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We report on proton acceleration from intense CO2 laser-irradiated hydrogen plasmas at near-critical densities, with the density gradient steepened by Nd:YAG laser ablation-driven hydrodynamic shocks. While the experimental results, such as the quasi-monoenergetic proton spectra and their scaling with respect to the laser energy, are generally in agreement with the simulations, certain laser shots produced significantly higher proton energies than anticipated during the experiment. The increased proton energy may be linked to nonlinear propagation effects in the steepened plasma density ramp before the critical surface, including relativistic self-focusing and, for the case of temporally-structured laser pulses observed in the experiment, focusing of the trailing pulse through the plasma channel formed by the leading pulse 25 ps ahead. The occurrence of channel focusing in the underdense hydrogen plasma is supported by a subsequent pump-probe experiment with a dark-field imaging technique, where the formation of ion channels was observed after the passage of an intense CO2 laser pulse. [ABSTRACT FROM AUTHOR]

Experiments are performed to characterize the electrical properties of plasma filaments that are generated by self-guided femtosecond laser pulses propagating in air. A single plasma filament passes through a high-voltage sphere pulsed at--100 kV to a grounded electrode, which serves as a current monitor. The experiments utilize moderate electric fields to probe the filament conductivity, thereby avoiding the strong perturbations caused by electric discharges. The measured filament current decreases as ~1/[R.sup.2] as the separation R between the electrodes is increased up to 1.5 m. The pulselength of the filament current signal is 2 ns (full-width at half-maximum), but the time resolution is limited by the bandwidth of the oscilloscope. The typical plasma density in the conducting filament is 9 x [10.sup.15] [cm.sup.-3], which is inferred from the conductivity measurements and the size of the optical filaments. Comparisons are made with mobility values derived from electron swarm data, where the mobility depends upon the applied electric field. The conductivity of the filament is measured as the laser pulselength is varied from 50 fs to 1.5 ps. We find that relatively long laser pulses (1 ps) produce filaments with the largest conductivity. A model is used to predict the longitudinal position where the plasma filament forms and is in reasonably good agreement with measurements. Index Terms--Air propagation, filament conductivity, filamentation, laser--plasma interaction, ultrashort pulses.

Imaging of femtosecond laser filaments is accomplished by utilizing a recently developed diagnostic which is not damaged by the filaments and which greatly reduces the nonlinearities in the detection system. This calibrated detection system allows quantities such as the filament energy and fluence distribution to be determined with greater detail accuracy, and confidence than was previously possible. The diagnostic is placed on a rail so that filament images can be obtained at a large number of positions along the propagation path. lt is found that filaments formed from 450-fs pulses carry higher fluence and propagate farther than filaments formed from 50-fs pulses. Index Terms--Air propagation, filamentation, laser-plasma interaction, ultrashort pulses.

High-energy electrons were generated by the synchronized interaction of a 2-TW laser beam with a nitrogen gas jet and a 10-TW laser beam with helium gas jet. The jets centers are separated by 0.5 mm distance and the propagation of the lasers are collinear. Plasma electrons were trapped and accelerated by the wakefields generated in the gas jet plasmas to above 20 MeV. Index Terms--Electron accelerators, lasers, plasmas.

Most conceptual designs for future laser wakefield accelerators (LWFA) require external injection of precisely-phased, monoenergetic, ultrashort bunches of MeV electrons. This paper reports simulation and Hamiltonian models of several nonideal injection schemes that demonstrate strong phase bunching and good accelerated beam quality in a channel-guided LWFA. For the case of monoenergetic, unphased (long bunch) injection, there is an optimum range of injection energies for which the LWFA can trap a significant fraction of the injected pulse while producing an ultrashort, high-quality accelerated pulse. These favorable results are due to a combination of pruning of particles at unfavorable phases, rapid acceleration, and strong phase bunching. Also, the plasma channel introduces a favorable shift in the region of accelerating phase where electrons are focused, which can significantly reduce the required injection energy. Simulation results agree well with the predictions of the Hamiltonian model. Simulations of phased injection with a broad injected energy spread also exhibit final accelerated bunches with small energy spread. These results suggest that relatively poor quality injection pulses may still be useful in LWFA demonstration experiments. Index Terms--Accelerators, laser applications, optical propagation in plasma media, pulsed lasers.

High-power femtosecond laser pulses propagating in air form self-guided filaments that can persist for many meters. Characterizing these filaments has always been challenging owing to their high intensity. An apparently novel diagnostic is used to directly measure the fluence distribution of femtosecond laser pulses after they have formed self-guided optical filaments in air. The diagnostic is unique in that the information contained in the filaments is not lost owing to the interaction with the apparatus. This allows filament characteristics such as energy and size to be unambiguously determined for the first time. OCIS codes: 120.1880, 190.3270, 190.4420, 320.2250, 320.7110.

The original implementation of the ponderomotive guiding center algorithm is not suitable for modeling the propagation of short laser pulses over long distances. An improved algorithm is given, which improves the accuracy of the calculation, allowing much longer propagation distances to be modeled. Index Terms--Particle code, plasma simulation.

A novel particle simulation code is described that self-consistently models certain classes of laser--plasma interactions without resolving the optical cycles of the laser. This is accomplished by separating the electromagnetic field into a laser component and a wake component. Although the wake component is treated as in a fully explicit particle-in-cell (PIC) code, the laser component is treated in the high-frequency limit, which allows the optical cycles to be averaged out. This leads to enormous reductions in computer time when the laser frequency is much greater than all other frequencies of interest. This work is an extension of the work of Mora and Antonsen, Jr. [1], [2], who derived the time-averaged equations coupling the laser with the particles and developed a code to solve these equations in the quasi-static limit. The code presented here is distinguished by the fact that it is useful when the plasma length is much less than the laser pulse length. Also, it is already parallelized and should be straightforward to extend to three dimensions. Index Terms--Particle code, plasma simulation.

High-intensity laser-plasma interaction experiments were performed using high-power laser pulses (greater than 8 TW) at a wavelength of 527 nm. These pulses were focused to intensity greater than [10.sup.18] W/[cm.sup.2] into underdense helium plasma at a density of [n.sub.e] [is less than or equal to] 5 x [10.sup.19] [cm.sup.-3]. Systematic examination of the forward Raman scattering instability was performed by measuring the spectrum of the forward scattered light. It was found that the three-wave instability occurs first, which then proceeds to a nonlinear regime involving four-wave processes. Measurements of the accelerated electron spectrum suggest that higher phase velocity plasma waves are generated than when using lower frequency light. Index Terms--Electron accelerator, electromagnetic scattering by plasma media, plasma waves.

The interaction of an intense short pulse laser ([is greater than] 5 x [10.sup.18] W[cm.sup.-2]) with underdense plasma was extensively studied. The beam is found to be highly susceptible to the forward Raman scattering instability. At sufficiently high growth rates, this can lead to wavebreaking with the resultant production of a high flux of accelerated electrons ([is greater than] [10.sup.11] for E [is greater than] 2 MeV). Some electrons are found to be accelerated well above the dephasing energy, up to 94 MeV. Self-scattered images intimate the presence of high-intensity channels that extend more than 3.5 mm or 12 Rayleigh lengths. These filaments do not follow the axis of laser propagation, but are seen to be emitted within an f4 cone centered around this axis. Spectra of the self-scattered light show that the main contribution of the scattering is not from light captured within these filaments. But there is evidence for self-phase modulation from effects such as ionization and relativistic self-focusing. However, no clear correlation is observed between channel length and the number or energies of accelerated electrons. Evidence for high intensities within the channels is given by small-angle Thomson scattering of the plasma wave generated therein. With this method, the intensity is found to be of the order of [10.sup.18] W[cm.sup.-2] greater than 12 Rayleigh lengths from focus. Index Terms--Electromagnetic propagation in plasma media, electromagnetic scattering electron accelerators, plasma waves.

Enhanced proton acceleration to high energy by relatively modest ultrashort laser pulses and structured dynamic plasma snow targets was demonstrated experimentally. High proton yield emitted to narrow solid angle with energies of up 25 MeV were detected from interaction of a 5 TW laser with snow targets. The high yield was attributed to a carefully planned prepulse and microstructured snow targets. We studied experimentally the minimal energy requirements for the adequate prepulse and we are using PIC simulations to study the dynamics of acceleration process. Based on our simulations, we predict that using the proposed scheme protons can be accelerated to energies above 150 MeV by 100 TW laser systems. [ABSTRACT FROM AUTHOR]