K. Rohlena

Production of ultrahigh ion current densities at skin-layer subrelativistic laser–plasma interaction

Some applications of fast ions driven by a short (≤1 ps) laser pulse (e.g. fast ignition of ICF targets, x-ray laser pumping, laboratory astrophysics research or some nuclear physics experiments) require ion beams of picosecond (or shorter) time durations and of very high ion current densities (∼10 10 A cm -2 or higher). A possible way of producing ion beams with such extreme parameters is ballistic focusing of fast ions generated by a target normal sheath acceleration (TNSA) mechanism at relativistic laser intensities. In this paper we discuss another method, where the production of short-pulse ion beams of ultrahigh current densities is possible in a planar geometry at subrelativistic laser intensities and at a low energy (≤ 1 J) of the laser pulse. This method-referred to as skin-layer ponderomotive acceleration (S-LPA)-uses strong ponderomotive forces induced at the skin-layer interaction of a short laser pulse with a proper preplasma layer in front of a solid target. The basic features of the high-current ion generation by S-LPA were investigated using a simplified theory, numerical hydrodynamic simulations and measurements. The experiments were performed with subjoule 1 ps laser pulses interacting with massive or thin foil targets at intensities of up to 2 x 10 17 W cm -2 . It was found that both in the backward and forward directions highly collimated high-density ion beams (plasma blocks) with current densities at the ion source (close to the target) approaching 10 10 A cm -2 are produced, in accordance with the theory and numerical calculations. These ion current densities were found to be comparable to (or even higher than) those estimated from recent short-pulse TNSA experiments with relativistic laser intensities. Apart from the simpler physics of the laser-plasma interaction, the advantage of the considered method is the low energy of the driving laser pulses allowing the production of ultrahigh-current-density ion beams with a high repetition rate. It opens a prospect for unique tabletop experiments in various fields of physical and technological research.

Stable dense plasma jets produced at laser power densities around 1014W∕cm2

The results of investigations are presented that are connected with defocused laser beam–planar target interaction. Following the very large focus laser-plasma interaction experiments on the Nova [H. T. Powell, J. A. Caird, J. E. Murray, and C. E. Thompson, 1991 ICF Annual Report UCRL-LR-105820-91, p. 163 (1991)] and GEKKO-XII [C. Yamanaka, Y. Kato, Y. Izawa, K. Yoshida, T. Yamanaka, T. Sasaki, T. Nakatsuka, J. Kuroda, and S. Nakai, IEEE J. Quantum Electron. QE-17, 1639 (1981)] lasers, as well as on the National Ignition Facility (NIF) laser [W. J. Hogan, E. I. Moses, B. E. Warner, M. S. Sorem, and J. M. Soures, Nucl. Fusion 41, 567 (2001)] with generation of high Mach number jets, this paper is devoted to similar jet generation with very detailed measurements of density profiles by using high-power lasers at large focus conditions. The experiment was carried out with target materials of different mass densities (Al, Cu, Ag, Ta, and Pb) using the Prague Asterix Laser System (PALS) iodine laser [K. Jungwir...

Laser produced Ag ions for direct implantation

The amount and properties of ions produced by laser ablation of Ag targets have been analyzed. The maximum ion current density jmax=21.0 mA and maximum charge state Ar37+ of the ions produced by a laser power density of about 1×1014 W cm−2 at 1.315 and 0.657 μm on an Ag target have been determined. Direct implantation of the Ag ions from the laser-produced plasma has also been studied. An implanted ion density of 3.5×1016 cm−2 at a depth of 500 nm in Al samples was determined by RBS.

IODINE LASER PRODUCTION OF HIGHLY CHARGED Ta IONS

The results of systematic studies of multiply charged Ta ion production with the fundamental frequency of an iodine laser (λ=1.315μm), and its 2nd (0.657μm) and 3rd (0.438μm) harmonics are summarized and discussed. Short laser pulse (350 ps) and a focus spot diameter of 100μm allowed for the laser power densities in the range of 5×1013–1.5×1015 W/cm2. Corpuscular diagnostics were based on time-of-flight methods; two types of ion collectors and a cylindrical electrostatic ion energy analyzer were used. The Ta ions with charge state up to 55+ were registered in the distance of 210 cm; the maximum amplitude of the signal of a high energy ion group was found to belong to the ions with the charge state around 43+, depending on the laser power density. The ion energy distribution was measured for all three wavelengths, however, in a different energy range; the maximum registered ion energy was 8.8 MeV. The occurrence of highly charged ions in the far expansion zone is discussed in view of the mechanism of charge distribution “freezing” during two-temperature isothermal plasma expansion.

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.

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

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 superfast (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 of the course of some experimental dependencies obtained.

Equivalent ion temperature in Ta plasma produced by high energy laser ablation

High energy laser, 400ps pulse duration, irradiating heavy targets in vacuum produce intense plasma and generate emission of various energetic ion groups. The ion intensity is high along the normal to the irradiated target surface and high charge state and high velocity ions are produced. The characteristics of the ion streams were investigated by using an electrostatic ion energy analyzer and different ion collectors were placed at various angles with respect to the target normal. The ion energy distribution as a function of the ion charge state was measured and the comparison of the properties of different ion groups generated by laser beams at two different energies was carried out. Measurements point out that five ion groups or more can be generated by the laser interaction with the preformed plasma, with different “equivalent ion temperatures.” Slow, thermal, fast, and very fast ions follow a Boltzmann-like distribution; the equivalent temperatures of different ion groups were estimated to reach valu...

Charge-state and energy enhancement of laser-produced ions due to nonlinear processes in preformed plasma

At laser intensities above IL∼1×1014W∕cm2(ILλ2∼1×1014Wμm2∕cm2), nonlinear processes in preformed plasma, such as self-focusing, influence ion generation significantly and ions with higher charge states and energies can be produced than without interaction with preformed plasma. The step (spread) in plots of experimental data of ion energy per nucleon versus ILλ2 reported by other researchers most likely reflects high-intensity laser interactions with and without preformed plasma.

Ion production by lasers using high-power densities in a near infrared region : Laser-Ion Sources

Results are presented of experiments on ion production from Ta targets using a short pulse (350-600 ps in focus) illumination with focal power densities exceeding 10 14 Wcm -2 at the wavelength of an iodine photodissociation laser (1.315 μm) and its harmonics. Strong evidence of the existence of tantalum ions with the charge state +45 near the target surface was obtained by X-ray spectroscopy methods. The particle diagnostics point to the existence of frozen high charge states ( 4 MeV) for the highest observed charge states. A tentative theoretical explanation of the observed anomalous charge state freezing phenomenon in the expanding plasma produced by a subnanosecond laser pulse is given.

Plasma jets produced in a single laser beam interaction with a planar target

A laser experiment on a plasma jet formation and its multidimensional numerical analysis are presented. Under suitable conditions on the laser intensity, focal spot radius, and target atomic number a radiative jet could be launched from a simple planar target. The jet lasts more than 10ns and extends over several millimeters. It has a velocity of around 500km∕s, a Mach number greater than 10, and a density above 1018cm−3. It is concluded from the dimensional analysis of the experiment and from numerical simulations that the x-ray emission has a dominant effect on the jet formation and collimation. The similarity criteria for scaling of the laser experiment to larger systems are verified. Using a relatively low-energy laser pulse, below 500J, one can produce a laboratory jet that presents similarities with astrophysical objects such as protostellar jets.