A. Velyhan

Mass-loss rate for MF resin microspheres

This paper deals with the influence of experimental conditions on properties of melamine formaldehyde resin particles. Motivation for this study was the fact that many laboratories and experimenters use these particles as samples for dust plasma investigations. We have found that the mass of these grains decreases during long-time exposure in vacuum. This decrease proceeds much faster if the temperature of grains is increased.

Full characterization of laser-accelerated ion beams using Faraday cup, silicon carbide, and single-crystal diamond detectors

Multi-MeV beams of light ions have been produced using the 300 picosecond, kJ-class iodine laser, operating at the Prague Asterix Laser System facility in Prague. Real-time ion diagnostics have been performed by the use of various time-of-flight (TOF) detectors: ion collectors (ICs) with and without absorber thin films, new prototypes of single-crystal diamond and silicon carbide detectors, and an electrostatic ion mass spectrometer (IEA). In order to suppress the long photopeak induced by soft X-rays and to avoid the overlap with the signal from ultrafast particles, the ICs have been shielded with Al foil filters. The application of large-bandgap semiconductor detectors (>3 eV) ensured cutting of the plasma-emitted visible and soft-UV radiation and enhancing the sensitivity to the very fast proton/ion beams. Employing the IEA spectrometer, various ion species and charge states in the expanding laser-plasma have been determined. Processing of the experimental data based on the TOF technique, including est...

Generation of high pressure shocks relevant to the shock-ignition intensity regime

An experiment was performed using the PALS laser to study laser-target coupling and laser-plasma interaction in an intensity regime ≤1016 W/cm2, relevant for the “shock ignition” approach to Inertial Confinement Fusion. A first beam at low intensity was used to create an extended preformed plasma, and a second one to create a strong shock. Pressures up to 90 Megabars were inferred. Our results show the importance of the details of energy transport in the overdense region.

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.

Single crystal silicon carbide detector of emitted ions and soft x rays from power laser-generated plasmas

A single-crystal silicon carbide (SiC) detector was used for measurements of soft x rays, electrons, and ion emission from laser-generated plasma obtained with the use of the Prague Asterix Laser System (PALS) at intensities of the order of 1016 W/cm2 and pulse duration of 300 ps. Measurements were performed by varying the laser intensity and the nature of the irradiated target. The spectra obtained by using the SiC detector show not only the photopeak due to UV and soft x-ray detection, but also various peaks due to the detection of energetic charged particles. Time-of-flight technique was employed to determine the ion kinetic energy of particles emitted from the plasma and to perform a comparison between SiC and traditional ion collectors. The detector was also employed by inserting absorber films of different thickness in front of the SiC surface in order to determine, as a first approximation, the mean energy of the soft x-ray emission from the plasma.

Ion field emission from micrometer-sized spherical glass grains

An experimental investigation of ion field emission from spherical glass grains in a 1-10 /spl mu/m size range is described. A single dust grain is trapped in an electrodynamic quadrupole and charged positively by a monoenergetic ion beam. The energy of these ions (5 keV) results in a high surface potential and high electric field intensity. A limitation of the surface potential due to ion field emission was observed for the surface field intensity of 3/spl times/10/sup 8/ V/m, which is a considerably lower value than expected. The density of the discharging current was as high as 10/sup -5/ A/m/sup 2/, and increased exponentially with increasing intensity of the electric field. As there are no data of the ion field emission from insulators, we have used the zinc grains for a comparison. Taking into account the irregular shape of used metallic grains, we conclude that the ion field emission from the glass grains starts at a significantly (two orders of magnitude) lower electric field intensity than that from metals. This may lead to the limitation of the attainable surface potential of small insulator grains. The possible effect of the ion field emission on the charging of dust grains in the interplanetary space is discussed.

Advanced scheme for high-yield laser driven nuclear reactions

The use of a low contrast nanosecond laser pulse with a relatively low intensity (3?????1016?W?cm?2) allowed the enhancing of the yield of induced nuclear reactions in advanced solid targets. In particular the ?ultraclean? proton?boron fusion reaction, producing energetic alpha particles without neutron generation, was chosen. A spatially well-defined layer of boron dopants in a hydrogen-enriched silicon substrate was used as a target. A combination of the specific target composition and the laser pulse temporal shape allowed the enhancing of the yield of alpha particles up to 109 per steradian. This result can be ascribed to the interaction of the long-laser pre-pulse with the target and to the optimal target geometry and composition.

Neutron Energy Distribution Function Reconstructed From Time-of-Flight Signals in Deuterium Gas-Puff

The implosion of a solid deuterium gas-puff Z-pinch was studied on the S-300 pulsed power generator [A. S. Chernenko, , Proceedings of 11th Int. Conf. on High Power Particle Beams, 154 (1996)]. The peak neutron yield above 1010 was achieved on the current level of 2 MA. The fusion neutrons were generated at about 150 ns after the current onset, i.e., during the stagnation and at the beginning of the expansion of a plasma column. The neutron emission lasted on average 25 ns. The neutron energy distribution function was reconstructed from 12 neutron time-of-flight signals by the Monte Carlo simulation. The side-on neutron energy spectra peaked at 2.42 plusmn 0.04 MeV with about 450-keV FWHM. In the downstream direction (i.e., the direction of the current flow from the anode toward the cathode), the peak neutron energy and the width of a neutron spectrum were 2.6 plusmn 0.1 MeV and 400 keV, respectively. The average kinetic energy of fast deuterons, which produced fusion neutrons, was about 100 keV. The generalized beam-target model probably fits best to the obtained experimental data.

Z

A deconvolution of time-resolved ion collector signals was used to determine the temperature and the centre-of-mass (COM) velocity of H+ and Cq+ (1 ? q ? 6) ions emitted from a polyethylene target irradiated by the 3rd harmonics (? = 438?nm) of an iodine laser up to the laser-beam intensity value of about 2 ? 1016?W?cm?2. The intensity dependence of the COM energy for single charge-states is shown. A plasma block, which precedes the main ion group, is characterized. The properties of the main ion group are compared with those of fast ions produced by a CO2 laser [1]. It is shown that the separation of peaks, i.e. their visualization in the time-resolved ion collector signal, can be characterized by the ratio of thermal velocity to COM velocity. The charge-state dependence of the COM velocity is discussed.

-Pinch

Measurements of the return-current flowing through a solid target irradiated with the sub-nanosecond kJ-class Prague Asterix Laser System is reported. A new inductive target probe was developed which allows us measuring the target current derivative in a kA/ns range. The dependences of the target current on the laser pulse energy for cooper, graphite, and polyethylene targets are reported. The experiment shows that the target current is proportional to the deposited laser energy and is strongly affected by the shot-to-shot fluctuations. The corresponding maximum target charge exceeded a value of 10 μC. A return-current dependence of the electromagnetic pulse produced by the laser-target interaction is presented.