The energetic laser–cluster interaction has opened the door to nuclear fusion experiments using table-top laser systems operating at high repetition rates. The D + T -> n + He4 reaction has by far the largest cross-section for the range of ion energies accessible in laser–cluster experiments, but tritium poses a serious radiological hazard and so the D–D reaction has been investigated instead. Half the D–D fusion events follow the D + D -> n(2.45 MeV) + 3He reaction, which is convenient for study since the neutron can escape the inter- action chamber and, being of well-defined energy, can be identified unambiguously through TOF measurements.
The first definitive observation of a cluster based fusion reaction was reported in 1999 [68]. A dense D2 cluster target was irradiated with a 35 fs, 200 mJ laser pulse from a Ti:S CPA system. In the target, deuterons ejected from different clusters collide with sufficient energy (multi-keV) to trigger fusion events. The low value of the condensation parameter, k, for D2 requires it to be cooled below 80 K to allow large cluster formation. Around 50,000 neutrons were observed per shot (at a repetition rate of 10 Hz), which is a relatively large number when normalized to the input laser energy. In contrast to high Z clusters (Ar, Kr, Xe), D2 clusters can be stripped of almost all their electrons by a sufficiently short laser pulse (<50 fs is required). Hence a Coulomb explosion picture, rather than a nanoplasma model is appropriate. For a fully stripped D2 cluster of radius r, the maximum ion energy from the Coulomb explosion is given by
formula (9) page 321, TD69.pdf
where nD is the initial ion density (~3 x 1022 cm-3 for D2). From this it can be calculated that a D2 cluster of radius r > 2.5 nm is required to produce multi-keV deuterons necessary for fusion. A detailed description of this work can be found in [69].
Modelling of the neutron yield within this Coulomb explosion picture has been carried out [70] providing good agreement with the experimental data, and a comparison with the case of a hydrodynamic cluster expansion is given in [71].
Also, direct measurements of proton energies up to 7 keV from exploding hydrogen clusters have been made [72], showing a combination of Coulomb explosion and hydrodynamic mechanisms for somewhat longer laser pulses (90 fs).
Fusion in clusters has attracted considerable interest because it provides a pulsed source of neutrons of a small source size. The neutron pulse duration is basically set by the time taken for an energetic deuteron to leave the laser focus, and has been measured to be less than 500 ps [73], which is considerably shorter than Z-pinches and spallation devices. A number of schemes have been proposed to increase the number of neutrons per laser shot. The use of mixed species (or heteronuclear) clusters comprising deuterium and a higher Z atomic species (e.g. DI, D20, CD4) is likely to increase the deuteron explosion energies and hence the neutron yields owing to the presence of the higher ionic charge [74–76]. A further advantage is that many of these molecular gases have much larger condensation parameters than D/2 , eliminating the need for cryogenic cooling to produce large clusters. Another scheme recently demonstrated is the ‘‘machining’’ of a clustered gas jet target using a second laser pulse [77] to disassemble clusters in the lower density entrance wing of the jet, ahead of the arrival of the main heating pulse. By reducing absorption in this wing – known to limit the fusion yield [69] – this has been shown to significantly increase the deposition of laser energy in the high density, central part of the gas jet. A significant increase in both the number of neutrons per shot, and the pulse repetition rate of the neutron source (1 kHz is already feasible) is likely to usher in applications of this table-top source, including short pulse neutron imaging, material probing and long term testing of materials under high fluxes of neutrons.
See Also
3.14 - Vortex Theory of Atomic Motions 13.04 - Atomic Subdivision atomic Atomic Cluster Application Atomic Cluster Charge build-up Atomic Cluster Expansion Atomic Cluster Experimental Apparatus Atomic Cluster Heating Atomic Cluster Ionization Atomic Cluster X-Ray Emission Atomic Clusters Atomic Force atomic mass atomic number atomic theory atomic triplet atomic weight diatomic Figure 13.06 - Atomic Subdivision Force-Atomic Formation of Atomic Clusters Interaction of Intense Laser Pulses with Atomic Clusters - Measurements of Ion Emission Simulations and Applications TD69.pdf InterAtomic Ion Energies from Atomic Cluster Explosions Laser Cluster Interactions Law of Atomic Dissociation Law of Atomic Pitch Law of Oscillating Atomic Substances Law of Pitch of Atomic Oscillation Law of Variation of Atomic Oscillation by Electricity Law of Variation of Atomic Oscillation by Sono-thermism Law of Variation of Atomic Oscillation by Temperature Law of Variation of Atomic Pitch by Electricity and Magnetism Law of Variation of Atomic Pitch by Rad-energy Law of Variation of Atomic Pitch by Temperature Law of Variation of Pitch of Atomic Oscillation by Pressure Models of Laser Cluster Interactions monatomic Numerical Simulation of an Atomic Cluster Explosion Sonofusion subatomic