J. Krása

The Prague Asterix Laser System

The Prague Asterix Laser System (PALS) is a new international laboratory where research teams are invited to compete for the beam time. The PALS Center runs an iodine photodissociation high-power laser system delivering up to 1.2 kJ of energy in ∼400 ps pulses at the wavelength of 1.315 μm. Optional doubling and tripling of the frequency is assured by large-diameter nonlinear crystals. The ASTERIX IV laser [H. Baumhacker et al., Appl. Phys. B 61, 325 (1995)], transferred from Garching into a new laser hall in Prague, was updated and put into operation on 8 June 2000. These upgrades include new beam delivery options and a twin interaction chamber, which is designed flexibly for a broad spectrum of applications. Results of the first series of experiments are presented and some planned upgrades are briefly described. These include implementation of adaptive optics, replacement of the iodine master oscillator by a more flexible solid state oscillator based on fiber optics, and a femtosecond extension of the l...

Effects of ps and ns laser pulses for giant ion source

Abstract Laser driven ion sources produce giant ion emission current densities at the emission area which exceed the few mA/cm 2 of classical ion sources (MEVVA or ECR) by many orders of magnitude. Very energetic highly charged fast ions, separated by their charge number Z , from ns laser pulses were explained by relativistic self-focusing and nonlinear force effects. Recently a strong difference with ps pulses was found in contrast to the ns pulses depending on the prepulses. We present an explanation based on a skin layer process. This has consequences to sub-picosecond laser-plasma interaction for the studies of the fast ignitor physics for laser fusion and to the new field of nuclear physics opened by these laser pulses which produce up to 100 MeV particles and gammas of high density as well as for ion source applications.

Ablation of organic polymers by 46.9-nm-laser radiation

We report results of the exposure of poly(tetrafluoroethylene) -(PTFE), poly(methyl methacrylate) -(PMMA), and polyimide -(PI) to intense 46.9-nm-laser pulses of 1.2-ns-duration at fluences ranging from ∼0.1 to ∼10J∕cm2. The ablation rates were found to be similar for all three materials, ∼80–90nm∕pulse at 1J∕cm2. The results suggest that the ablation of organic polymers induced by intense extreme ultraviolet laser radiation differs from that corresponding to irradiation with longer wavelengths.

Ion and neutral emission from pulsed laser irradiation of metals

Abstract An Nd:YAG pulsed laser at 1064 nm wavelength with 9 ns pulse width and a maximum pulse energy of 900 mJ is focused on different metallic targets (Al, Ti, Ni, Cu, Nb, Sn, Ta, W, Au and Pb) placed in vacuum. The interaction produces a high etching for a pulse energy higher than a threshold value typical of each metal. Near the threshold a strong neutral emission takes place; at high pulse energy a stronger ionic emission occurs. The experimental thresholds of the ion emission are very similar to the threshold of the neutral emission. The atomic neutral emission is monitored by a mass quadrupole spectrometer and by the vapor thin film deposition technique. The ionic emission is detected through ion collectors (IC) using Faraday cups and time-of-flight measurements. The energy thresholds, the emission yields, the angular distribution, the fractional ionization, the kinetics and characteristics of the plasma production and the ion charge state are presented and discussed.

Angular distribution of ions emitted from Nd:YAG laser-produced plasma

Angular distribution of ion currents emitted from laser-produced plasmas are reported for a Nd:YAG laser with intensities lower than 1×1010 W/cm2. This distributions are strongly peaked along the normal to the target surface for Cu, Sn, Ta, W, Au, and Pb ion streams, independent of the incidence angle of the irradiated target. For Al, Ni, and Nb the main axis tends to decline to about −10°. The comparison of fits of Gaussian function and cosP(α−α0)+y0 to the experimental data verified the formal equivalency of both the functions. Fitted values of the FWHM and of the exponent P are compared for all the elements used. The angular distribution of mean ion velocity 〈v〉 and ion kinetic energy 〈E〉 are presented.

Fast ion emission from the plasma produced by the PALS laser system

This paper presents the results of studies of fast ion emission from the multiply charged high-Z number plasma generated using the PALS high-energy iodine laser system (?1.2?kJ, 0.4?ns) at the PALS Research Center in Prague. The properties of the emitted ion streams were investigated using ion collectors located at various angles with respect to the target normal and an electrostatic energy analyser. The x-ray emission from the plasma was measured using semiconductor detectors. Different groups of ions (slow, thermal and fast) were observed in the ion collector signals. Ion current densities higher than 80?mA?cm?2 at ~1?m from the target were demonstrated. The charge velocity distribution, ion current density and angular distribution of ion charge emission, as well as total charge and average ion energy were obtained from these signals. Using the electrostatic ion-energy analyser, the emission of highly charged heavy ions (Ta52+, Ag38+) with energies up to 7?MeV for Ta ions was demonstrated. The dependence of ion stream parameters on the experimental conditions is discussed. We also report the results of preliminary experiments on the direct implantation of laser-produced ions into various materials.

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.

Production of low energy, high intensity metal ion beams by means of a laser ion source

The ECLISSE (ECR coupled to Laser Ion Source for charge State Enhancement) project started in 1999 with the aim to obtain an intense beam of highly charged ions (pulsed mode) by means of the coupling between a laser ion source (LIS) and an electron cyclotron resonance (ECR) ion source. The major points to be investigated appeared to be the coupling efficiency between the ion beam produced by the LIS and the ECR plasma, as well as the possibility to enhance the available charge state by an ECRIS with respect to the standard methods which are used to produce ion beams from solid samples (e.g., evaporation, sputtering). The calculations have confirmed that this concept may be effective, provided that the ion energy from the LIS is lower than a few hundred eV. The main features of the calculations will be shown, along with the results obtained in the off-line test facility at laser power densities below 1011 W/cm2.

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.

Charge losses in expanding plasma created by an XeCl laser

The emission of multiply charged Cuq+ ions from a plasma produced by 308 nm excimer laser is analyzed with respect to the distance from the irradiated target. The critical zone, outside which the charge states of ions of the expanding plasma are frozen, was determined to be approximately 20 cm from the target. This value was estimated using a charge-freezing criterion expressed by a distance dependence of the total charge carried by the ions Q∝L−2, which describes the dilution of plasma by its expansion into a vacuum without collisional recombination processes.