Dmitri Kaganovich

Shaping gas jet plasma density profile by laser generated shock waves

The Gaussian plasma density profile from a simple cylindrical nozzle jet was modified using laser generated shock wave. This modification provided great variety of density profiles suitable for different applications. The Gaussian plasma density distribution was modified into fast-rise slow-fall profile with adjustable gradients, almost flat-top profiles, and profiles with variable lengths. Position of the shock wave center and time delay were the major parameters used for shaping the density profiles. Other easily adjustable parameters such as shock wave energy and backing pressure provided linear scaling of the modified plasma densities.

Measurements of intense femtosecond laser pulse propagation in air

The nonlinear self-focusing of an intense femtosecond pulse propagating in air can be balanced by the plasma defocusing as the laser intensity is increased above the threshold for multiphoton ionization. The resultant laser∕plasma filament can extend many meters, suitable for many applications such as remote atmospheric breakdown, laser induced electrical discharge, and femtosecond laser material interactions. Direct (bore-sight) measurements of filament size and fluence over 4 m showed a preservation of the total energy in the filament during propagation. This indicates the energy lost in creating the central plasma column through multiphoton ionization was continuously being replenished from the surrounding radiation. Electrical measurement of the filament conductivity estimated the plasma density to be 1×1016cm−3 and electrical discharges triggered by a femtosecond laser filament were found to occur at substantially reduced breakdown fields.

Formation and propagation of meter-scale laser filaments in water

We report the demonstration, characterization, and modeling of meter-scale underwater optical filaments using a nanosecond pulsed laser. We observed single filament formation for P/PCRIT = 1–5, where PCRIT ∼ 1 MW in water. We employed a variable distance water tube to characterize laser pulse evolution and filament formation. Filaments with uniform radius 50 ± 10 μm persisted for 55 cm (>35 Rayleigh lengths). Significant forward Stimulated Raman Scattering (up to 60%) was observed and characterized. Simulation results for propagation distances and radii were in agreement with experiment, and predict a structured plasma with peak density of 1.5 × 1018 cm−3.

First demonstration of a staged all-optical laser wakefield acceleration

A proof-of-principle experiment on staged all-optical laser wakefield acceleration was performed at the Naval Research Laboratory. Electrons with <1MeV energy created by the interaction of a 2TW laser beam with a nitrogen gas jet were injected and accelerated to more than 20MeV in a plasma wakefield generated by a 10TW laser beam in a helium gas jet. The energy gain occurred in a narrow time window of 3ps between the injection and acceleration laser beams, and within a tight spatial alignment of ∼10μm.

Longitudinal profiles of plasma parameters in a laser-ignited capillary discharge and implications for laser wakefield accelerator applications

The evolution of longitudinal electron density and temperature profiles in plasma channel produced by a low-current Plexiglas capillary discharge with laser ignition was investigated by spectroscopic methods. The plasma was produced by an electric discharge using a 0.5 mm diameter, 15 mm long Plexiglas capillary. The electron density measured in near-outlet region was found to be lower by 30%. Simulations show that this variation of the plasma density near the entrance of the capillary can pose substantial difficulties for external injection of electrons for laser wakefield accelerator applications.

High intensity focusing of laser pulses using a short plasma channel lens

Plasma channels have been used to guide intense laser pulses over distances of many Rayleigh lengths. This paper investigates the possibility of using a short plasma channel to provide focusing or control of the spot size of a laser pulse at intensities far above the usual damage limits of conventional optical elements. Analytical models for the focal length and focused spot size of a single plasma channel lens and a nonconverging laser pulse are presented, and results are compared with the two-dimensional simulation code LEM [J. Krall et al., Phys. Rev. E 48, 2157 (1993)]. Several advanced thin lens configurations, including multiple lens transport systems, and both focusing and defocusing lenses for externally focused converging laser pulses are also analyzed. Experimental techniques for producing appropriate plasma profiles are reviewed, and evidence for plasma channel focusing in a capillary discharge guiding experiment is analyzed. Thick “overmoded” lenses offer a possible alternative if there are ex...

Trapping and acceleration of nonideal injected electron bunches in laser wakefield accelerators

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.

Origin and control of the subpicosecond pedestal in femtosecond laser systems

The picosecond time scale pedestal of a multiterawatt femtosecond laser pulse is investigated experimentally and analytically. The origin of the pedestal is related to the finite bandwidth of the laser system. By deliberately introducing a modulated spectrum with minima that match this limited bandwidth, the pedestal can be reduced, with no deleterious effect on the main pulse. Using this technique, we experimentally demonstrate a subpicosecond scale order of magnitude enhancement of contrast ratio while preserving the energy in the main pulse.

Electron density in low density capillary plasma channel

A low density plasma channel of 1017cm−3 is created in a capillary discharge. The plasma channel is characterized through the hydrogen plasma spectra in the Balmer Hα line. The measured plasma density profiles were found to be in qualitative agreement with the hydrodynamic simulations. Guiding over many vacuum diffraction length using a plasma channel operating in the low density regime is demonstrated.

Long plasma channels in segmented capillary discharges

Guided propagation of ultrashort (100fs) high intensity (1016Wcm−2) laser pulses over distances up to 12.6cm using optimized segmented capillary is reported. A new diagnostic technique is presented in which the transport of a guided laser pulse at different delay times from the initiation of the discharge is sampled on a single discharge shot. The current waveform was optimized to obtain a long lasting, deep radial profile. Radial profiles with the maximal electron density from 4×1017 to 2×1018cm−3 and up to 25% deep were obtained thereby, whereas longitudinal profiles were found to be remarkably uniform. The potential application of these long channels to the laser wake field accelerator is discussed.