We report on simultaneous measurements of backward- and forward-accelerated protons spectra when an ultrahigh intensity (approximately 5 x 10(18) W/cm(20), ultrahigh contrast (>10(10)) laser pulse interacts with foils of thickness ranging from 0.08 to 105 microm. Under such conditions, free of preplasma originating from ionization of the laser-irradiated surface, we show that the maximum proton energies are proportional to the p component of the laser electric field only and not to the ponderomotive force and that the characteristics of the proton beams originating from both target sides are almost identical. All these points have been corroborated by extensive 1D and 2D particle-in-cell simulations showing a very good agreement with the experimental data.
Two-color two-photon femtosecond ionization experiments have been
performed on NaI. The wave packet
evolution of the A excited state has been followed by detecting
photoions and photoelectrons. The results
indicate that the Na+ ions are formed when the wave
packet is located at the outer turning point of the
excited state. Surprisingly, the NaI+ ions are also
observed to be in phase with the Na+ signal.
Photoelectron
spectra show that high kinetic energy electrons are produced when
ionizing around the outer turning point,
in agreement with the NaI+ formation. The absence of
signal corresponding to ionization from the covalent
part of the excited state potential can only be understood if the
absolute ionization cross section is much
smaller in the covalent region of the A state (where the molecule can
be considered as a van der Waals
complex) than in the ionic Na+···I-
part of the A state potential (where the interatomic distance is such
that
the ionization process may be considered as a photodetachment of the
electron from I- anion). Simulations
taking into account that ionization occurs only when the wave packet is
in the ionic region of the A state are
in good agreement with experimental data.
The interaction of laser pulses with thin grating targets, having a periodic groove at the irradiated surface, is experimentally investigated. Ultrahigh contrast (~10(12)) pulses allow us to demonstrate an enhanced laser-target coupling for the first time in the relativistic regime of ultrahigh intensity >10(19) W/cm(2). A maximum increase by a factor of 2.5 of the cutoff energy of protons produced by target normal sheath acceleration is observed with respect to plane targets, around the incidence angle expected for the resonant excitation of surface waves. A significant enhancement is also observed for small angles of incidence, out of resonance.
We present and characterize a very efficient optical device that employs the plasma mirror technique to increase the contrast of high-power laser systems. Contrast improvements higher than 10(4) with 50% transmission are shown to be routinely achieved on a typical 10 TW laser system when the pulse is reflected on two consecutive plasma mirrors. Used at the end of the laser system, this double plasma mirror preserves the spatial profile of the initial beam, is unaffected by shot-to-shot fluctuations, and is suitable for most high peak power laser systems. We use the generation of high-order harmonics as an effective test for the contrast improvement produced by the double plasma mirrors.
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