Abstract
Low-intensity laser prepulses (<1013 W cm−2, nanosecond duration) are a major issue in experiments on laser-induced generation of protons, often limiting the performances of proton sources produced by high-intensity lasers (≈1019 W cm−2, picosecond or femtosecond duration). Depending on the intensity regime, several effects may be associated with the prepulse, some of which are discussed in this paper: (i) destruction of thin foil targets by the shock generated by the laser prepulse; (ii) creation of preplasma on the target front side affecting laser absorption; (iii) deformation of the target rear side; and (iv) whole displacement of thin foil targets affecting the focusing condition. In particular, we show that under oblique high-intensity irradiation and for low prepulse intensities, the proton beam is directed away from the target normal. Deviation is towards the laser forward direction, with an angle that increases with the level and duration of the ASE pedestal. Also, for a given laser pulse, the beam deviation increases with proton energy. The observations are discussed in terms of target normal sheath acceleration, in combination with a laser-controllable shock wave locally deforming the target surface.
Highlights
Laser-produced shocks and proton generationThe effects related to the shock induced by the laser prepulse depend on the following: 1) Shock pressure (which increases with laser intensity): at high pressure the shock will produce vaporization of the target rear side and plasma formation
We will show how, depending on target thickness and prepulse intensity, three different regimes can be obtained
The effects related to the shock induced by the laser prepulse depend on the following: 1) Shock pressure: at high pressure the shock will produce vaporization of the target rear side and plasma formation
Summary
The effects related to the shock induced by the laser prepulse depend on the following: 1) Shock pressure (which increases with laser intensity): at high pressure the shock will produce vaporization of the target rear side and plasma formation. The Hugoniot curve is the ensemble of states in a material that can be reached by shock compression This is named as shock adiabat in figure 2. We must consider the fact that, as described in [3], at the time at which the shock breaks out on the target rear side, the shock pressure will not be maintained since the shocked material will face vacuum (or a gas at very low pressure). Instead shock compression of Al to 1.2 Mbar will lead to an unloading that ends at zero pressure below the evaporation curve of Al. Despite the small difference in pressure, the two shocks produce dramatically different effects. They may be termed as the case of strong shock pressure and the case of small shock pressure
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