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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.

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.