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At temperatures and densities that are typical of plasmas produced by lasers pulses interacting with solid targets, at power intensities I > 1012W/cm2, the classical Debye screening factor in nuclear reactions becomes comparable with the one of the solar core. Preliminary calculations about the total number of fusion reactions have been performed following an hydrodynamical approach for the description of the plasma dynamics. This approach is propaedeutic for future measurements of D-D fusion reaction rates.
Angular distributions of currents and velocities (energies) of ions produced at various target irradiation angles and laser intensities ranged from 1010 W/cm2 to 1017 W/cm2 were analyzed. It was confirmed that for low laser intensities the ion current distributions are always peaked along the target normal. However, at laser intensities comparable to or higher than 1014 W/cm2, the preferred direction of ion emission strongly depends on the irradiation geometry (laser focus setting, the irradiation angle), and can be off the target normal. This is very likely caused by the non-linear interaction of the laser beam with produced plasma, in particular, by the action of ponderomotive forces and the laser beam self-focusing.
Intense laser-beam interactions with preformed plasma, preceding the laser-target interactions, significantly influence both the ion and X-ray generation. It is due to the laser pulse (its total length, the shape of the front edge, its background, the contrast, the radial homogeneity) as well as plasma (density, temperature) properties. Generation of the super fast (FF) ion groups is connected with a presence of non-linear processes. Saturated maximum of the charge states (independently on the laser intensity) is ascribed to the constant limit radius of the self-focused laser beam. Its longitudinal structure is considered as a possible explanation for the course of some experimental dependencies obtained.
Laser-generated plasma is obtained in high vacuum (10−7 mbar) by irradiation of metallic targets (Al, Cu, Ta) with laser beam with intensities of the order of 1010 W/cm2. An Nd:Yag laser operating at 1064 nm wavelength, 9 ns pulse width, and 500 mJ maximum pulse energy is used. Time of flight measurements of ion emission along the direction normal to the target surface were performed with an ion collector. Measurements with and without a 0.1 Tesla magnetic field, directed along the normal to the target surface, have been taken for different target-detector distances and for increasing laser pulse intensity. Results have demonstrated that the magnetic field configuration creates an electron trap in front of the target surface along the axial direction. Electric fields inside the trap induce ion acceleration; the presence of electron bundles not only focuses the ion beam but also increases its energy, mean charge state and current. The explanation of this phenomenon can be found in the electric field modification inside the non-equilibrium plasma because of an electron bunching that increases the number of electron-ion interactions. The magnetic field, in fact, modifies the electric field due to the charge separation between the clouds of fast electrons, many of which remain trapped in the magnetic hole, and slow ions, ejected from the ablated target; moreover it increases the number of electron-ion interactions producing higher charge states.
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.
Laser-beam interaction with expanding plasma was investigated using
the PALS high-power iodine-laser system. The interaction conditions are
significantly changing with the laser focus spot position. The decisive
role of the laser-beam self-focusing, participating in the production of
ions with the highest charge states, was proved.
We report on the results concerning the characteristics and the
behavior of expanding plasma generated by a Laser Ion Source (LIS). The
LIS technique is an efficient means in producing of multi-charged ions
utilizing pulsed laser beams. In order to extract Cu ions, in this
experiment an XeCl excimer UV laser was employed, providing a power
density on the target surface up to 5 × 108
W/cm2. Two typologies of diagnostic systems were
developed in order to detect the plasma current and the ion energy. The
time-of-flight (TOF) measurements were performed exploiting either a
Faraday cup or an Ion Energy Analyzer (IEA). This latter allowed
getting quantitative information about the relative ion abundances,
their kinetic energy and their charge state. To study the plasma
characteristics we measured the total etched material per pulse at 70
mJ. It was 0.235 μg and the overall degree of ionization, 16%. The
angular distribution of the ablated material was monitored by optical
transmission analysis of the deposited film as a function of the angle
with respect to the normal to the target surface. Applying a high
voltage to an extraction gap a multi-charged ion beam was obtained;
different peaks could be distinguished in the TOF spectrum, resulting
from the separation of ions of hydrogen, adsorbed compounds in the
target and copper.
The experimental results of the investigations on the influence
of external magnetic and electric fields on the characteristics
of a tungsten ion stream emitted from a plasma produced by the
Nd:glass laser (1 J, 1 ns) performed at IPPLM, Warsaw are
presented. A negatively biased target up to −15 kV and
a magnetic field up to 0.45 T were used in the experiment. A
set of ion collectors and an electrostatic cylindrical ion energy
analyzer located at small angles with respect to the laser beam
axis and at large distances from the target were applied for
ion measurements. The effect of an external magnetic field is
essential to plasma expansion, but the effect of the retarding
potential of the target is very weak in our experimental
conditions. The aim of the studies was to prove the possibility
of the optimization of ion beam parameters from laser-produced
plasma for the particular application as a laser ion source
coupled with the electron cyclotron resonance ion source for
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