Laser-Driven Proton Beams: Acceleration Mechanism, Beam Optimization, and Radiographic Applications

This paper reviews recent experimental activity in the area of optimization, control, and application of laser accelerated proton beams, carried out at the Rutherford Appleton Laboratory and the Laboratoire pour l’Utilisation des Lasers Intenses 100 TW facility in France. In particular, experiments have investigated the role of the scale length at the rear of the plasma in reducing target-normal-sheath-acceleration acceleration efficiency. Results match with recent theoretical predictions and provide information in view of the feasibility of proton fast-ignition applications. Experiments aiming to control the divergence of the proton beams have investigated the use of a laser-triggered microlens, which employs laser-driven transient electric fields in cylindrical geometry, enabling to focus the emitted
protons and select monochromatic beam lets out of the broad spectrum beam. This approach could be advantageous in view
of a variety of applications. The use of laser-driven protons as a particle probe for transient field detection has been developed and
applied to a number of experimental conditions. Recent work in this area has focused on the detection of large-scale self-generated magnetic fields in laser-produced plasmas and the investigation of fields associated to the propagation of relativistic electron both on the surface and in the bulk of targets irradiated by high-power laser pulses.

SLA-driven flexible bandwidth reservation negotiation schemes for QoS aware IP networks

We present a generic Service Level Agreement (SLA)-driven service provisioning architecture, which enables dynamic and flexible bandwidth reservation schemes on a per-user or a per-application basis. Various session level SLA negotiation schemes involving bandwidth allocation, service start time and service duration parameters are introduced and analysed. The results show that these negotiation schemes can be utilised for the benefits of both end user and network provide such as getting the highest individual SLA optimisation in terms of Quality of Service (QoS) and price. A prototype based on an industrial agent platform has also been built to demonstrate the negotiation scenario and this is presented and discussed.

Seasonal patterns in activity and habitat use by bats (Pipistrellus spp. and Nyctalus leisleri) in Northern Ireland, determined using a driven transect

The seasonal activity of Leisler's bat Nyctalus leisleri and pipistrelle bats Pipistrellus spp. with respect to minimum bat numbers and habitat use were investigated in County Down, Northern Ireland using a driven transect from April 1998 to October 1998. Data were collected in lowland farmland near Belfast, Northern Ireland using two BatBox III bat detectors tuned to detect both species and species groups simultaneously. The number of bats/km increased during April, May and June, peaking in July and tailed off after this period. The main peak in July is assumed to reflect the occurrence of newly volant young. An increase in the number of pipistrelle social calls during August and September probably represented mating activity. Bat activity correlated with temperature in both N. leisleri and Pipistrellus spp., although bat numbers were independent of temperature after the middle of June. There was significant variation in habitat use by pipistrelle bats along roads over the study period. Pipistrelle bats were observed in greater numbers in areas of tree-line, cut hedge (

Ultrafast Laser-Driven Excitation Dynamics in Ne: An Ab Initio Time-Dependent R-Matrix Approach

An attosecond pump-probe scheme that combines the use of a free-electron laser pulse with an ultrashort pulse is applied in order to explore the ultrafast excitation dynamics in Ne. We describe the multielectron dynamics using a new nonperturbative time-dependent R-matrix theory. This theory enables the interaction of ultrashort light fields with multielectron atoms and atomic ions to be determined from first principles. By probing the emission of an inner 2s electron from Ne we are also able to study the bound state population dynamics during the free-electron laser pulse.

Current-driven atomic waterwheels

A current induces forces on atoms inside the conductor that carries it. It is now possible to compute these forces from scratch, and to perform dynamical simulations of the atomic motion under current. One reason for this interest is that current can be a destructive force—it can cause atoms to migrate, resulting in damage and in the eventual failure of the conductor. But one can also ask, can current be made to do useful work on atoms? In particular, can an atomic-scale motor be driven by electrical current as it can be by other mechanisms. For this to be possible, the current-induced forces on a suitable rotor must be non-conservative, so that net work can be done per revolution. Here we show that current-induced forces in atomic wires are not conservative and that they can be used, in principle, to drive an atomic-scale waterwheel.

The dynamics of radio-frequency driven atmospheric pressure plasma jets

The complex dynamics of radio-frequency driven atmospheric pressure plasma jets is investigated using various optical diagnostic techniques and numerical simulations. Absolute number densities of ground state atomic oxygen radicals in the plasma effluent are measured by two-photon absorption laser induced fluorescence spectroscopy (TALIF). Spatial profiles are compared with (vacuum) ultra-violet radiation from excited states of atomic oxygen and molecular oxygen, respectively. The excitation and ionization dynamics in the plasma core are dominated by electron impact and observed by space and phase resolved optical emission spectroscopy (PROES). The electron dynamics is governed through the motion of the plasma boundary sheaths in front of the electrodes as illustrated in numerical simulations using a hybrid code based on fluid equations and kinetic treatment of electrons.

Ultrafast Probing of Collective Electron Dynamics Driven by Dielectronic Repulsion

We use the time-dependent R-matrix approach to investigate an ultrashort pump-probe scheme to observe collective electron dynamics in C(+). The ionization probability of a coherent superposition of the 2s2p(2) (2)D and (2)S states shows rapid modulation due to collective dynamics of the two equivalent 2p electrons, with the modulation frequency linked to the dielectronic repulsion. The best insight into this collective dynamics is achieved by a transformation from LS symmetry to the uncoupled basis. Such dynamics may be important in high-harmonic generation using open-shell atoms and ions.

Laser-Driven Ultrafast Field Propagation on Solid Surfaces

The interaction of a 3x10(19) W/cm(2) laser pulse with a metallic wire has been investigated using proton radiography. The pulse is observed to drive the propagation of a highly transient field along the wire at the speed of light. Within a temporal window of 20 ps, the current driven by this field rises to its peak magnitude similar to 10(4) A before decaying to below measurable levels. Supported by particle-in-cell simulation results and simple theoretical reasoning, the transient field measured is interpreted as a charge-neutralizing disturbance propagated away from the interaction region as a result of the permanent loss of a small fraction of the laser-accelerated hot electron population to vacuum.

One-dimensional particle simulation of the filamentation instability: Electrostatic field driven by the magnetic pressure gradient force

Two counterpropagating cool and equally dense electron beams are modeled with particle-in-cell simulations. The electron beam filamentation instability is examined in one spatial dimension, which is an approximation for a quasiplanar filament boundary. It is confirmed that the force on the electrons imposed by the electrostatic field, which develops during the nonlinear stage of the instability, oscillates around a mean value that equals the magnetic pressure gradient force. The forces acting on the electrons due to the electrostatic and the magnetic field have a similar strength. The electrostatic field reduces the confining force close to the stable equilibrium of each filament and increases it farther away, limiting the peak density. The confining time-averaged total potential permits an overlap of current filaments with an opposite flow direction.