Factors influencing parameters of laser ion sources

Abstract

Various applications demand various kinds of ions. Charge state, energy and the amount of laser produced ions depend, primary, on the wavelength, the energy, the pulse duration, and the focusing ability of the laser used. Angle of the target irradiation, angle of the ion extraction (recording), and mainly the focus setting may significantly influence especially the portion of ions with the highest charge states. The participation of non-linear processes on the generation of ions with extremely high parameters is demonstrated. The observed effects support the idea of a longitudinal structure of the self-focused laser beam with a space period of ∼200 µm.

Investigations on low temperature laser-generated plasmas

“…This field is responsible for the ion acceleration and for the ion energy distributions separation as a function of the charge state (Laska et al, 2007;. The thermal processes occurring in the Knudsen layer, close to the target surface, can be understood evaluating the plasma temperature by assuming that the ionized gas is in a local thermal equilibrium (LTE) condition.…”

Laser contrast and other key parameters enhancing the laser conversion efficiency in ion acceleration regime

“…The laser contrast, in terms of ratio between the intensity of the main pulse to that of the pedestal or prepulse, in fact, plays a fundamental role in the forward TNSA ion acceleration mechanism and the pedestal must be controlled accurately in duration and in intensity before to be applied to a given target irradiation. I some cases the laser prepulse or the use of a laser focal positioning in front of the target may produce a preplasma in front of the solid target very useful to induce self-focusing effect [6], consisting in the increment of the refractive index in which the laser light may be further focalized up to spot dimensions comparable with the laser wavelength, so that the intensity increases, the accelerated electron energy also and consequently the final ion acceleration enhanced. However the key parameters to enhance the particle acceleration are many and some of them concern the target composition and structure, that must be employed to improve the TNSA mechanism, to enhance the yield of the ions that we want accelerate and to improve their ion energy and angular emission distributions.…”

Generation of high-energy neutrons with the 300-ps-laser system PALS

“…Fast neutrons with energies in excess of 10 MeV resulting from the 7 Li(d, xn) 8 Be reaction (Q = 15.03 MeV) have been reported by several authors [6][7][8][9][10][11] . Acceleration of deuterons is mostly reported in experiments using lasers with intensities of 10 19 W cm −2 . The deuterons are accelerated in focal spots on thin-film targets through either the target-normal sheath acceleration (TNSA) mechanism or the newly recognized break-out afterburner (BOA) mechanism [2] .…”

“…In contrast to ultra-short high-intensity lasers which allow the generation of beams of protons and deuterons possessing kinetic energies 1 MeV, sub-nanosecond lasers of the kJclass capable of delivering a moderate intensity onto a target make it possible to accelerate ions up to MeV energies per nucleon [19][20][21][22] . The clear-cut evidence that the fastest protons accelerated by the laser system PALS (1.315 µm, 300 ps, 3×10 16 W cm −2 ) have energies up to ∼4 MeV [20,21] creates a way to accelerate a high number of deuterons from the front side of a target and exploit them in the production of high-energy (∼15 MeV) neutrons through the 7 Li(d, xn) nuclear reaction even if the mean kinetic energy of the bunch of deuterons is <1 MeV.…”