Spectral Modification of Shock Accelerated Ions Using a Hydrodynamically Shaped Gas Target.

We report on reproducible shock acceleration from irradiation of a λ=10 µm CO_{2} laser on optically shaped H_{2} and He gas targets. A low energy laser prepulse (I≲10^{14} W cm^{-2}) is used to drive a blast wave inside the gas target, creating a steepened, variable density gradient. This is followed, after 25 ns, by a high intensity laser pulse (I>10^{16} W cm^{-2}) that produces an electrostatic collisionless shock. Upstream ions are accelerated for a narrow range of prepulse energies. For long density gradients (≳40 µm), broadband beams of He^{+} and H^{+} are routinely produced, while for shorter gradients (≲20 µm), quasimonoenergetic acceleration of protons is observed. These measurements indicate that the properties of the accelerating shock and the resultant ion energy distribution, in particular the production of narrow energy spread beams, is highly dependent on the plasma density profile. These findings are corroborated by 2D particle-in-cell simulations.

MeV-energy x rays from inverse compton scattering with laser-wakefield accelerated electrons.

We report the generation of MeV x rays using an undulator and accelerator that are both driven by the same 100-terawatt laser system. The laser pulse driving the accelerator and the scattering laser pulse are independently optimized to generate a high energy electron beam (>200 MeV) and maximize the output x-ray brightness. The total x-ray photon number was measured to be ∼1×10(7), the source size was 5 µm, and the beam divergence angle was ∼10 mrad. The x-ray photon energy, peaked at 1 MeV (reaching up to 4 MeV), exceeds the thresholds of fundamental nuclear processes (e.g., pair production and photodisintegration).

Large-angle electron diffraction structure in laser-induced rescattering from rare gases.

We have measured full momentum images of electrons rescattered from Xe, Kr, and Ar following the liberation of the electrons from these atoms by short, intense laser pulses. At high momenta the spectra show angular structure (diffraction) which is very target dependent and in good agreement with calculated differential cross sections for the scattering of free electrons from the corresponding ionic cores.

Effects of molecular structure on ion disintegration patterns in ionization of O2 and N2 by short laser pulses.

We demonstrate that the structure of the outermost orbitals of oxygen and nitrogen can be observed in the angular distribution of coincident ion pairs generated by the double ionization of these molecules by 8 fs laser pulses. We do this by establishing that these ions emerge from well defined excited electronic states of O2+2 and N2+2 respectively and that they are produced dominantly through a process which involves electron rescattering. The angular distributions of the ions from the two targets are very different, reflecting the different structures of the outermost orbitals of the two molecules.

Routes to control of H2 Coulomb explosion in few-cycle laser pulses.

We have measured coincident ion pairs produced in the Coulomb explosion of H2 by 8-30 fs laser pulses at different laser intensities. We show how the Coulomb explosion of H2 can be experimentally controlled by tuning the appropriate pulse duration and laser intensity. For laser pulses less than 15 fs, we found that the rescattering-induced Coulomb explosion is dominated by first-return recollisions, while for longer pulses and at the proper laser intensity, the third return can be made to be the major one. Additionally, by choosing suitable pulse duration and laser intensity, we show H2 Coulomb explosion proceeding through three distinct processes that are simultaneously observable, each exhibiting different characteristics and revealing distinctive time information about the H2 evolution in the laser pulse.