Angular distribution for the elastic scattering of electrons from Ar+(3s^2 3p^5 ^2P) above the first inelastic threshold

The measured angular differential cross section (DCS) for the elastic scattering of electrons from Ar+(3s2 3p5 2P) at the collision energy of 16 eV is presented. By solving the Hartree-Fock equations, we calculate the corresponding theoretical DCS including the coupling between the orbital angular momenta and spin of the incident electron and those of the target ion and also relaxation effects. Since the collision energy is above one inelastic threshold for the transition 3s2 3p5 2P–3s 3p6 2S, we consider the effects on the DCS of inelastic absorption processes and elastic resonances. The measurements deviate significantly from the Rutherford cross section over the full angular range observed, especially in the region of a deep minimum centered at approximately 75°. Our theory and an uncoupled, unrelaxed method using a local, spherically symmetric potential by Manson [Phys. Rev. 182, 97 (1969)] both reproduce the overall shape of the measured DCS, although the coupled Hartree-Fock approach describes the depth of the minimum more accurately. The minimum is shallower in the present theory owing to our lower average value for the d-wave non-Coulomb phase shift s2, which is due to the high sensitivity of s2 to the different scattering potentials used in the two models. The present measurements and calculations therefore show the importance of including coupling and relaxation effects when accurately modeling electron-ion collisions. The phase shifts obtained by fitting to the measurements are compared with the values of Manson and the present method.

Angular distributions of electrons in photoionization of 3s subshell of atomic chlorine

R-matrix calculated photoelectron angular distribution asymmetry parameters, beta for Cl+ 3s3p(5) P-3(o) and 3s(2)3p(3) (D-2(o))3d P-1(o) final ionic states in photoionization of the ground state of atomic Cl are presented in the photon energy range from threshold to 80 eV. The results, characterized by prominent autoionization structures which are sensitive to multielectron correlations, are compared with those recently measured by Whitfield et al (Whitfield S B, Kehoe K, Krause M 0 and Caldwell C D 2000 Phys. Rev. Lett. 84 4818). Contrary to experiment and previous theoretical calculations, our detailed CIV3 structure calculation (Deb N C, Crothers D S F, Felfli Z and Msezane A Z 2002 J. Phys. B: At. Mol. Opt. Phys. submitted) has identified the lowest P-1(o) level of Cl+ as 3S(2)3p(3)(D-2(o))3d P-1(o) rather than 3s3p(5) P-1(o). The implications and consequences of the measured data for the 3s P-1(o) level are also discussed in the context of our calculated energies for Cl+ and beta for 3d P-1(o).

3D Simulations of Surface Harmonic Generation with few-cycle laser pulses

The mechanism of harmonic generation in the interaction of short laser pulses with solid targets holds the promise for the production of intense attosecond pulses. Using the three dimensional code ILLUMINATION we have performed simulations pertaining to an experimentally realizable parameter range by high power laser systems to become available in the near future. The emphasis of the investigation is on the coherent nature of the emission. We studied the influence of the plasma scale length on the harmonic efficiency, angular distribution and the focusability using a post processing scheme in which the far-field of the emission is calculated. It is found that the presence of an extended density profile reduces significantly the transverse coherence length of the emitted XUV light. The different stages of the interaction for two particular cases can be followed with the help of movies.

Intense-field dissociation dynamics of D2+ molecular ions using ultrafast laser pulses

The dynamics of dissociation of pre-ionized D2+ molecules using intense (10^12–10^15 W cm-2), ultrashort (50 fs), infrared (? = 790 nm) laser pulses are examined. Use of an intensity selective scan technique has allowed the deuterium energy spectrum to be measured over a broad range of intensity. It is found that the dominant emission shifts to lower energies as intensity is increased, in good agreement with corresponding wavepacket simulations. The results are consistent with an interpretation in terms of bond softening, which at high intensity (approximately >3 × 10^14 W cm-2) becomes dominated by dissociative ionization. Angular distribution measurements reveal the presence of slow molecular dissociation, an indication that vibrational trapping mechanisms occur in this molecule.

Differential cross section measurements for elastic scattering of electrons from Ar2+ and Xe2+

A crossed-beams energy-loss spectrometer has been used to investigate angular distributions for electron scattering from Ar2+ and Xe2+ ions, at a collision energy of 16 eV. For Ar2+ the measurements are compared with the predictions of a partial waves calculation based on a semi-empirical potential, where it is shown that the interference term governs the position of the observed minimum in the angular distribution.

LASING PROPERTIES OF THE J = 0-1 AND THE J = 2-1 LINES OF A NEON-LIKE GERMANIUM SOFT-X-RAY LASER

Lasing properties of a collisional-excitation Ne-like Ge soft-x-ray laser have been studied with exploding-foil, single-slab, and double-slab targets under identical pumping conditions. Experimental results for the angular intensity distributions and the temporal variations of the lasing intensities are examined with a hydrodynamic code and ray-trace calculations. The observed angular distribution are well reproduced by these analyses, and it is found that the effective gain regions are located on the high-density side of the expected gain regions. It is shown that the observed lasing intensity of the J = 0 to J = 1 line is strongly correlated with the temporal change of the calculated electron temperature for both the slab and the exploding-foil targets.

Characterization of Li-7(p,n)Be-7 neutron yields from laser produced ion beams for fast neutron radiography

Investigations of Li-7(p,n)Be-7 reactions using Cu and CH primary and LiF secondary targets were performed using the VULCAN laser [C.N. Danson , J. Mod. Opt. 45, 1653 (1997)] with intensities up to 3x10(19) W cm(-2). The neutron yield was measured using CR-39 plastic track detector and the yield was up to 3x10(8) sr(-1) for CH primary targets and up to 2x10(8) sr(-1) for Cu primary targets. The angular distribution of neutrons was measured at various angles and revealed a relatively anisotropic neutron distribution over 180degrees that was greater than the error of measurement. It may be possible to exploit such reactions on high repetition, table-top lasers for neutron radiography. (C) 2004 American Institute of Physics.

Characterization of a gamma-ray source based on a laser-plasma accelerator with applications to radiography

The application of high intensity laser-produced gamma rays is discussed with regard to picosecond resolution deep-penetration radiography. The spectrum and angular distribution of these gamma rays is measured using an array of thermoluminescent detectors for both an underdense (gas) target and an overdense (solid) target. It is found that the use of an underdense target in a laser plasma accelerator configuration produces a much more intense and directional source. The peak dose is also increased significantly. Radiography is demonstrated in these experiments and the source size is also estimated. (C) 2002 American Institute of Physics.

LINEAR ACCELERATION EMISSION IN PULSAR MAGNETOSPHERES

Linear acceleration emission occurs when a charged particle is accelerated parallel to its velocity. We evaluate the spectral and angular distribution of this radiation for several special cases, including constant acceleration (hyperbolic motion) of finite duration. Based on these results, we find the following general properties of the emission from an electron in a linear accelerator that can be characterized by an electric field E acting over a distance L: (1) the spectrum extends to a cutoff frequency (h) over bar omega(c)/mc(2) approximate to L(E/E(Schw))(2)/(lambda) over bar (C), where E(Schw) = 1.3 x 10(18) V m(-1) is the Schwinger critical field and (lambda) over bar (C) = (h) over bar /mc = 3.86 x 10(-13) m is the Compton wavelength of the electron, (2) the total energy emitted by a particle traversing the accelerator is 4/3 alpha(f)(h) over bar omega(c) in accordance with the standard Larmor formula where alpha(f) is the fine-structure constant, and (3) the low frequency spectrum is flat for hyperbolic trajectories, but in general depends on the details of the accelerator. We also show that linear acceleration emission complements curvature radiation in the strongly magnetized pair formation regions in pulsar magnetospheres. It dominates when the length L of the accelerator is less than the formation length rho/gamma of curvature photons, where rho is the radius of curvature of the magnetic field lines and gamma the Lorentz factor of the emitting particle. In standard static models of pair creating regions linear acceleration emission is negligible, but it is important in more realistic dynamical models in which the accelerating field fluctuates on a short length scale.

ELIMED:A new hadron therapy concept based on laser driven ion beams

Laser accelerated proton beams have been proposed to be used in different research fields. A great interest has risen for the potential replacement of conventional accelerating machines with laser-based accelerators, and in particular for the development of new concepts of more compact and cheaper hadrontherapy centers. In this context the ELIMED (ELI MEDical applications) research project has been launched by INFN-LNS and ASCR-FZU researchers within the pan-European ELI-Beamlines facility framework. The ELIMED project aims to demonstrate the potential clinical applicability of optically accelerated proton beams and to realize a laser-accelerated ion transport beamline for multi-disciplinary user applications. In this framework the eye melanoma, as for instance the uveal melanoma normally treated with 62 MeV proton beams produced by standard accelerators, will be considered as a model system to demonstrate the potential clinical use of laser-driven protons in hadrontherapy, especially because of the limited constraints in terms of proton energy and irradiation geometry for this particular tumour treatment. Several challenges, starting from laser-target interaction and beam transport development up to dosimetry and radiobiology, need to be overcome in order to reach the ELIMED final goals. A crucial role will be played by the final design and realization of a transport beamline capable to provide ion beams with proper characteristics in terms of energy spectrum and angular distribution which will allow performing dosimetric tests and biological cell irradiation. A first prototype of the transport beamline has been already designed and other transport elements are under construction in order to perform a first experimental test with the TARANIS laser system by the end of 2013. A wide international collaboration among specialists of different disciplines like Physics, Biology, Chemistry, Medicine and medical doctors coming from Europe, Japan, and the US is growing up around the ELIMED project with the aim to work on the conceptual design, technical and experimental realization of this core beamline of the ELI Beamlines facility. © 2013 SPIE.

High order harmonics from relativistic electron spikes

A new regime of relativistic high-order harmonic generation has been discovered (Pirozhkov 2012 Phys. Rev. Lett. 108 135004). Multi-terawatt relativistic-irradiance (>1018 W cm−2) femtosecond (~30–50 fs) lasers focused to underdense (few × 1019 cm−3) plasma formed in gas jet targets produce comb-like spectra with hundreds of even and odd harmonic orders reaching the photon energy of 360 eV, including the 'water window' spectral range. Harmonics are generated either by linearly or circularly polarized pulses from the J-KAREN (KPSI, JAEA) and Astra Gemini (CLF, RAL, UK) lasers. The photon number scalability has been demonstrated with a 120 TW laser, producing 40 μJ sr−1 per harmonic at 120 eV. The experimental results are explained using particle-in-cell simulations and catastrophe theory. A new mechanism of harmonic generation by sharp, structurally stable, oscillating electron spikes at the joint of the boundaries of the wake and bow waves excited by a laser pulse is introduced. In this paper, detailed descriptions of the experiments, simulations and model are provided and new features are shown, including data obtained with a two-channel spectrograph, harmonic generation by circularly polarized laser pulses and angular distribution.

Gamma-ray emission in near critical density plasmas at laser intensities of 1021 W/cm2

We study synchrotron radiation emission from laser interaction with near critical density (NCD) plasmas at intensities of 1021 W∕cm2 using three-dimensional particle-in-cell simulations. It is found that the electron dynamics depend on the laser shaping process in NCD plasmas, and thus the angular distribution of the emitted photons changes as the laser pulse evolves in space and time. The final properties of the resulting synchrotron radiation, such as its overall energy, the critical photon energy, and the radiation angular distribution, are strongly affected by the laser polarization and plasma density. By using a 420 TW∕50 fs laser pulse at the optimal plasma density (∼1nc ), about 108 photons/0.1% bandwidth are produced at multi-MeV photon energies, providing a route to ultraintense, femtosecond gamma ray pulses.

Vacuum birefringence in strong inhomogeneous electromagnetic fields

Birefringence is one of the fascinating properties of the vacuum of quantum electrodynamics (QED) in strong electromagnetic fields. The scattering of linearly polarized incident probe photons into a perpendicularly polarized mode provides a distinct signature of the optical activity of the quantum vacuum and thus offers an excellent opportunity for a precision test of nonlinear QED. Precision tests require accurate predictions and thus a theoretical framework that is capable of taking the detailed experimental geometry into account. We derive analytical solutions for vacuum birefringence which include the spatio-temporal field structure of a strong optical pump laser field and an x-ray probe. We show that the angular distribution of the scattered photons depends strongly on the interaction geometry and find that scattering of the perpendicularly polarized scattered photons out of the cone of the incident probe x-ray beam is the key to making the phenomenon experimentally accessible with the current generation of FEL/high-field laser facilities.

Relativistic radiatively damped R -matrix calculations of the electron-impact excitation of helium-like iron and krypton

Electron-impact excitation data for He-like ions are of significant importance for diagnostic applications to both laboratory and astrophysical plasmas. Here we report on the first fully relativistic R -matrix calculations with radiation damping for the He-like ions Fe 24+ and Kr 34+ . Effective collision strengths for these two ions have been determined with and without damping over a wide temperature range for all transitions between the 49 levels through n = 5. We find that damping has a pronounced effect on the effective collision strengths for excitation to some of the low-lying levels, but its effect on excitation to the vast majority of levels is small. At the energy of a resonance peak, we also investigate the effect of radiation damping on the angular distribution of scattered electrons. Finally, we compare our results for Fe 24+ with an earlier intermediate coupling frame transformation R -matrix calculation with radiation damping by Whiteford et al ( J. Phys. B: At. Mol. Opt. Phys. 34 3179) and find good agreement, especially for excitation to the lower levels.

The Vertical Distribution and Evolution of Slam Pressure on an Oscillating Wave Surge Converter

The accurate definition of the extreme wave loads which act on offshore structures represents a significant challenge for design engineers and even with decades of empirical data to base designs upon there are still failures attributed to wave loading. The environmental conditions which cause these loads are infrequent and highly non-linear which means that they are not well understood or simple to describe. If the structure is large enough to affect the incident wave significantly further non-linear effects can influence the loading. Moreover if the structure is floating and excited by the wave field then its responses, which are also likely to be highly non-linear, must be included in the analysis. This makes the description of the loading on such a structure difficult to determine and the design codes will often suggest employing various tools including small scale experiments, numerical and analytical methods, as well as empirical data if available.
Wave Energy Converters (WECs) are a new class of offshore structure which pose new design challenges, lacking the design codes and empirical data found in other industries. These machines are located in highly exposed and energetic sites, designed to be excited by the waves and will be expected to withstand extreme conditions over their 25 year design life. One such WEC is being developed by Aquamarine Power Ltd and is called Oyster. Oyster is a buoyant flap which is hinged close to the seabed, in water depths of 10 to 15m, piercing the water surface. The flap is driven back and forth by the action of the waves and this mechanical energy is then converted to electricity.
It has been identified in previous experiments that Oyster is not only subject to wave impacts but it occasionally slams into the water surface with high angular velocity. This slamming effect has been identified as an extreme load case and work is ongoing to describe it in terms of the pressure exerted on the outer skin and the transfer of this short duration impulsive load through various parts of the structure.
This paper describes a series of 40th scale experiments undertaken to investigate the pressure on the face of the flap during the slamming event. A vertical array of pressure sensors are used to measure the pressure exerted on the flap. Characteristics of the slam pressure such as the rise time, magnitude, spatial distribution and temporal evolution are revealed. Similarities are drawn between this slamming phenomenon and the classical water entry problems, such as ship hull slamming. With this similitude identified, common analytical tools are used to predict the slam pressure which is compared to that measured in the experiment.