Test ion acceleration in a dynamic planar electron sheath

New exact results are obtained for relativistic acceleration of test positive ions in the laminar zone of a planar electron sheath evolving from an initially mono-energetic electron distribution. The electron dynamics is calculated against the background of motionless foil ions. The limiting gamma-factor γ_p∞ of accelerated ions is shown to be determined primarily by the values of the ion-electron charge-over-mass ratio μ=m_eZ_p/m_p and the initial gamma-factor γ₀ of the accelerated electrons. For μ> 1/8 a test ion always overtakes the electron front and attains γ_p∞> γ₀. For μ< 1/8 a test ion can catch up with the electron front only when γ₀ is above a certain critical value γ_cr, which for μ≪1 can most often be evaluated as . In this model the protons and heavier test ions, for which γ_cr> 10³⁹⁸ is enormous, always lag behind the front edge of the electron sheath and have γ_p∞< γ₀; for their maximum energy an appropriate intermediate asymptotic formula is derived. The domain of applicability of the laminar-zone results is analyzed in detail.     

Experimental studies of emission of highly charged Au-ions and of X-rays from the laser-produced plasma at high laser intensities

Au ions with maximum charge states 58+ and with the kinetic energy in a region of ∼100 MeV were recorded. The influence of various experimental conditions is presented and discussed.     

The first target experiments on the National Ignition Facility

A first set of shock timing, laser-plasma interaction, hohlraum energetics and hydrodynamic experiments have been performed using the first 4 beams of the National Ignition Facility (NIF), in support of indirect drive Inertial Confinement Fusion (ICF) and High Energy Density Physics (HEDP). In parallel, a robust set of optical and X-ray spectrometers, interferometer, calorimeters and imagers have been activated. The experiments have been undertaken with laser powers and energies of up to 8 TW and 17 kJ in flattop and shaped 1–9 ns pulses focused with various beam smoothing options. The experiments have demonstrated excellent agreement between measured and predicted laser-target coupling in foils and hohlraums, even when extended to a longer pulse regime unattainable at previous laser facilities, validated the predicted effects of beam smoothing on intense laser beam propagation in long scale-length plasmas and begun to test 3D codes by extending the study of laser driven hydrodynamic jets to 3D geometries.     

Laser-plasma interaction in the context of inertial fusion: experiments and modeling

Many nonlinear processes may affect the laser beam propagation and the laser energy deposition in the underdense plasma surrounding the pellet. These processes, associated with anomalous and nonlinear absorption mechanisms, are fundamental issues in the context of Inertial Confinement Fusion. The work presented in this article refers to laser-plasma interaction experiments which were conducted under well-controlled conditions, and to their theoretical and numerical modeling. Thanks to important diagnostics improvements, the plasma and laser parameters were sufficiently characterized in these experiments to make it possible to carry out numerical simulations modeling the laser plasma interaction in which the hydrodynamics conditions were very close to the experimental ones. Two sets of experiments were carried out with the LULI 2000 and the six beam LULI laser facilities. In the first series of experiments, the interaction between two single hot spots was studied as a function of their distance, intensity and light polarization. In the second series, the intensity distribution of stimulated Brillouin scattering (SBS) inside the plasma was studied by means of a new temporally resolved imaging system. Two-dimensional (2D) simulations were carried out with our code Harmony2D in order to model these experiments. For both series of experiments, the numerical results show a very good agreement with the experimental ones for what concerns the main SBS features, namely the spatial and temporal behavior of the SBS-driven acoustic waves, as well as the average SBS reflectivities. Thus, these well diagnosed experiments, carried out with well defined conditions, make it possible to benchmark our theoretical and numerical modelings and, hence, to improve our predictive capabilities for future experiments.     

Multistability in the laser cooling of atomic beams

Summary We describe the interaction of a beam of two-level atoms with multiple laser beams propagating in the opposite direction. The lasers are frequency-chirped in order to remain close to resonance and they continuously decelerate the atoms by radiation pressure. In a deterministic treatment, the average atomic velocity exhibits multistability cycles as a function of the chirping rate. A statistical (Fokker-Planck) treatment shows the corresponding splitting of the atomic velocity distribution into well-separate, quasi-monokinetic bunches of atoms. In the laboratory reference frame, multistability shows up through the increasingly decelerated motion of these atomic bunches. Analytical results are presented, as well as numerical simulations for a beam of cesium atoms, both in the laboratory frame and in the non-inertial frame. The conditions for the observation of multiple cooling with frequency-chirped lasers are discussed. A comparison with standard chirped cooling is outlined. Possible applications are indicated.     

Diagnostics of picosecond laser pulse absorption in preformed plasma

We present results where highly supersonic plasma jets and accelerated plasma fragments are generated by interaction of an intense picosecond laser pulse with a metallic target (Al, Cu, W, and Ta) in gas atmosphere. The formation of jets and well-localized massive plasma fragments occurs when a strong forward shock from a main laser pulse and a reverse shock from a pre-pulse meet to. Interferometric and shadow graphic measurements with high temporal (100 ps) and spatial (1 μm) resolution yield information about the formation and evolution of plasma jets and plasma fragments. The excitation of the electric and self-generated magnetic field by ponderomotive force during propagation of the laser pulse in a gas atmosphere was investigated as well. It had been shown previously that under certain conditions a hollow current channel can be generated in laser-produced plasma. The azimuthal magnetic field in such a micro-channel was determined by Faraday rotation of a probing laser beam to be 7.6 MGauss (MG). Ion acceleration in a pinched annular current channel up to 8 MeV analogous to micro-“plasma focus” conditions, may be realized at lengths of 100 μm. Self-generated magnetic fields of 4-7 MG have also been measured in thin skin layers in front of shock waves, where well-collimated plasma blocks were separated and accelerated away from the plasma body. The velocity of dense plasma blocks reaches values of order of 3 × 10⁸ cm/s and they are stable during acceleration and propagation in gas.     

Head-on collision of ion-acoustic solitary waves in an unmagnetized electron-positron-ion plasma

The head-on collision between two ion-acoustic solitary waves in an unmagnetized electron-positron-ion plasma has been investigated. By using the extended Poincaré-Lighthill-Kuo perturbation method, we obtain the KdV equation and the analytical phase shift after the head-on collision of two solitary waves in this three-component plasma. The effects of the ratio of electron temperature to positron temperature, and the ratio of the number density of positrons to that of electrons on the phase shift are studied. It is found that these parameters can significantly influence the phase shifts of the solitons. Moreover, the compressive solitary wave can propagate in this system.     

Kinetic energy oscillations in annular regions of ultracold neutral plasmas

A study of ion equilibration in annular regions of ultracold strontium plasmas is reported. Plasmas are formed by photoionizing laser-cooled atoms with a pulsed dye laser. The experimental probe is spatially-resolved absorption spectroscopy using the ²S_1/2-²P_1/2 transition of the Sr⁺ ion. The kinetic energy of the ions is calculated from the Doppler broadening of the spectrum, and it displays clear oscillations during the first microsecond after plasma formation. The oscillations, which are a characteristic of strong coulomb coupling, are fit with a simple phenomenological model incorporating damping and density variation in the plasma.     

Black holes under external influence

The work on black holes immersed in external fields is reviewed in both test-field approximation and within exact solutions. In particular we pay attention to the effect of the expulsion of the flux of external fields across charged and rotating black holes which are approaching extremal states. Recently this effect has been shown to occur for black hole solutions in string theory. We also discuss black holes surrounded by rings and disks and rotating black holes accelerated by strings. The content corresponds to the lecture given at ICGC 2000 in Kharagpur. Sections 2–6 are based on the text of the lecture on ‘Electromagnetic fields around black holes and Meissner effect’ given at the 3rd ICRA workshop in Pescara 1999 (to be published with T Ledvinka in Nuovo Cimento).     

The role of experimental science in ICF – examples from X-ray diagnostics and targets

The USA Inertial Confinement Fusion (ICF) Program evolved from the Nuclear Test Program which had restricted shot opportunities for experimentalists to develop sophisticated experimental techniques. In contrast the ICF program in the US was able to increase the shot availability on its large facilities, and develop sophisticated targets and diagnostics to measure and understand the properties of the high energy density plasmas (HEDP) formed. Illustrative aspects of this evolution at Lawrence Livermore National Laboratory (LLNL), with examples of the development of diagnostics and target fabrication are described.     

Microscopic calculation of the magnetic field of neutron stars

Based on the Dirac equation describing an electron moving in a uniform and cylindrically symmetric magnetic field which may be the result of the self-consistent mean field of the electrons themselves in a neutron star, we have obtained the eigen solutions and the orbital magnetic moments of electrons in which each eigen orbital can be calculated. From the eigen energy spectrum we find that the lowest energy level is the highly degenerate orbitals with the quantum numbers pZ=0, n=0, and m≥0. At the ground state, the electrons fill the lowest eigen states to form many Landau magnetic cells and each cell is a circular disk with the radius λfree and the thickness λe, where λfree is the electron mean free path determined by Coulomb cross section and electron density and λe is the electron Compton wavelength. The magnetic moment of each cell and the number of cells in the neutron star are calculated, from which the total magnetic moment and magnetic field of the neutron star can be calculated. The results are compared with the observational data and the agreement is reasonable.     

Fringe contrast in three grating Mach-Zehnder atomic interferometers

Several three-grating Mach-Zehnder atomic interferometers have been built and operated in recent years but no general theory of the contrast of the fringes produced by these apparatus is available. The purpose of this paper is to develop this theory, based on the Fresnel-Kirchoff approximate treatment of diffraction. Such a theory has been developed by Turchette et al. [JOSA B 9, 1601 (1992)] but because the necessary multiple integrals were evaluated in a purely numerical way, this treatment was not fully general. We show here how to reduce the computation by analytic means and we are thus able to calculate the contrast with a modest numerical effort. Moreover, we get a simple insight of the contrast reduction related to several defects of a real apparatus. We apply our calculations to existing interferometers as well as to an apparatus working with lithium which is under construction in our laboratory. Received: 24 April 1998 / Revised: 25 October 1998 / Accepted: 11 December 1998     

Microscopic calculation of the magnetic field of neutron stars

Based on the Dirac equation describing an electron moving in a uniform and cylindrically symmetric magnetic field which may be the result of the self-consistent mean field of the electrons themselves in a neutron star, we have obtained the eigen solutions and the orbital magnetic moments of electrons in which each eigen orbital can be calculated. From the eigen energy spectrum we find that the lowest energy level is the highly degenerate orbitals with the quantum numbers pz = 0, n = 0, and m ≥0. At the ground state, the electrons fill the lowest eigen states to form many Landau magnetic cells and each cell is a circular disk with the radius λfree and the thickness λe, where λfree is the electron mean free path determined by Coulomb cross section and electron density and λe is the electron Compton wavelength. The magnetic moment of each cell and the number of cells in the neutron star are calculated, from which the total magnetic moment and magnetic field of the neutron star can be calculated. The results are compared with the observational data and the agreement is reasonable.     

Accumulation of chromium metastable atoms into an optical trap

We report the fast accumulation of a large number of metastable ⁵²Cr atoms in a mixed trap, formed by the superposition of a strongly confining optical trap and a quadrupolar magnetic trap. The steady state is reached after about 400 ms, providing a cloud of more than one million metastable atoms at a temperature of about 100 μK, with a peak density of 10¹⁸ atoms m^-3. We have optimized the loading procedure, and measured the light shift of the ⁵D₄ state by analyzing how the trapped atoms respond to a parametric excitation. We compare this result to a theoretical evaluation based on the available spectroscopic data for chromium atoms.     

Cooling mechanisms in potassium magneto-optical traps

We report on a theoretical and experimental investigation of ³⁹K magneto-optical trapping. The small hyperfine splitting characterizing the upper level of the cooling transition affects the cooling mechanism. In order to model the atom-laser interaction, the whole level structure of the D₂ line has to be taken into account. Two different regimes have been recognized, one optimizing the loading of the trap, the second minimizing the temperature of the atoms. We investigated these two regimes experimentally and found results in agreement with the theoretical predictions. Received: 6 March 1998 / Received in final form: 13 May 1998 / Accepted: 13 May 1998     

Channeling chaotic transport in a wave-particle experiment

A numerical and experimental study of a control method aimed at channeling chaos by building barriers in phase space is performed on a paradigm for wave-particle interaction, i.e., a traveling wave tube. Control of chaotic diffusion is achieved by adding small apt modifications to the system with a low additional cost of energy. This modification is realized experimentally through additional waves with small amplitudes. Robustness of the method is investigated both numerically and experimentally. An erratum to this article is available at .     

A focussing funnel for metastable helium

We have constructed a magneto-optical funnel for He atoms and studied its properties using a laser cooled, highly mono-energetic atomic beam. A simple model of its action allows us to quantitatively understand the observed spot size and “focal length”. We show that for a fast beam, the velocity damping coefficient plays an important role in determining the focal length of the device. The observed spot size is limited mainly by transverse heating processes which impose a transverse velocity spread. The device also permits easy scanning of the focussed spot. Received 30 October 1998 and Received in final form 27 January 1999     

Particle accelerator physics and technology for high energy density physics research

Interaction phenomena of intense ion- and laser radiation with matter have a large range of application in different fields of science, extending from basic research of plasma properties to applications in energy science, especially in inertial fusion. The heavy ion synchrotron at GSI now routinely delivers intense uranium beams that deposit about 1 kJ/g of specific energy in solid matter, e.g. solid lead. Our simulations show that the new accelerator complex FAIR (Facility for Antiproton and Ion Research) at GSI as well as beams from the CERN large hadron collider (LHC) will vastly extend the accessible parameter range for high energy density states. A natural example of hot dense plasma is provided by our neighbouring star the sun, and allows a deep insight into the physics of fusion, the properties of matter at high energy density, and is moreover an excellent laboratory for astroparticle physics. As such the sun's interior plasma can even be used to probe the existence of novel particles and dark matter candidates. We present an overview on recent results and developments of dense plasma physics addressed with heavy ion and laser beams combined with accelerator- and nuclear physics technology.     

Electronic correlation effects on the properties of an half-filled octahedron cluster in a magnetic field

The present study focuses on electronic correlation effects on magnetic energy, the spin-spin correlation function of an octahedron cluster in the (3↑, 3 ↓) electronic configuration threaded by a magnetic field. Some other spin configurations are also discussed and various field directions are considered. An accurate diagonalisation technique has been used to solve the Hubbard Hamiltonian. A result is analysed on a linear energy stabilisation at low magnetic flux. Moreover, two types of antiferromagnetic transition versus the flux occurring for a correlation term larger than a critical one have been observed, i.e. the likelihood of a charge excitation before the antiferromagnetic transition. Finally, a comparison between the results obtained from the exact diagonalisation and the Gutzwiller method has been carried out, leading to a suggested modification of the Gutzwiller approach in order to improve it. Received 23 June 1999 and Received in final form 28 July 2000