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An experimental observation has been made by using aluminum-coated Mylar foils, which strongly supports that in the case of plastic target, the energetic part of the proton beam originates from the front-side of the target. When a 30 fs laser pulse with an intensity of 1.6×1019 W/cm2 was irradiated on the 12.5-μm-thick Mylar side of the aluminum-coated Mylar foil, the maximum proton energy was reduced by a factor 5.5 as compared to that of 3.3 MeV observed from the single layer of the Mylar foil. With the help of a two-dimensional particle-in-cell simulation, these observations can be interpreted that in the case of plastic target, the energetic proton beam originates from the front-side of the target. In the case of an aluminum-coated 6-μm-thick Mylar foil, more energetic proton beams of 4.7 MeV were also observed when the laser pulse was irradiated on the aluminum side as compared to those of 3.4 MeV from the single Mylar foil. [ABSTRACT FROM AUTHOR]

The effect of a laser prepulse on the generation of proton beams is compared between plastic and metal targets by irradiating a 30 fs, 2.4×1018 W/cm2 Ti:sapphire laser pulse. Proton energies generated from both target materials increase as the pulse duration of the laser prepulse decreases. However, it was found that there are distinct differences with respect to target materials. In the case of aluminum targets, as target thickness decreases, proton energy gets higher, which is well described by an isothermal expansion model. However, in the case of Mylar® targets, no such dependence on target thickness could be observed, and the highest maximum proton energies are higher by factors of 1.5 to 3 than those from aluminum targets or those predicted by the isothermal expansion model. Such characteristics of the proton beams from Mylar® targets can be accounted for by a bulk acceleration model, or acceleration by a resistively induced electric field. [ABSTRACT FROM AUTHOR]

Soft x-ray spectra (40-180 Å) produced by the interaction of 25 fs laser pulses at an intensity of ∼7x10[sup 16] W/cm[sup 2] with a cryogenically cooled Ar gas jet have been measured. New spectral lines from Ar[sup 8+] and Ar[sup 9+] charge states appeared with decreasing gas temperature. Nonlinear increase of x-ray line emission from Ar[sup 7+], Ar[sup 8+], and Ar[sup 9+] was observed with cooling, which saturated below certain temperature. The drastic change in the spectrum is attributed to efficient collisional heating and collisional ionization of growing (10[sup 2]-10[sup 3] atoms) Ar clusters from the cooled jet. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]