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12OPTIMAL REGIMES OF LASER TRIGGERED PROTON ACCELERATION FROM LOW-DENSE TARGETS
A. V. Brantov, E. A. Govras, V. Yu. Bychenkov
P. N. Lebedev Physical Institute, Russian Academy of Sciences, Moscow
brantov@sci. lebedev. ru
Proton acceleration by short relativistic laser pulses has potential to many applications in nuclear physics, warm dense matter and astrophysics researches, radiography, testing for radiation resistance of microchips, nuclear pharmacology and proton therapy.
In this work we have studied dependence of maximum proton energy on both the target thickness and density. To obtain maximum ion energy target thickness should be properly matched to the laser intensity. For the foils with thickness smaller than a skin depth, the optimal target thickness corresponds to maximum possible electron evacuation from the target focal volume. This provides maximum charge separation field which effectively accelerates ions. In this case the controlling parameter is the areal electron density (the product of target density and thickness), rather than two parameters, thickness and density. This parameter defines transparency of a foil and maximizes ion energy for the semi-transparent target. With density decreasing the optimal target thickness increases and becomes larger than a skin depth. Further reduction of foil thickness may result in induced relativistically transparency. Therefore, for the foils with thickness larger than a skin depth, density and thickness become independent controlling parameters for optimization of laser target interaction with aim to maximize ion energy. In this work, by using 3D PIC code MANDOR we find the optimum foil thickness for given target density and laser pulse energy that maximizes energy of the accelerated ions. The energy of femtosecond laser pulse was varying from several mJ to tens of joules and target density was varying from solid to sub-critical density. It has been shown that reduction of target density may result in considerable increase of ion energy (up to 2 times) for laser energy of tens of joules. This validates an appropriateness of using low-dense homogeneous targets although their implementation is still awaited.
We also proposed analytical model which describes ion acceleration from thin foils for arbitrary temperature of laser-heated electrons. It has been shown that charge density waves play important role in formation of electric field distribution that results in modification of ion spectra. Derived ion spatial-temporal and spectral characteristics are consistent with numerical simulations for wide range of acceleration regimes from quasi-neutral expansion to Coulomb explosion.
This work was supported by the Russian Science Foundation.
