensitively on the size and composition of the cluster 1-3 . It is desirable to assemble the cluster-based materials from properly designed clusters so that the unique properties of these individual clusters can be retained 6,7 . To be a building block of clusterassembled materials, the cluster should be highly stable and relatively unreactive. Thus, the clusters would interact weakly with each other and maintain their identities when they are brought together in the cluster-assembled solids. A well-known example is the C 60 solids 8 . Besides C 60

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 ζ = ρBqB/ρAqA > 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.

An experimental investigation on the interaction of an ultraintense femtosecond laser pulse at the intensity of 2×1017 W/cm2 (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 fusion neutron 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 fusion neutron 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 fusion neutron yields have reached the maximum of 2.0×105 per shot at the backing pressure of 74 bars. Consequently, an efficiency of 1.6×106 neutrons per joule of incident laser energy was realized.

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We report the first observation of the high vibronic levels of the Na3 ground state, performed by stimulated emission spectroscopy. The highest cross sections of the process, which uses the C 2 E" state as an intermediate state, are obtained for combinations of symmetric stretch and bending modes. These experimental results are of primary importance for the understanding of energy-level structures in systems exhibiting strong vibronic coupling.PACS numbers: 36.40.+d, 31.50,+w, 35.20.-i Energy levels of diatomic molecules are a well-known quantum-mechanical problem, except for highly excited Rydberg states, in which quantum chaos 1 may appear. For molecules having three atoms or more, the energy levels in the whole potential surface cannot be easily analytically calculated, except in certain circumstances, like the well-defined vibronic-mode approximation or the free pseudorotation in Jahn-Teller systems. 2 Because of their simplicity, the alkali-metal trimer systems are the most promising candidates for achieving deeper insight in this very active field. 34 Among these, Na3 is the best known system: Numerous theoretical calculations have been carried out over the last ten years 5 " 8 and, with the advent of highly cooled supersonic beams, well-resolved spectra of the electronically excited states are available. 9 " 13 However, experimental information about the theoretically better known ground state was, up to now, rather scarce. Lindsay and Thompson 14 have analyzed the matrix spin resonance in terms of pseudorotation. Broyer et a/. 15 investigated the hot bands that appear just below the origin of each of the visible systems. In this way they were able to observe three fundamental frequencies (49, 87, and 139 cm -1 ), and these were associated in a straightforward way with the normal-mode frequencies of vibration computed for the shallow minima of Civ symmetry found in ab initio calculations. 6,7 At present, we are not aware of any precise computations, based on ab initio potential-energy surfaces, of the ground-state energy levels. However, recent classical and quantum calculations have been performed on H3 in a chaotic region. 4 We report in this paper a stimulated emission pumping (SEP) experiment 16 on Na 3 , which has, in the past, been very successfully used to study highly excited vibrational levels at high resolution. The stimulated emission rate is detected by depletion of the photoionization signal. Under these conditions, this technique becomes highly sensitive and cluster-size specific. Our experiment is the first SEP experiment performed on clusters. We have chosen the well-resolved bands of the Na 3 C system as intermediate state for stimulated emission back to ex-cited vibronic levels of the ground state.The principle of the SEP experiment on Na 3 uses the ion-depletion technique 17 (Fig. 1). The Na 3 molecules, produced in a molecular-beam apparatus, are excited in an intermediate level of the C system 11,13 by a first laser co\. A second laser (02 is then used for inducing two competitive p...