Optimization of the neutron yield in fusion plasmas produced by Coulomb explosions of deuterium clusters irradiated by a petawatt laser

Abstract

The kinetic energy of hot (multi-keV) ions from the laser-driven Coulomb explosion of deuterium clusters and the resulting fusion yield in plasmas formed from these exploding clusters has been investigated under a variety of conditions using the Texas Petawatt laser. An optimum laser intensity was found for producing neutrons in these cluster fusion plasmas with corresponding average ion energies of 14 keV. The substantial volume (1-10 mm 3 ) of the laser-cluster interaction produced by the petawatt peak power laser pulse led to a fusion yield of 1.6 ×10 7 neutrons in a single shot with a 120 J, 170 fs laser pulse. Nuclear fusion from laser-heated deuterium clusters has been studied since 1999 [1]. Deuterium clusters are nanometerscale assemblies of atoms bound at liquid density by van der Waals forces, which can be produced by forcing cold deuterium gas under high pressure through a supersonic nozzle into vacuum. In these experiments, the deuterium clusters are irradiated by an intense ultrashort laser pulse. The clusters absorb the pulse energy very efficiently [2] and the process by which the ions attain their high kinetic energies has been well explained by the Coulomb explosion model [3,4]. In this model, the electrons in the atomic cluster first absorb the laser pulse energy as the atoms are ionized. The electrons further gain energy through other absorption mechanisms such as above-threshold ionization [5], inverse bremsstrahlung heating [6], resonant heating [6][7][8], and escape from the space-charge forces of the cluster, on the time scale of tens of fs. At high enough laser intensity, almost all of the electrons are removed from the cluster on a time scale short relative to the ion motion. What remains is a highly charged cluster of ions at liquid density, which promptly explodes by Coulomb repulsion.In experiments with peak laser intensities of 10 16 -10 18 W/cm 2 , deuterium ions with average kinetic energies up to about 10 keV have been observed, which were energetic enough to drive DD fusion events in a plasma with an average ion density near 10 19 cm −3 [9][10][11][12]. DD fusion can also occur when energetic ions collide with cold atoms in the background gas jet [13]. As a result of both of these fusion reactions, quasimonoenergetic 2.45 MeV neutrons are produced from the localized fusion plasma in a subnanosecond burst until the plasma disassembles in about 100 ps.Neutron yields greater than 10 8 n/shot would yield neutron fluences near the cluster jet greater than 10 10 n/cm 2 enabling subnanosecond time-resolved pump-probe experiments of neutron damage studies [14]. The petawatt lasers currently operating and being built with pulse durations below 200 fs have the potential to drive such sources. Therefore, the laser-cluster-generated fusion plasma is attractive as a bright, * Author to whom correspondence should be addressed: dws223@physics.utexas.edu short, and localized neutron source that is potentially useful for material damage studies.In this paper, we describe the scaling of cluster pl...

Experimental study of fusion neutron and proton yields produced by petawatt-laser-irradiated D2-3He or CD4

“…A ground mesh placed in front of the cup maintained a field-free region near the Faraday cup, while a negative 400 V bias on the collector repelled the slow electrons that could affect the TOF measurements arriving at the same time as the ions. We calculated the total number and energy spectrum of deuterium ions in the laser plasma, with the assumption of isotropic emission [10,21]. This assumption is reasonable because the clusters undergo Coulomb explosion in this experiment rather than the ambipolar expansion according to the criteria in Ref.…”

Characterization of deuterium clusters mixed with helium gas for an application in beam-target-fusion experiments

“…We compare this cluster size with that found in Ref. [16] for pure deuterium clusters at a similar laser intensity.…”

“…The TPW irradiated the clusters with 140-180 J, 150-200 fs, 1.06 μm wavelength pulses. An f/40 focusing mirror with a 10 m focal length created a large interaction volume with laser intensity sufficiently high for a near-complete Coulomb explosion of deuterium clusters [16,44].…”

“…Production of energetic ions from laser-heated atomic or molecular clusters has been an active area of research in laser-plasma physics over the past two decades [1][2][3][4][5][6][7][8]. In particular, theoretical and experimental studies have been made of nuclear fusion reactions generated using deuterium clusters or deuterated methane clusters [7][8][9][10][11][12][13][14][15][16][17][18]. In a typical laser-cluster-fusion experiment involving deuterium, an intense femtosecond laser pulse irradiates deuterium clusters, 1-10 nm radius aggregates of deuterium molecules, bound at liquid density by van der Waals forces.…”

The Texas petawatt laser and current experiments

“…He [7]. These interactions produced deuterium ions energetic enough to initiate D-3 He nuclear fusion reactions as well as the DD fusion reactions inside the fusion plasma.…”