T. Ditmire

Nuclear fusion from explosions of femtosecond laser-heated deuterium clusters

As a form of matter intermediate between molecules and bulk solids, atomic clusters have been much studied. Light-induced processes in clusters can lead to photo-fragmentation, and Coulombic fission, producing atom and ion fragments with a few electronvolts (eV) of energy. However, recent studies of thephotoionization of atomic clusters with high intensity (>1016 W cm−2) femtosecond laser pulses have shown that these interactions can be far more energetic—excitation of large atomic clusters can produce a superheated microplasma that ejects ions with kinetic energies up to 1 MeV (ref. 10). This phenomenon suggests that through irradiation of deuterium clusters, it would be possible to create plasmas with sufficient average ion energy for substantial nuclear fusion. Here we report the observation of nuclear fusion from the explosions of deuterium clusters heated with a compact, high-repetition-rate table-top laser. We achieve an efficiency of about 105 fusion neutrons per joule of incident laser energy, which approaches the efficiency of large-scale laser-driven fusion experiments. Our results should facilitate a range of fusion experiments using small-scale lasers, and may ultimately lead to the development of a table-top neutron source, which could potentially find wide application in materials studies.

Quasi-monoenergetic laser-plasma acceleration of electrons to 2 GeV

Laser-plasma accelerators of only a centimetre’s length have produced nearly monoenergetic electron bunches with energy as high as 1 GeV. Scaling these compact accelerators to multi-gigaelectronvolt energy would open the prospect of building X-ray free-electron lasers and linear colliders hundreds of times smaller than conventional facilities, but the 1 GeV barrier has so far proven insurmountable. Here, by applying new petawatt laser technology, we produce electron bunches with a spectrum prominently peaked at 2 GeV with only a few per cent energy spread and unprecedented sub-milliradian divergence. Petawatt pulses inject ambient plasma electrons into the laser-driven accelerator at much lower density than was previously possible, thereby overcoming the principal physical barriers to multi-gigaelectronvolt acceleration: dephasing between laser-driven wake and accelerating electrons and laser pulse erosion. Simulations indicate that with improvements in the laser-pulse focus quality, acceleration to nearly 10 GeV should be possible with the available pulse energy.

Demonstration of a 1.1 petawatt laser based on a hybrid optical parametric chirped pulse amplification/mixed Nd:glass amplifier

We present the design and performance of the Texas Petawatt Laser, which produces a 186 J 167 fs pulse based on the combination of optical parametric chirped pulse amplification (OPCPA) and mixed Nd:glass amplification. OPCPA provides the majority of the gain and is used to broaden and shape the seed spectrum, while amplification in Nd:glass accounts for >99% of the final pulse energy. Compression is achieved with highly efficient multilayer dielectric gratings.

High energy electrons, nuclear phenomena and heating in petawatt laser-solid experiments

The Petawatt laser at LLNL has opened a new regime of laser-matter interactions in which the quiver motion of plasma electrons is fully relativistic with energies extending well above the threshold for nuclear processes. In addition to -few MeV ponderomotive electrons produced in ultra-intense laser-solid interactions, we have found a high energy component of electrons extending to -100 MeV apparently from relativistic self-focusing and plasma acceleration in the underdense pre-formed plasma. The generation of hard bremsstrahlung, photo-nuclear reactions, and preliminary evidence for positron-electron pair production will be discussed.

Characterization of a cryogenically cooled high-pressure gas jet for laser/cluster interaction experiments

We have developed and carried out detailed characterization of a cryogenically cooled (34–300 K), high-pressure (55 kTorr) solenoid driven pulsed valve that has been used to produce dense jets of atomic clusters for high intensity laser interaction studies. Measurements including Rayleigh scattering and short pulse interferometry show that clusters of controlled size, from a few to >104 atoms/cluster can be produced from a broad range of light and heavy gases, at average atomic densities up to 4×1019 atoms/cc. Continuous temperature and pressure control of the valve allows us to vary mean cluster size while keeping the average atomic density constant, and we find that many aspects of the valves behavior are consistent with ideal gas laws. However, we also show that effects including the build up of flow on milliseconds time scales, the cooling of gas flowing into the valve, and condensation of gas inside the valve body at temperatures well above the liquefaction point need to be carefully characterized in...

Self-phase modulation in chirped-pulse amplification

The effect of self-phase modulation in chirped-pulse amplification is investigated. A numerical model is used to predict the effects of phase modulation on pulse recompression, and experimental results are presented that agree well with the calculations. We show that even moderate self-phase modulation can significantly distort the recompressed pulse after amplification, thereby reducing the peak power and degrading the pulse contrast.

Fusion neutron and ion emission from deuterium and deuterated methane cluster plasmas

Experiments on the interaction of intense, ultrafast pulses with large van der Waals bonded clusters have shown that these clusters can explode with substantial kinetic energy and that the explosion of deuterium clusters can drive nuclear fusion reactions. Producing explosions in deuterated methane clusters with a 100 fs, 100 TW laser pulse, it is found that deuterium ions are accelerated to sufficiently high kinetic energy to drive deuterium nuclear fusion. From measurements of cluster size and ion energy via time of flight methods, it is found that these exploding deuterated methane clusters exhibit higher ion energies than explosions of comparably sized neat deuterium clusters, in accord with recent theoretical predictions. From measurements of the plume size and peak density, the relative contribution to the fusion yield from both beam target and intrafilament fusion is discussed.

Detailed study of nuclear fusion from femtosecond laser-driven explosions of deuterium clusters

Recent experiments on the interaction of intense, ultrafast pulses with large van der Waals bonded clusters have shown that these clusters can explode with sufficient kinetic energy to drive nuclear fusion. Irradiating deuterium clusters with a 35 fs laser pulse, it is found that the fusion neutron yield is strongly dependent on such factors as cluster size, laser focal geometry, and deuterium gas jet parameters. Neutron yield is shown to be limited by laser propagation effects as the pulse traverses the gas plume. From the experiments it is possible to get a detailed understanding of how the laser deposits its energy and heats the deuterium cluster plasma. The experiments are compared with simulations.

Explosion of atomic clusters heated by high-intensity femtosecond laser pulses

We have experimentally and theoretically studied the high-intensity ( .10 W cm), femtosecond photoionization of inertially confined noble-gas clusters. We have examined the energies of electrons and ions ejected during these interactions and found that particles with substantial kinetic energy are generated. Electrons with energies up to 3 keV and ions with energies of up to 1 MeV have been observed. These experimental observations are well explained by a theoretical model of the cluster as a small plasma sphere that explodes following rapid electron collisional heating by the intense laser pulse. @S1050-2947 ~97!02912-0#

INVESTIGATION OF HIGH-HARMONIC GENERATION FROM XENON ATOM CLUSTERS

We report on the generation of harmonic radiation (in the 70 - 90 nm range) from clusters of Xe atoms formed in a gas jet. We find that the harmonic yield from the clusters exhibits an anomalous cubic scaling with backing pressure to the gas jet. This scaling is consistent with a cluster dipole moment resulting from collective oscillations of electrons around the central ions of the cluster. Using a nanosecond ultraviolet prepulse to dissociate the clusters, we have also attempted to compare harmonic yields from clusters with those produced from monatomic Xe, under otherwise identical conditions. Our results suggest that yields from clusters might exceed those from monomers by up to a factor of five.