A self-modulated laser wakefield accelerator (SM-LWFA) experiment was performed at the Naval Research Laboratory. Large amplitude plasma wakefields produced by a sub-picosecond, high intensity laser pulse (7×1018 W/cm2) in an underdense plasma (ne≈1019 cm−3) were measured with a pump–probe coherent Thomson scattering (CTS) technique to last for less than 5 ps, consistent with the decay of large amplitude plasma waves due to the modulational instability. A plasma channel was observed to form in the wake of the pump laser pulse, and its evolution was measured with the pump–probe CTS diagnostic. The trailing probe laser pulse was observed to be guided by this channel for about 20 Rayleigh lengths. High energy electrons (up to 30 MeV) have been measured using an electro-magnetic spectrometer, with the energy spectra and divergence of lower energy (up to 4 MeV) electrons obtained using photographic films. Highly nonlinear plasma waves were also detected using forward Raman scattering diagnostics and were observed to correlate with the electron signals. Simulations of self-trapping of plasma electrons from the interaction of the laser wakefield with the slow plasma wave generated by Raman backscattering are also presented.

No abstract

No abstract

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.

A laser synchrotron source (LSS) [P. Sprangle, A. Ting, E. Esarey, and A. Fisher, J. Appl. Phys. 72, 5032 (1992)] was proposed to generate short-pulsed, tunable x rays by Thomson scattering of laser photons from a relativistic electron beam. A proof-of-principal (p.o.p.) experiment on this LSS configuration is performed. An intense laser pulse (λ0=1.053 μm) is Thomson backscattered from a focused relativistic electron beam. Time integrated x-ray signals from a photocathode/electron multiplier, at an electron beam energy of 650 keV and an x-ray photon energy of 20 eV, indicate an increase in the x-ray signals above the baseline by an amount comparable to the theoretically predicted value.