D. Klir

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

Fusion neutron detector for time-of-flight measurements in z-pinch and plasma focus experiments

We have developed and tested sensitive neutron detectors for neutron time-of-flight measurements in z-pinch and plasma focus experiments with neutron emission times in tens of nanoseconds and with neutron yields between 10(6) and 10(12) per one shot. The neutron detectors are composed of a BC-408 fast plastic scintillator and Hamamatsu H1949-51 photomultiplier tube (PMT). During the calibration procedure, a PMT delay was determined for various operating voltages. The temporal resolution of the neutron detector was measured for the most commonly used PMT voltage of 1.4 kV. At the PF-1000 plasma focus, a novel method of the acquisition of a pulse height distribution has been used. This pulse height analysis enabled to determine the single neutron sensitivity for various neutron energies and to calibrate the neutron detector for absolute neutron yields at about 2.45 MeV.

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

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.

Efficient production of 100?keV deuterons in deuterium gas puff Z-pinches at 2?MA current

Deuterium gas puff experiments were carried out on the S-300 Z-pinch at the Kurchatov Institute in Moscow. Gas puffs imploded onto the axis before a current peak at about 100 ns. Fusion neutrons were generated after the gas puff implosion during global expansion of a plasma column. Neutron emission lasted on average 35 ± 5 ns (full width half maximum, FWHM). In the downstream direction (on the Z-pinch axis behind the cathode), a mean neutron energy was 2.6 ± 0.1 MeV. Side-on neutron energy spectra peaked at 2.40 ± 0.05 MeV with about 600 ± 150 keV FWHM. A broad width of side-on neutron spectra implied a high radial component of deuteron velocities. An average kinetic energy of fast deuterons, which produced fusion neutrons, was 150 keV. A peak neutron yield reached a value of 6 × 1010 on a current level of 1.5 MA. It was by one order higher in comparison with other deuterated loads used on the same current generator. On the basis of experimental observations, we concluded that a total energy of deuterons accelerated to fusion energies was above 1.5 kJ. It is more than 15% of the energy input into a plasma. Therefore gas puff Z-pinches seem to be not only powerful sources of x-ray radiation but also efficient sources of 100 keV deuterons. Such a result is consistent with high neutron yields observed on the Angara Z-pinch and plasma foci with similar currents.

Neutron emission generated during wire array Z-pinch implosion onto deuterated fiber

The implosion of both cylindrical and conical wire arrays onto a deuterated polyethylene fiber was studied on the S-300 pulsed power generator [A. S. Chernenko et al., Proceedings of the 11th International Conference on High Power Particle Beams (Academy of Science of Czech Republic, Prague, 1996), p. 154]. Neutron measurements were used to obtain information about acceleration of fast deuterons. An average neutron yield approached 109 on the current level of 2MA. In the case of conical wire arrays, side-on neutron energy spectra peaked at 2.48±0.05MeV with 450±100keV full width at half-maximum. In the downstream direction, the peak neutron energy and the width of a neutron spectrum were 2.65±0.10MeV and 350±100keV, respectively. The total number of fast deuterons was 1015 and their average kinetic energy was about 150keV. Most of the deuterons were directed toward the cathode. The broad width of neutron spectra in the side-on direction implied a high radial component of deuteron velocity. With regard to ...

Anisotropy of the emission of DD-fusion neutrons caused by the plasma-focus vessel

The anisotropic emission of DD-fusion neutrons emitted by the plasma focus device PF-1000 at the Institute of Plasma Physics and Laser Microfusion, Warsaw, was observed with the use of thermoluminescent dosimeters and fast-neutron moderators. The observed anisotropy is compared with the computational prediction of the anisotropy caused by the PF-1000 vessel, which scatters the expanding DD-neutrons. The resulting changes in the group energy spectrum of neutrons induced by their scattering on the discharge vessel are presented and their effect on time-of-flight spectra of neutrons is discussed.

Laboratory modeling of supersonic radiative jets propagation in plasmas and their scaling to astrophysical conditions

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.

Existence of a return direction for plasma escaping from a pinched column in a plasma focus discharge

The use of multi-frame interferometry used on the PF-1000 device with the deuterium filling showed the existence of a return motion of the top of several lobules of the pinched column formed at the pinched plasma column. This phenomenon was observed in the presence of an over-optimal mass in front of the anode, which depressed the intensity of the implosion and the smooth surface of the pinched plasma column. The observed evolution was explored through the use of closed poloidal currents transmitted outside the pinched plasma. This interpretation complements the scenario of the closed currents flowing within the structures inside the pinched column, which has been published recently on the basis of observations from interferometry, neutron, and magnetic probe diagnostics on this device.