Spatial evolution of a q-Gaussian laser beam in relativistic plasma

In a recent experimental study, the beam intensity profile of the Vulcan petawatt laser beam was measured; it was found that only 20% of the energy was contained within the full width at half maximum of 6.9 mu m and 50% within 16 mu m, suggesting a long-tailed non-Gaussian transverse beam profile. A q-Gaussian distribution function was suggested therein to reproduce this behavior. The spatial beam profile dynamics of a q-Gaussian laser beam propagating in relativistic plasma is investigated in this article. A non-paraxial theory is employed, taking into account nonlinearity via the relativistic decrease of the plasma frequency. We have studied analytically and numerically the dynamics of a relativistically guided beam and its dependence on the q-parameter. Numerical simulation results are shown to trace the dependence of the focusing length on the q-Gaussian profile.

Spatially Resolved Measurements of Laser Filamentation in Long Scale Length Underdense Plasmas with and without Beam Smoothing

The onset of filamentation, following the interaction of a relatively long (tau(L) similar or equal to 1 ns) and intense (I-L similar or equal to 5 x 10(14) W/cm(2)) laser pulse with a neopentane filled gas bag target, has been experimentally studied via the proton radiography technique, in conditions of direct relevance to the indirect drive inertial confinement fusion scheme. The density gradients associated with filamentation onset have been spatially resolved yielding direct and unambiguous evidence of filament formation and quantitative information about the filamentation mechanism in agreement with previous theoretical modelings. Experimental data confirm that, once spatially smoothed laser beams are used, filamentation is not a relevant phenomenon during the heating laser beams propagation through typical hohlraum gas fills.

Electron-beam treatment of poly(lactic acid) to control degradation profiles

Bioresorbable polymers such as polylactide (PIA) and polylactide-co-glycolide (PLGA) have been used successfully as biomaterials in a wide range of medical applications. However, their slow degradation rates and propensity to lose strength before mass have caused problems. A central challenge for the development of these materials is the assurance of consistent and predictable in vivo degradation. Previous work has illustrated the potential to influence polymer degradation using electron beam (e-beam) radiation. The work addressed in this paper investigates further the utilisation of e-beam radiation in order to achieve a more surface specific effect. Variation of e-beam energy was studied as a means to control the effective penetrative depth in poly-L-lactide (PLEA). PLEA samples were exposed to e-beam radiation at individual energies of 0.5 MeV, 0.75 MeV and 1.5 MeV. The near-surface region of the PLEA samples was shown to be affected by e-beam irradiation with induced changes in molecular weight, morphology, flexural strength and degradation profile. Moreover, the depth to which the physical properties of the polymer were affected is dependent on the beam energy used. Computer modelling of the transmission of each e-beam energy level used corresponded well with these findings. (C) 2010 Elsevier Ltd. All rights reserved.

Experimental setup and first measurement of DNA damage induced along and around an antiproton beam

Radiotherapy employs ionizing radiation to induce lethal DNA lesions in cancer cells while minimizing damage to healthy tissues. Due to their pattern of energy deposition, better therapeutic outcomes can, in theory, be achieved with ions compared to photons. Antiprotons have been proposed to offer a further enhancement due to their annihilation at the end of the path. The work presented here aimed to establish and validate an experimental procedure for the quantification of plasmid and genomic DNA damage resulting from antiproton exposure. Immunocytochemistry was used to assess DNA damage in directly and indirectly exposed human fibroblasts irradiated in both plateau and Bragg peak regions of a 126 MeV antiproton beam at CERN. Cells were stained post irradiation with an anti-gamma-H2AX antibody. Quantification of the gamma-H2AX foci-dose relationship is consistent with a linear increase in the Bragg peak region. A qualitative analysis of the foci detected in the Bragg peak and plateau region indicates significant differences highlighting the different severity of DNA lesions produced along the particle path. Irradiation of desalted plasmid DNA with 5 Gy antiprotons at the Bragg peak resulted in a significant portion of linear plasmid in the resultant solution.

On the investigation of fast electron beam filamentation in laser-irradiated solid targets using multi-MeV proton emission

The transverse filamentation of beams of fast electrons transported in solid targets irradiated by ultraintense (5 x 10(20) W cm(-2)), picosecond laser pulses is investigated experimentally. Filamentation is diagnosed by measuring the uniformity of a beam of multi-MeV protons accelerated by the sheath field formed by the arrival of the fast electrons at the rear of the target, and is investigated for metallic and insulator targets ranging in thickness from 50 to 1200 mu m. By developing an analytical model, the effects of lateral expansion of electron beam filaments in the sheath during the proton acceleration process is shown to account for measured increases in proton beam nonuniformity with target thickness for the insulating targets.

Frequency selective surface using nested split ring slot elements as a lens with mechanically reconfigurable beam steering capability

The design is described of a double layer frequency selective surface which can produce a differential phase shift of 180 ° as the wave propagates through it at normal incidence. The hand of an applied circularly polarized signal is reversed due to the 180° phase shift, and it is demonstrated that the exit circularly polarized output signal can be phase advanced or phase retarded by 180 ° upon rotation of the elements comprising the structure. This feature allows the surface to act as a spatial phase shifter. In this paper the beam steering capabilities of a 10 × 10 array of such elements are demonstrated. Here the individual elements comprising the array are rotated relative to each other in order to generate a progressive phase shift. At normal incidence the 3 dB Axial Ratio Bandwidth for LHCP to RHCP conversion is 5.3% and the insertion loss was found to be -2.3 dB, with minimum axial ratio of 0.05 dB. This array is shown to be able to steer a beam from -40 ° to +40 ° while holding axial ratio at the pointing angle to below 4 dB. The measured radiation patterns match the theoretical calculation and full-wave simulation results. © 2010 IEEE.

A preliminary structural design procedure for laser beam welded airframe stiffened panels

Initial sizing procedures for aircraft stiffened panels that include the influence of welding fabrication residual process effects are missing. Herein, experimental and Finite Element analyses are coupled to generate knowledge to formulate an accurate and computationally efficient sizing procedure which will enable designers to routinely consider panel fabrication, via welding, accounting for the complex distortions and stresses induced by this manufacturing process. Validating experimental results demonstrate the need to consider welding induced material property degradation, residual stresses and distortions, as these can reduce static strength performance. However, results from fuselage and wing trade-studies, using the validated sizing procedure, establish that these potential reductions in strength performance may be overcome through local geometric tailoring during initial sizing, negating any weight penalty for the majority of design scenarios.

Use of a random phase plate as a KrF laser beam homogenizer for thin film deposition applications

Fabrication of devices based on thin film structures deposited using the pulsed laser deposition technique relies on reproducibility and control of deposition rates over substrate areas as large as possible. Here we present an application of the random phase plate technique to smooth and homogenize the intensity distribution of a KrF laser footprint on the surface of a target which is to be ablated. It is demonstrated that intensity distributions over millimeter-sized spots on the target can be made insensitive to the typical changes that occur in the near-field intensity distribution of the ultraviolet output from a KrF laser. (C) 1999 American Institute of Physics. [S0034-6748(99)02504-6].

A saturated X-ray laser beam at 7 nanometers

A saturated nickel-like samarium x-ray laser beam at 7 nanometers has been demonstrated with an output energy of 0.3 millijoule in 50-picosecond pulses, demonstrating that saturated operation of a laser at wavelengths shorter than 10 nanometers can be achieved. The narrow divergence, short wavelength, short pulse duration, high efficiency, and high brightness of this samarium laser make it an ideal candidate for many x-ray laser applications.

Study of beam aberrations in a germanium XXIII XUV laser amplifier

A beam of amplified spontaneous emission at 23.2/23.6 nm from a GeXXIII XUV laser has been injected into a separate amplifier plasma and the astigmatic aberrations introduced by plasma density gradients in the amplifier have been estimated from analysis of images of the amplified beam.