This paper (paper I) presents the first part of results obtained with the PF-1000 facility for the first time at its upper energy limit (≈1 MJ). Special attention is paid here to plasma (‘pinch’) dynamics, which was investigated in relation to its electro-technical and radiation (especially neutron) characteristics with the help of a number of diagnostics, both time-integrated and with nanosecond temporal resolution. In these methods we utilized a Rogowski coil for the routine electro-technical measurements, visual multi-frame and streak cameras, soft x-ray pin-hole multi-frame cameras, PIN-diode assembly and PM tubes with scintillators for soft and hard x-rays as well as for neutron investigations together with a set of activation counters. In particular, the temporal cross correlation of different phenomena taking place during the discharge was investigated. The pinch's longevity appears to be 10–15 times larger than the ideal magnetohydrodynamic growth time (ratio of the pinch radius to the ion thermal velocity). It is demonstrated how the ‘target’ dynamics (pinch plasma of the dense plasma focus (DPF)) depends on and may be controlled by the electrode's size and the geometry of the chamber in this large-scale device. Diffraction of a shock wave together with a current sheath on an obstacle made at the DPF anode cap opens an opportunity for an inertial electrode to be used in future at larger DPF devices.

No abstract

The PF-1000 plasma focus was modified by adding the cathode disk 3 cm in front of the anode. This modification facilitated the evaluation of neutron energy spectra. Two neutron pulses were distinguishable. As regards the first neutron pulse, it lasted 40 ns during the plasma stagnation and it demonstrated high isotropy of neutron emission. A peak neutron energy detected upstream was 2.46±0.02 MeV. The full width of neutron energy spectra of 90±20 keV enabled to calculate an ion temperature of 1.2 keV. These parameters and a neutron yield of 109 corresponded to theoretical predictions for thermonuclear neutrons.

No abstract

Plasma focus experiments were carried out at a modified PF-1000 where the cathode disc was added in front of the anode. Experimental results indicated a fraction of thermonuclear neutrons on the mega-ampere current level. In order to prove the thermonuclear mechanism, the time of neutron production and the neutron energy spectrum were measured by time-of-flight (TOF) diagnostics. Neutron TOF signals showed that the neutron production was a multiphase process and more than one mechanism occurred simultaneously. The occurrence of the thermonuclear mechanism was most evident during the plasma stagnation at low deuterium pressures. At low filling pressures, the narrow width of the neutron energy spectra demonstrated an ion temperature of about 1 keV. The possibility of thermonuclear neutrons was studied also after the stagnation, during the main neutron emission. In this case, the thermonuclear mechanism could be verified by calculating the number of deuterons that participate in the fusion process. For the bulk of thermonuclear plasmas, a significant fraction of plasma should participate in fusion. Finally, the basic consideration of the thermonuclear mechanism in Z-pinches showed the reasonableness of the MagLIF concept.

The application of a mixture of nitrogen and deuterium for the gas-puffing along the anode axis in deuterium plasma-focus discharges, as carried out at megaampere-level currents, enabled observations of the filamentary structure, and the decrease in the transformation velocity of the plasma column to be performed. It made possible to investigate the instability evolution during the production of hard X-rays and fast neutrons in more detail. The constriction of a plasma column transforms itself during the final phase of the compression into one or more small dense plasmoid-like structures which are separated by narrow necks. During the next phase, these structures start to decay by an expansion, in which a part of the plasma volume maintains its compactness. This evolution is explained by an increase and later decrease in the internal poloidal current component by reconnections of the associated magnetic lines, which are responsible for the acceleration of electron and ion beams.

No abstract

This paper reports on experiments undertaken to compare the radiation resistance of two types of ceramics, boron nitride (BN) and pure alumina (Al 2 O 3 ), which are used in a TAEA antenna coil installed in the MAST spherical tokamak. Samples of the investigated materials (bulk BN and a 20 µm film of Al 2 O 3 on Al substrate) were exposed on the axis of the plasma-focus PF-1000 device, which can emit intense streams of hot plasma (v ≈ 10 7 cm s −1 and N pl ≈ 10 18 cm −3 ) and fast deuteron beams (E i ≈ 100 keV). The most powerful plasma-ion pulse lasted 0.2-1.0 µs and its intensity decayed in about 100 µs. The irradiation process was diagnosed using fast optical cameras, laser interferometry and optical spectrometry. Experiments were performed at power flux densities equal to 10 9 -10 10 W cm −2 or 10 8 -10 9 W cm −2 during the most powerful stage of the interaction process. The irradiated specimens were investigated by means of optical microscopy and x-ray structure analysis (XRSA). It was shown that at 10 10 W cm −2 pulses the Al 2 O 3 coating was completely evaporated, whereas a surface of the BN sample became smoother than in the virgin one. A direct comparison of both samples after the action of 10 8 W cm −2 pulses demonstrated a wave-like structure (more distinct on Al 2 O 3 ). Weighing of these samples showed, however, that the evaporation of BN was about two times stronger than that of Al 2 O 3 in spite of the lower irradiation flux; the XRSA showed no evidence of cracking of Al 2 O 3 after these pulses. The insulation properties of Al 2 O 3 did not decline, and the Al 2 O 3 coating may be potentially more beneficial, provided that it is kept below its melting point. Characteristic features of damages of a material based on the carbon-fiber composite with additions of silicium carbide (SiC; 8-40% volumetric) were also investigated. It was found that at q = 10 9 W cm −2 , the surface erosion is associated with sputtering and evaporation. The degree of this erosion depends on the fibers' orientation in relation to the direction of the plasma-ion streams, and on the percentage of the SiC admixture.