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Collimated jets of beryllium, carbon, oxygen, fluorine, and palladium ions with >1 MeV/nucleon energies are observed from the rear surface of thin foils irradiated with laser intensities of up to 5×1019 W/cm2. The normally dominant proton acceleration is suppressed when the target is subjected to Joule heating to remove hydrogen-bearing contaminant. This inhibits screening effects and permits effective energy transfer to and acceleration of heavier ion species. The influence of remnant protons on the spectral shape of the next highest charge-to-mass ratio species is shown. Particle-in-cell simulations confirming the experimental findings are presented. [ABSTRACT FROM AUTHOR]

In experiments, the feasibility was demonstrated of generating multi-MeV electrons in a form of a collimated beam utilizing a table-top laser system delivering 200 fs pulses with P[sub L] = 1.2 TW and 10 Hz capability. The method uses the process of relativistic self-channeling in a high-density gas jet producing electron densities in the range of 3 x 10[sup 19] - 6 x 10[sup 20] cm]sup -3]. In a thorough investigation, angularly resolved and absolutely calibrated electron spectra were measured and their dependence on the plasma density, laser intensity, and gas medium was studied. For the optimum electron density of n[sub e] = 2 x 10[sup 20] cm[sup -3] the effective temperature of the electron energy distribution and the channel length exhibit a maximum of 5 MeV and 400 μm respectively. The laser-energyto-MeV-electron efficiency is estimated to be 5%. In a second step, utilizing the multi-MeV electron beam anti-particles, namely positrons, were successfully generated in a 2 mm Pb converter. The average intensity of this new source of positrons is estimated to be equivalent to a radioactivity of 2 x 10[sup 8] Bq and it exhibits a very favorable scaling for higher laser intensities. [ABSTRACT FROM AUTHOR]

In high-power photodissociation lasers such as the iodine laser, spatially inhomogeneous pump light deposition causes temperature and pressure gradients. These gradients set the gaseous laser medium into motion, thereby changing its density and thus also its refractive index. These perturbations may seriously degrade the laser beam quality. A pulse propagation model is presented, which for the first time incorporates the gasdynamic disturbances in an appropriate manner. Thus any deterioration of the beam quality resulting from gasdynamic effects and diffraction can be precisely evaluated. The model is shown to compare well with experiment. It is also suited for predicting the performance of high-power amplifier chains. [ABSTRACT FROM AUTHOR]

Stimulated Brillouin scattering in the highly transient regime is experimentally investigated using a multipulse reflection scheme. In high pressure N2 and SF6 backward pulses with reflection coefficients approaching 1 are demonstrated employing 1 ns pump pulses at 1.32 μm. The experimentally observed reflection behavior is satisfactorily reproduced by a numerical solution of the equations describing the scattering process. The phase conjugated fractions in the backscattered light is large. [ABSTRACT FROM AUTHOR]

Utilizing a femtosecond table-top laser system, we have succeeded in converting via electron acceleration in a plasma channel, low-energy photons into antiparticles, namely positrons. The average intensity of this source of positrons is estimated to be equivalent to 2x10[sup 8] Bq and it exhibits a very favorable scaling for higher laser intensities. The advent of positron production utilizing femtosecond laser pulses may be the forerunner to a table-top positron source appropriate for applications in material science, and fundamental physics research like positronium spectroscopy. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have developed a magnetic spectrometer to characterize the hot electrons generated in high-intensity (>10[sup 18] W/cm[sup 2]) laser-plasma interaction experiments. It comprises a dispersive element consisting of a permanent dipole magnet and an electron detector incorporating 45 scintillating/light-guiding plastic fibers connected to a cooled charged-coupled device camera. The main features of this instrument are high spectral resolution, low sensitivity to x and γ rays, and versatility due to its compact design. Performance and operational capabilities are illustrated based on experimental results in which electron energy-spectra in the range of 520 keV to 12.6 MeV were obtained with an energy resolution of 10% and a detection threshold of 10[sup 6] electrons per MeV. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have investigated the MeV bremsstrahlung which is emitted when fast electrons generated by the interaction of 200 mJ, 130 fs Ti:sapphire laser pulses with a preformed plasma penetrate into a solid target. Employing different targets the dependence of the γ-ray spectrum on the atomic number was studied. We detected single γ photons with an energy up to 2.5 MeV and found a maximum conversion efficiency of the laser energy into MeV bremsstrahlung of 4×10[sup -6]. Data analysis using a Monte Carlo code revealed a fast-electron temperature of 0.9 MeV.© 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]