Jozef Kravarik

Ablation of various materials with intense XUV radiation

Ablation behavior of organic polymer (polymethylmethacrylate) and elemental solid (silicon) irradiated by single pulses of XUV radiation emitted from Z-pinch, plasma-focus, and laser-produced plasmas was investigated. The ablation characteristics measured for these plasma-based sources will be compared with those obtained for irradiation of samples with XUV radiation generated by a free-electron laser.

Determination of Deuteron Energy Distribution From Neutron Diagnostics in a Plasma-Focus Device

Fast neutrons from deuteron-deuteron fusion reactions were used for a study of fast deuterons in the PF-1000 plasma-focus device. The energy spectrum of neutrons was determined by the time-of-flight method using ten scintillation detectors positioned downstream, upstream, and side-on the experimental facility. Neutron energy-distribution functions enabled the determination of axial and radial components of energy of deuterons producing the fusion neutrons, as well as a rough evaluation of the total energy distribution of all fast deuterons in the pinch. It was found that the total deuteron energy-distribution function decreases with the deuteron energy more slowly than the tail of the Maxwellian distribution for 1-2-keV deuterons.

Interferometric Study of Pinch Phase in Plasma-Focus Discharge at the Time of Neutron Production

A plasma column generated in the PF-1000 device working in deuterium gas at a current level of 1 MA was investigated with interferometric diagnostics and scintillation detectors. The beam of diagnostic laser of 527-nm wavelength was optically split into 16 beams with a time delay in the range from 0 to 220 ns. This diagnostic tool makes possible the imaging of the evolution of pinch geometry, the axial and radial distributions of plasma density in the column at the stagnation phase, and their comparison with the evolution of X-ray and neutron production. The evolution of dense structure is described with respect to its importance for fusion processes.

Spontaneous Transformation in the Pinched Column of the Plasma Focus

The laser interferometry and X-ray diagnostic studies were performed within the PF-1000 facility operated with the maximum current of 2 MA and the deuterium gas filling (ensuring neutron yield above ). At this current, the plasmoidal, helical and toroidal structures were formed inside the plasma column. Some of them penetrated the column surface and later on were dissolved inside the dense plasma column. The period of their life was from a few tens to hundreds of nanoseconds and a plasma density was higher than in neighbor regions. It could be explained as a result of the plasma pinching by a magnetic field originating from the internal currents. Hard X-rays and fusion neutrons were produced during four different phases of plasma column transformations, i.e., in the period of the formation of a dense plasmoid, in the period of an escape of the plasma from the region between the dense structure and anode, during the interruption of the constriction, and during the integration of a “plasma lobule” with the pinch column. Fast electrons and deuterons were probably accelerated at the same region, during the same period of explosions of the plasma structures with a density ranging above . The plasma evolution could be explained by a spontaneous transformation of azimutal and poloidal components of magnetic fields. The poloidal component could be self-generated during the implosion of the current sheath.

Transformation of the Pinched Column at a Period of the Neutron Production

A PF-1000 device working with a deuterium gas filling and a current on the order of 1 MA was used for studies of the pinch-column structure by means of a laser interferometric system at a period of hard X-ray (HXR) and neutron production. Three different phases of the plasma-column evolution, corresponding to the intense HXR and neutron emission, were studied for discharges with neutron yields equal to about 1011 neutrons/shot. First, the start of the stagnation of a pinch column was considered; as the second phase, the development and disruption of constrictions was studied, and as the third phase, the decrease of the plasma density in a part of the plasma column during its stagnation was considered. Regions of the probable electron and ion acceleration and possible neutron production were identified.

Correlation of Radiation With Electron and Neutron Signals Taken in a Plasma-Focus Device

In the PF 1000 plasma-focus device, deuterium is used as a filling gas for the study of fast neutrons (originated from D-D fusion reactions) and X-rays. The X-ray signals have two peaks. The first peak corresponds to the time of the minimum diameter of the pinch phase, as recorded by the visible frames. The second peak has its maximum 150 to 200 ns later. The electrons with energy above a few hundreds of kiloelectronvolts are registered mostly at the first peak in both axial directions. Upstream and downstream electrons differ in their intensity (ratio 3 : 1), temporal profile, and time of their maximum. The energy of the neutrons and the time of their generation are determined by the time-of-flight method using six or seven scintillation detectors positioned in the axial direction. Each neutron pulse has a dominant portion of beam-target origin with downstream energies up to 3.2 MeV and the final portion of the neutrons with energies in the range of 2.2 to 2.7 MeV. The evolution of the neutron pulses correlates with the visible frames. The first pulse correlates with the fast downstream motion of the intense radiating axis layer of the pinch and with the forming and existence of the radiating ball-shaped structure at the bottom of the dilating plasma sheath. The second neutron pulse correlates with the exploding of the plasma after the second pinching, and with the forming and existence of the structure of the dense plasma at the bottom of the dilating current sheath, which is similar to the first pulse

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

The interaction of laser driven jets with gas puffs at various pressures is investigated experimentally and is analyzed by means of numerical tools. In the experiment, a combination of two complementary diagnostics allowed to characterize the main structures in the interaction zone. By changing the gas composition and its density, the plasma cooling time can be controlled and one can pass from a quasiadiabatic outflow to a strongly radiation cooling jet. This tuning yields hydrodynamic structures very similar to those seen in astrophysical objects; the bow shock propagating through the gas, the shocked materials, the contact discontinuity, and the Mach disk. From a dimensional analysis, a scaling is made between both systems and shows the study relevance for the jet velocity, the Mach number, the jet-gas density ratio, and the dissipative processes. The use of a two-dimensional radiation hydrodynamic code, confirms the previous analysis and provides detailed structure of the interaction zone and energy repartition between jet and surrounding gases.

-Pinch

Supersonic jets propagation over considerable distances and their interactions with surrounding media is one of the important subjects in astrophysics. Laboratory-created jets have completely different scales, however, typical velocities are the same, and the similarity criteria can be applied to scale them to astrophysical conditions. Moreover, by choosing appropriate pairs of colliding plasmas, one can fulfil the scaling conditions for the radiation emission rates. In this paper we present the results of studies of interaction of laser-created jets with gas-puff plasmas at the PALS laser facility. By varying the gas pressure and composition, the nature of the interaction zone changes from a quasi-adiabatic outflow to a strongly radiation cooling jet. The fine scale structures of the interaction zone are studied by means of optical and x-ray diagnostics, and they are interpreted with a semi-analytical model and 2D radiation hydrodynamic simulations. The conclusions from the laboratory experiment are scaled to the astrophysical conditions.

Studies of supersonic, radiative plasma jet interaction with gases at the Prague Asterix Laser System facility

Laser-produced multimaterial jets have been investigated at the Prague Asterix Laser System laser [K. Jungwirth et al., Phys. Plasmas 8, 2495 (2001)]. The method of jet production is based on the laser-plasma ablation process and proved to be easy to set up and robust. The possibility of multimaterial laboratory jet production is demonstrated and complex hydrodynamic flows in the jet body are obtained. Two complementary diagnostics in the optical ray and x-ray ranges provide detailed information about jet characteristics. The latter are in agreement with estimates and two-dimensional radiation hydrodynamic simulation results. The experiment provides a proof of principle that a velocity field could be produced and controlled in the jet body. It opens a possibility of astrophysical jet structure modeling in laboratory.