This paper discusses the momentum coupling coefficient in the relativistic, collisionless realm as contrasted with the collisional, ablation dominated regime. It is shown the total momentum coupling coefficient is a result of the combined ion and photon momenta.
Interaction of TW-ps laser with plasma results in a skin layer mechanism for nonlinear (ponderomotive) force driven two dimensional plasma blocks (pistons) if a very high contrast ratio is provided for suppression of relativistic self-focusing. This Skin layer acceleration (SLA) [1] results in space charge neutral plasma blocks with ion current densities larger than 1010 Amp/cm2 [1-3]. Using Ions in the MeV range results in 1000 times higher proton or DT current densities [3] than the proton fast igniter [4] is using and may result in better conditions of this fast ignitor scheme. Using ballistic focusing of the generated plasma blocks and a short time thermal expansion of these blocks for increasing their thickness while keeping the high ion current densities, results in conditions favourable for this option of fast ignition of a fusion target. Some details of the interaction processes are still to be analysed but the solutions studies to date are most encouraging.
It is being clarified why the observations of plane wave geometry interaction within the skin depth of a laser irradiated target are very unique exceptions from the broad stream of the usual experiments of laser plasma interaction. This permits a much more simplified description by plane wave interaction theory for laser pulses of about ps or shorter duration and powers above TW and simplifies computations in contrast to the usual cases with relativistic self-focusing. After establishing theoretically and experimentally the generation of highly directed plasma blocks with ion current densities above 1010 A/cm2 moving against the laser light or into the target, applications for laser fusion, and a completely new improvement of ion sources for the next generation of accelerators are discussed.
The studies of laser ablation have lead to a new theory of nuclei, endothermic nuclei generation and quark-gluon plasmas. The surface of ablated plasma expanding into vacuum after high power laser irradiation of targets, contains an electric double layer having the thickness of the Debye length. This led to the discovery of surface tension of plasmas and to the internal dynamic electric fields in all inhomogeneous plasmas. The surface causes stabilization by short length surface waves smoothing the expanding plasma plume. Generalizing this to the degenerate electrons in a metal with the Fermi energy instead of the temperature, resulted in the surface tension of metals in agreement with measurements. Taking then the Fermi energy in the Debye length for nucleons results in a theory of nuclei with stable confinement of protons and neutrons just at the well known nuclear density, and in the Debye length equal to Hofstadter's decay of the nuclear surface. Increasing the nuclear density by a factor of 6 leads to the change of the Fermi energy into its relativistic branch where no surface energy is possible and the particle mass is not defined, permitting the quark-gluon plasma. Expansion of this higher density at the big band or in a supernova results in nucleation and element generation. The Boltzmann equilibrium permits the synthesis of nuclei even in the endothermic range limited to about uranium.
Energy requirements for near-Earth object (NEO) threat mitigation are addressed. Due to a large NEO mass range and the possibility of limited available time for orbit change, energy requirements can be large (~petajoules). To quantitatively determine energy requirements for a given momentum change use is made of the momentum coupling coefficient, CM, that depends on the type of energy transfer, energy density, interaction time scale, and target material properties. Inhomogeneous and flawed NEO materials are empirically confronted through experimental methodologies that generate parameters on high energy density mechanical and radiative induced momentum coupling to (primarily) meteorite targets serving as near-Earth asteroid surrogates. Interactants include high speed (> 8 km/s) mechanical projectiles, high intensity (~ GW/cm2) infra-red laser radiation, and (~200 GW/cm2) soft X-ray radiation. Issues exists and are addressed regarding momentum coupling scaling from coupon sized (~mm2) to much larger (~m2) targets. Applications include high energy planetary and astrophysical momentum coupling dynamics.
Plasma emission or ablation from laser-irradiated targets shows very complicated properties. One novelty was observed at irradiation of neodymium glass laser pulses of ps duration and TW power if there was a very strong suppression of prepulses by a contrast ratio of about 108 until 100 ps before the main pulse arrived. The emitted ion maximum energy was more than 50 times below the values observed in all the comparable numerous experiments. The other anomaly is that the number of the fast ions did not change when the laser intensity varied by a factor 30. This permitted a separation of the usual effects of self-focusing and permitted an analysis fully based on simplified plane geometry as a skin layer interaction mechanism. The consequence is that plasma blocks are accelerated by the nonlinear (ponderomotive) force with ion current densities above 1010 A/cm2. This provides basically new aspects for laser fusion using uncompressed solid DT fuel and a new kind of x-ray laser process may be possible.
Experiments on pure and inhomogeneous materials (meteorite samples) have been performed at the Sandiad National Laboratory (SNL) using a hohlraum source of soft X-rays. It is of interest to deduce the scaling of the ablation pressure with the radiation temperature in this class of experiment. This paper uses similarity relations deduced from a radiation diffusion model together with several theoretical expressions for the Rosseland mean free path of the target material. The resulting scaling laws are compared. The momentum coupling coefficient scaling with input radiation temperature is also given. As an example of the methodology, scaling results are given for the types of meteorites used in these recent experiments as well as for a comet model based on opacities obtained from the LANL TOPS code. This work is part of an on-going program to model the dymanic properties and interactions of Near Earth Object (NEO) materials in the context of NEO hazard mitigation.
When a single-pulse high-power laser irradiates a surface at atmospheric pressure, a laser supported detonation (LSD) wave can form above the target surface. The high-pressure gas behind the LSD wave transfers momentum to the target. The laser target coupling is substantially reduced in vacuum, the coupling coefficient typically being an order of magnitude less than that when an atmosphere is present. Another pressure enhancement technique is to confine the laser-target interface. Confinement or 'tamping' also can substantially increase the momentum coupling to the target. Experiments tend to differ from one another based on the target size (thickness) and confinement geometry. This work describes and compares some experimental results for metallic targets irradiated by 1054 nm radiation in the GW/cm2 range and interprets them in terms of simple models. As will be discussed in this paper, such models predict a weak sensitivity to target materials but results are likely to be different for inhomogeneous materials as has been seen in recent experiments on iron-nickel and stony meteorites.
This paper analyzes laser induced impulse and associated scaling laws appropriate to the intensity range of interest for laser space debris clearing (106 - 1010 W/cm2). A simple radiative fluence model is used augmented by empirical values from some recent experiments.
The hydrodynamic and implosion efficiencies, defined respectively as the ratios of the thermal energy in the fuel and the kinetic energy in the shell to the absorbed laser energy, have been estimated in a series of experiments using = 0.26 im laser illumination of D-T filled glass microballons.
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