Laser-nuclear interactions

Laser-Nuclear Interactions and Laser-Driven Fusion... 

Gamma-ray laser action is a further related field which has attracted much efforts [9.306], Laser-driven (inertial confinement) fusion using the largest laser installation is a huge research field for laser-nuclear interaction [9.307]. The laser energy requirements for conventional laser-driven deuterium-tritium fusion seem to be difficult to reach. This has stimulated the development of a modified concept, the /ast igniter scheme, where an extremely intense (petawatt) laser pulse is fired into the pre-compressed plasma [9.308]. 

Altogether, we thus see a striking similarity, up to details, between the nuclear and the cluster responses for this clean Coulomb excitation mechanism with the prominent feature that the resonance modes turn out to represent robust, harmonic oscillations. It remains to be seen to what extent this behavior persists in other experimental situations. In the following we focus on heavy-ion collisions and laser-cluster interactions as complementary and widely studied excitation mechanisms. 

R. Gijbels continued to follow this double track theory and different practical applications by NAA in a variety of nuclear reactors. Modeling received again a boost with the arrival of a postdoc from the Hungarian Academy of Sciences, A. Vertes who started a 1-D model for laser-solid interaction. Another even more fruitful line of research started with the arrival, in 1986, of Jan M. L. Martin for his master thesis in Antwerp. R. Gijbels had seen a large variety of carbon cluster ions in spark source and laser induced mass spectra, and wondered what their structure could be. J. Martin performed quantum chemical calculations to model these clusters, in close collaboration with J.-P. Frangois, at the University of Hasselt. 

This chapter discusses the apphcation of femtosecond lasers to the study of the dynamics of molecular motion, and attempts to portray how a synergic combination of theory and experiment enables the interaction of matter with extremely short bursts of light, and the ultrafast processes that subsequently occur, to be understood in terms of fundamental quantum theory. This is illustrated through consideration of a hierarchy of laser-induced events in molecules in the gas phase and in clusters. A speculative conclusion forecasts developments in new laser techniques, highlighting how the exploitation of ever shorter laser pulses would permit the study and possible manipulation of the nuclear and electronic dynamics in molecules. 

A critical point in the retrieving of the number of nuclear reactions in laser-solid experiments is that there is no control on the spectrum of the electrons accelerated in the interaction, as well as the acceleration mechanism is uncertain and difficult to fit in a predictable scheme. In most cases, the electron energy distribution is assumed to be Boltzmann-like and deconvolutions are performed starting from this assumption. 

It should be noted that there is a limited number of works on classical relativistic dynamical chaos (Chernikov et.al., 1989 Drake and et.al., 1996 Matrasulov, 2001). However, the study of the relativistic systems is important both from fundamental as well as from practical viewpoints. Such systems as electrons accelerating in laser-plasma accelerators (Mora, 1993), heavy and superheavy atoms (Matrasulov, 2001) and many other systems in nuclear and particle physics are essentially relativistic systems which can exhibit chaotic dynamics and need to be treated by taking into account relativistic dynamics. Besides that interaction with magnetic field can also strengthen the role of the relativistic effects since the electron gains additional velocity in a magnetic field. 

With the availability of lasers, Brillouin scattering can now be used more confidently to study electron-phonon interactions and to probe the energy, damping and relative weight of the various hydro-dynamic collective modes in anharmonic insulating crystals.The connection between the intensity and spectral distribution of scattered light and the nuclear displacement-displacement correlation function has been extensively discussed by Griffin 236). 

The modification of the electronic potentials due to the interaction with the electric field of the laser pulse has another important aspect pertaining to molecules as the nuclear motion can be significantly altered in light-induced potentials. Experimental examples for modifying the course of reactions of neutral molecules after an initial excitation via altering the potential surfaces can be found in Refs 56, 57, where the amount of initial excitation on the molecular potential can be set via Rabi-type oscillations [58]. Nonresonant interaction with an excited vibrational wavepacket can in addition change the population of the vibrational states [59]. Note that this nonresonant Stark control acts on the timescale of the intensity envelope of an ultrashort laser pulse [60]. 

Of a special astronomical interest is the absorption due to pairs of H2 molecules which is an important opacity source in the atmospheres of various types of cool stars, such as late stars, low-mass stars, brown dwarfs, certain white dwarfs, population III stars, etc., and in the atmospheres of the outer planets. In short absorption of infrared or visible radiation by molecular complexes is important in dense, essentially neutral atmospheres composed of non-polar gases such as hydrogen. For a treatment of such atmospheres, the absorption of pairs like H-He, H2-He, H2-H2, etc., must be known. Furthermore, it has been pointed out that for technical applications, for example in gas-core nuclear rockets, a knowledge of induced spectra is required for estimates of heat transfer [307, 308]. The transport properties of gases at high temperatures depend on collisional induction. Collision-induced absorption may be an important loss mechanism in gas lasers. Non-linear interactions of a supermolecular nature become important at high laser powers, especially at high gas densities. 

We come now to the second study, described ten years later [23]. The main development was the employment of a tunable dye laser to pump the A <— X + transition. Rotational levels in the ground state with J = 1 to 29, in the v = 0 vibrational level, were pumped by the laser and radioffequency hyperfine transitions studied. The range of J levels studied meant that the effective Hamiltonian required the addition of terms describing the dipolar and scalar interactions between the 23Na nuclear spins. These terms were given earlier in our discussion of the D2 molecule, and the complete effective Hamiltonian is ... 

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