Self-guided wakefield experiments driven by petawatt class ultra-short laser pulses

We investigate the extension of self-injecting laser wakefield experiments to the regime that will be accessible with the next generation of petawatt class ultra-short pulse laser systems. Using linear scalings, current experimental trends and numerical simulations we determine the optimal laser and target parameters, i.e. focusing geometry, plasma density and target length, that are required to increase the electron beam energy (to > 1 GeV) without the use of external guiding structures.Comment: 15 pages, 8 figure

Laser-wakefield accelerators as hard x-ray sources for 3D medical imaging of human bone

A bright μm-sized source of hard synchrotron x-rays (critical energy Ecrit > 30 keV) based on the betatron oscillations of laser wakefield accelerated electrons has been developed. The potential of this source for medical imaging was demonstrated by performing micro-computed tomography of a human femoral trabecular bone sample, allowing full 3D reconstruction to a resolution below 50 μm. The use of a 1 cm long wakefield accelerator means that the length of the beamline (excluding the laser) is dominated by the x-ray imaging distances rather than the electron acceleration distances. The source possesses high peak brightness, which allows each image to be recorded with a single exposure and reduces the time required for a full tomographic scan. These properties make this an interesting laboratory source for many tomographic imaging applications

Influence of realistic parameters on state-of-the-art LWFA experiments

We examine the influence of non-ideal plasma-density and non-Gaussian transverse laser-intensity profiles in the laser wakefield accelerator analytically and numerically. We find that the characteristic amplitude and scale length of longitudinal density fluctuations impacts on the final energies achieved by electron bunches. Conditions that minimize the role of the longitudinal plasma density fluctuations are found. The influence of higher order Laguerre-Gaussian laser pulses is also investigated. We find that higher order laser modes typically lead to lower energy gains. Certain combinations of higher order modes may, however, lead to higher electron energy gains.Comment: 16 pages, 6 figures; Accepted for publication in Plasma Physics and Controlled Fusio

Electron trapping and acceleration on a downward density ramp: a two-stage approach

In a recent experiment at Lawrence Berkeley National Laboratory (Geddes et al 2008 Phys. Rev. Lett. 100 215004), electron bunches with about 1MeV mean energy and small absolute energy spread (about 0.3MeV) were produced by plasma wave breaking on a downward density ramp. It was then speculated that such a bunch might be accelerated further in a plasma of low constant density, while mostly preserving its small absolute energy spread. This would then lead to a bunch with a high mean energy and very low relative energy spread. In this paper, trapping of a low-energy, low-spread electron bunch on a downward density ramp, followed by acceleration in a constant-density plasma, has been explored through particle-in-cell simulations. It has been found that the scheme works best when it is used as a separate injection stage for a laserwakefield accelerator, where the injection and acceleration stages are separated by a vacuum gap

Longitudinal Ion Acceleration from High-Intensity Laser Interactions with Underdense Plasma

Longitudinal ion acceleration from high-intensity (I ~ 10^20 Wcm^-2) laser interactions with helium gas jet targets (n_e ~ 0.04 n_c) have been observed. The ion beam has a maximum energy for He^2+ of approximately 40 MeV and was directional along the laser propagation path, with the highest energy ions being collimated to a cone of less than 10 degrees. 2D particle-in-cell simulations have been used to investigate the acceleration mechanism. The time varying magnetic field associated with the fast electron current provides a contribution to the accelerating electric field as well as providing a collimating field for the ions. A strong correlation between the plasma density and the ion acceleration was found. A short plasma scale-length at the vacuum interface was observed to be beneficial for the maximum ion energies, but the collimation appears to be improved with longer scale-lengths due to enhanced magnetic fields in the ramp acceleration region.Comment: 18 pages, 6 figure

Optical and photoelectronic properties of a new material:Optoelectronic application

With the aim of studying the optical, electrochemical, and electronic properties of a new porphyrin-based material, we have synthesized a new porphyrinic complex, namely the (4,4$^{\prime}$-bipyridine)(meso-tetratrifluoromethylphenylporphyrinato)zinc(II) 4,4$^{\prime}$-bipyridine disolvate dihydrate complex with the formula [Zn(TFMPP)(4,4$^{\prime}$-bipy)]${\cdot }$2(4,4$^{\prime}$-bipy)${\cdot }$2H2O (I). This species is characterized by single-crystal X-ray molecular structure. The optical study is performed by UV–visible absorption and fluorescence spectroscopy. The fluorescence intensity presents an emission in the UV–visible range, indicating that this compound can be used as an optoelectronic material. The optical energy gap is 1.95 eV, and the current–voltage characteristics and impedance spectroscopy measurements have been studied to define the electronic properties of the zinc (II) porphyrin complex. The barrier height ${\phi }_{\mathrm{b}}$ is calculated, and the space-charge limited current mechanism is found to control the conductance. The results from the electronic study confirm that our porphyrin derivative can be used for various optoelectronic applications