Characterization of Fusion Burn Time in Exploding Deuterium Cluster Plasmas

J. Zweiback, T. E. Cowan, R. A. Smith, J. H. Hartley, R. Howell, C. A. Steinke, G. Hays, K. B. Wharton, J. K. Crane, T. Ditmire
2000 Physical Review Letters  
Exploiting the energetic interaction of intense femtosecond laser pulses with deuterium clusters, it is possible to create conditions in which nuclear fusion results from explosions of these clusters. We have conducted high-resolution neutron time-of-flight spectroscopy on these plasmas and show that they yield fast bursts of nearly monochromatic fusion neutrons with temporal duration as short as a few hundred picoseconds. Such a short, nearly pointlike source now opens up the unique
more » ... e unique possibility of using these bright neutron pulses, either as a pump or a probe, to conduct ultrafast studies with neutrons. PACS numbers: 52.58.Ns, 52.50.Jm, 52.70.Nc Fast neutrons are used in a wide array of applications including radiography [1] and materials science [2] . However, the possibility of ultrafast studies using neutrons, either as a pump or a probe, is usually not considered because no high flux sources of such short neutron pulses exist. Such a source needs to have not only a short initial pulse width, but also a small size and a monochromatic spectrum so that the ultrafast temporal structure is maintained over a reasonable distance from the source. High fluxes of energetic neutrons can be produced with a number of devices, including accelerators [3,4] and plasma pinches [5] . However, these devices yield neutron pulses with duration typically longer than a few nanoseconds. Neutron sources based on nuclear fusion in a hot plasma (like those produced in plasma pinches) produce nearly monochromatic neutrons (with energy near 2.45 MeV for neutrons produced from the fusion of deuterium). With a short lived fusion plasma it is possible to produce a short burst of fast neutrons. Fusion plasmas produced by large scale inertial confinement fusion experiments produce fast fusion neutron emission (ϳ100 ps) [6] with a fusion burn time set by the disassembly time of the inertially confined compressed plasma core (roughly equal to the plasma radius divided by the plasma sound speed). Intense, focused short pulse lasers now present another avenue to creating a short lived, hot fusion plasma [7] , and a number of recent experiments have observed beam-target fusion neutrons in intense laser solid interactions [8, 9] (typically produced at relativistic laser intensity with a rather broad neutron spectrum). Recent results indicate that nearly monochromatic fusion neutrons can be produced from the interaction of femtosecond laser pulses with deuterium clusters, even with laser pulses of quite modest energy (ϳ0.1 J per pulse) [10] . In this Letter we present measurements of the neutron pulse duration from plasmas created by the irradiation of deuterium clusters with an intense, 35 fs laser pulse. We find that the fusion neutrons produced in this interaction are nearly monochromatic around a neutron energy of 2.45 MeV and exhibit a fusion burn time pulse duration as short as 500 ps. This opens the potential of using tabletop scale neutron sources in sub-ns to ps scale pump-probe experiments for probing the dynamics of neutron interactions with materials. Nuclear fusion in a high temperature deuterium plasma will produce neutrons with 2.45 MeV of energy from the D 1 D ! He 3 1 n reaction. Ion temperatures above a few keV are needed to yield significant numbers of these fusion events in the plasma [11] . Recent experiments on the interaction of intense femtosecond laser pulses with gases of large deuterium clusters (clusters that contain many thousands of deuterium atoms each) have shown that a strong laser pulse can drive an energetic explosion of the clusters [12, 13] , ejecting ions with sufficient kinetic energy to fuse with ions ejected from other, nearby clusters [10] . In this experiment, the intense laser pulse is focused in a gas jet containing deuterium clusters. The clusters explode, accelerating the ions in a filament with diameter of roughly 200 mm (determined by the focused spot) and a filament length given by the absorption depth of the laser pulse in the cluster gas (about 2 mm). This cigar-shaped filament is illustrated in Fig. 1 . The fusion neutrons will, therefore, originate in a small volume bounded by this heated filament. Subsequent to the explosion of the clusters in this heated filament (the clusters explode on a sub-ps time scale) the fast ions ejected from clusters in the filament can fuse with each other. However, the burn time of this hot deuterium plasma is limited by the time it takes the fast ions to escape the heated filament. As illustrated in Fig. 1 , the fast ions will exit this filament on a time scale roughly given by the diameter of the filament (comparable to the laser focal spot diameter) divided by the ion velocity. Assuming 10 keV ions traversing a 200 mm plasma filament, the fusion burn time will be on the order of 200 ps. Furthermore, the emitted neutrons will have an energy centered at 2.45 MeV with a narrow energy spread arising from ion Doppler broadening. 3640 0031-9007͞00͞85(17)͞3640(4)$15.00
doi:10.1103/physrevlett.85.3640 pmid:11030970 fatcat:j3oluvftf5a65iktqaon23omve