Relativistic laser interactions with preformed plasma channels and gamma-ray measurements

The propagation of a 1-ps laser pulse at intensities exceeding 10(19) Wcm(-2) in a low-density plasma channel was experimentally tested. The channel was produced by a lower intensity preceding pulse of the same duration. Plasma electrons were accelerated during the propagation of the main pulse, and high energy gamma -ray detectors were used to detect their bremsstrahlung emission. The gamma -ray yield was studied for different channel conditions, by varying the delay between the channel forming pulse and the high intensity pulse. These results are correlated with the interferograms of the propagation region into the plasma.

Plasma ion evolution in the wake of a high-intensity ultrashort laser pulse

Experimental investigations of the late-time ion structures formed in the wake of an ultrashort, intense laser pulse propagating in a tenuous plasma have been performed using the proton imaging technique. The pattern found in the wake of the laser pulse shows unexpectedly regular modulations inside a long, finite width channel. On the basis of extensive particle in cell simulations of the plasma evolution in the wake of the pulse, we interpret this pattern as due to ion modulations developed during a two-stream instability excited by the return electric current generated by the wakefield.

Absolute atomic oxygen density profiles in the discharge core of a microscale atmospheric pressure plasma jet

The micro atmospheric pressure plasma jet is an rf driven (13.56 MHz, ~20 W) capacitively coupled discharge producing a homogeneous plasma at ambient pressure when fed with a gas flow of helium (1.4 slm) containing small admixtures of oxygen (~0.5%). The design provides excellent optical access to the plasma core. Ground state atomic oxygen densities up to 3x1016 cm-3 are measured spatially resolved in the discharge core by absolutely calibrated two-photon absorption laser-induced fluorescence spectroscopy. The atomic oxygen density builds up over the first 8 mm of the discharge channel before saturating at a maximum level. The absolute value increases linearly with applied power.

Dynamics of charge-displacement channeling in intense laser-plasma interactions

The dynamics of transient electric fields generated by the interaction of high intensity laser pulses with underdense plasmas has been studied experimentally with the proton projection imaging technique. The formation of a charged channel, the propagation of its front edge and the late electric field evolution have been characterized with high temporal and spatial resolution. Particle-in-cell simulations and an electrostatic, ponderomotive model reproduce the experimental features and trace them back to the ponderomotive expulsion of electrons and the subsequent ion acceleration.

Relativistic channeling of a picosecond laser pulse in a near-critical preformed plasma

Relativistic self-channeling of a picosecond laser pulse in a preformed plasma near critical density has been observed both experimentally and in 3D particle-in-cell simulations. Optical probing measurements indicate the formation of a single pulsating propagation channel, typically of about 5 mu m in diameter. The computational results reveal the importance in the channel formation of relativistic electrons traveling with the light pulse and of the corresponding self-generated magnetic field.

Microscopic Examination of Biofilm Destruction by Atmospheric Pressure Plasma

Objective: The aim of this study is to examine microscopically the destruction of bacterial biofilms mediated by atmospheric pressure non-thermal plasma (APNTP) at cellular level as well as at the level of biofilm structure as a whole. Methods: 3-day old bacterial biofilms were grown on polycarbonate coupons in a dual channel flow cell and were treated with an in-housed designed atmospheric pressure non-thermal plasma jet for up to 4 minutes of exposure before being examined by both confocal laser scanning microscopy (CLSM), preceded by Live/Dead bacterial viability staining, and scanning electron microscopy (SEM). Results: Differential live/dead staining followed by confocal microscopy examination revealed that biofilm eradication by APNTP was mediated by varying levels of both cell killing and physical removal. Relative extent of each mechanism was dependent on plasma operating conditions, bacterial species, growth conditions and biofilm thickness. On the other hand, SEM examination of plasma-exposed biofilms revealed a series of morphological changes exhibited by biofilm cells ranging from increased roughness of cell surface to complete cell lysis. Conclusions: Interesting mechanistic insights have been revealed by microscopic examination of plasma-treated bacterial biofilms that, when coupled with more specific biochemical studies, will not only contribute significantly to our understanding of the mechanism of plasma mediated biofilm destruction but also will help in better application-guided development of this novel anti-biofilm approach.

Magnetic field generation during intense laser channelling in underdense plasma

Channel formation during the propagation of a high-energy (120 J) and long duration (30 ps) laser pulse through an underdense deuterium plasma has been spatially and temporally resolved via means of a proton imaging technique, with intrinsic resolutions of a few micron and a few ps, respectively. Conclusive proof is provided that strong azimuthally symmetric magnetic fields with a strength of around 0.5 MG are created inside the channel, consistent with the generation of a collimated beam of
relativistic electrons. The inferred electron beam characteristics may have implications for the cone-free fast-ignition scheme of inertial confinement fusion