Self-focusing effect in Au-target induced by high power pulsed laser at PALS

AbstractSelf-focusing effects, induced by ASTERIX pulsed laser at PALS Laboratory of Prague, have been investigated. Laser was employed at the third harmonics (438 nm) and intensities of the order of 1016 W/cm2. Pure Au was used as thin target and irradiated with 30° incidence angle. An ion energy analyzer was employed to detect the energy-to-mass ratio of emitted ions from plasma. Measurements were performed by changing the focal point position with a high spatial resolution step-motor. Results demonstrated that non linear processes, due to self-focusing effects, occurs when the laser beam is focused at about 200 µm in front of the target surface. In such conditions, a new ion group, having high charge state and kinetic energy, is produced because of the increment in temperature of the laser-generated plasma

Expansion of tungsten ions emitted from laser-produced plasma in axial magnetic and electric fields

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 particle accelerators

Experimental studies of generation of ~100 MeV Au-ions from the laser-produced plasma

AbstractUsing the PALS iodine laser system, Au ions with the charge state up to 58+ and with the kinetic energy as high as ~300 MeV were generated. The production of these ions was tested in dependence on the laser frequency (1ω, 3ω), on the irradiation/detection angles (0°, 30°), on the focus position with regard to the target surface, and on the target thickness (500 µm, 200 µm, 80 µm). A larger amount of the fastest ions was produced with 1ω than with 3ω, the most of the fast ions were recorded in the direction ~10°from the target normal, the optimum focus position is in front of the target and should be set on with a precision of 50 µm. The forward emission is weaker than the backward one for both of the thinner targets (which burn through) at our experimental conditions

Self-focusing in processes of laser generation of highly-charged and high-energy heavy ions

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

Angular distributions of ions emitted from laser plasma produced at various irradiation angles and laser intensities

AbstractAngular 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

Factors influencing parameters of laser ion sources

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

The influence of an intense laser beam interaction with preformed plasma on the characteristics of emitted ion streams

AbstractIntense 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

Pulsed Laser Ablation of Gold at 1064 nm and 532 nm

A Nd:Yag laser, 9 ns pulse duration and 900 mJ maximum pulse energy, operating at the 1 064 nm fundamental wavelength and 532 nm second harmonic, is employed to irradiate Au targets placed in vacuum by using single laser pulses. The laser ablation is investigated in terms of ablation threshold, ablation yield, crater production and angular emission. The produced plasma emits ions at different charge state, from 1+ up to 6+, with supersonic velocity and kinetic energies up to about 3 keV, as demonstrated by time-of-flight measurements. The ion energy distributions vs. the charge state are obtained at the two wavelengths. The Boltzmann-Coulomb-shifted distributions depend regularly on the charge state, indicating the presence of a high electric field inside the non-equilibrium plasma. Measurements of plasma characterization, in terms of temperature, density and fractional ionization, are presented and discussed

Diamond detectors for characterization of laser-generated plasma

Diamond monocrystalline detectors were used to characterize radiation and particle emission from laser-generated plasma obtained at Laboratori Nazionali del Sud (LNS) and Plasma Asterix Laser Systems (PALS) laboratories by using a high power pulsed laser intensity of 10(10) W/cm(2) and 10(16) W/cm(2), respectively. Al, Ta, Au and CF2 plasmas were obtained in different irradiation conditions. Diamond detectors permitted to measure UV, X-rays, electrons and ions. Time-of-flight technique was employed to separate in time the different contributions. Results indicate that this detector has some advantages with respect to the others, such as the high energy gap, the high energy resolution, the low background current and the possibility to detect simultaneously photons, electrons and ions

Skin depth plasma front interaction mechanism with prepulse suppression to avoid relativistic self-focusing for high-gain laser fusion

Measurements of the ion emission from targets irradiated with neodymium glass and iodine lasers were analyzed and a very significant anomaly observed. The fastest ions with high charge number Z, which usually are of megaelectron volt energy following the relativistic self-focusing and nonlinear-force acceleration theory, were reduced to less than 50 times lower energies when 1.2 ps laser pulses of about 1 J were incident. We clarify this discrepancy by the model of skin depth plasma front interaction in contrast to the relativistic self-focusing with filament generation. This was indicated also from the unique fact that the ion number was independent of the laser intensity. The skin layer theory prescribes prepulse control and lower (near relativistic threshold) laser intensities for nonlinear-force-driven plasma blocks for high-gain ignition similar to light ion beam fusion