V. Ya. Nikulin

A simple criterion for the snowplowing efficiency of the working gas in a kJ plasma focus

Correlation between the appearance of the second peculiarity in the current-derivative signal and the drop in the neutron yield is observed experimentally in a plasma focus discharge. The time-resolved photographs of the current sheath lead to the conclusion that, at the instant of maximum compression, the main current is shunted by the second current sheath, which is formed from the rest of the gas after the passage of the first one. It is also suggested that the generation of the second current sheath is a consequence of two competing processes: filamentation of the first sheath, which thus becomes transparent to the working gas, and magnetization of the residual plasma by the field of the main discharge current.

Influence of the radiation of the plasma-focus current sheath on the implosion dynamics of condensed targets

Results are presented from experimental and theoretical studies of the influence of the radiation of the plasma-focus current sheath on the implosion dynamics of condensed targets. Radiative losses from the current sheath of a plasma focus in neon, argon, and hydrogen with a 2% admixture of Xe are calculated with allowance for the line, bremsstrahlung, and recombination radiation. It is shown that the temperature of the neon plasma (10–15 eV) is quite sufficient to evaporate Al2O3 grains of radii 10–20 μm. The use of neon as a working gas makes it possible to alter the cold-start condition in experiments on the implosion of foam liners.

Compact activation detectors for measuring of absolute neutron yield generated by powerful pulsed plasma installations

A system for measuring of absolute neutron yield and its spatial anisotropy is described. The system was developed in the LPI (Lebedev Physical Institute) and destined for study properties of neutron emission of pulse plasma installations such as Plasma Focus (PF),z-pinches and others. The system consists of transported compact activation detectors and small optically isolated neutron activation counter with 4th digit data output to LCD display. The electronic unit is built on the TTL standard. In power supply of the system the small-size accumulators are used.

Study of a cumulative jet in a plasma focus discharge by the method of shearing interferometry

The dynamics of the cumulative jet formed in the course of plasma compression in a plasma focus discharge is investigated by the method of differential optical interferometry. The jet propagation velocity is found to be V = (2.3–3) × 107 cm/s, which coincides with the results of calculations performed in a 2D ideal MHD model. Ejection of matter from the anode in the late stage of the discharge due to the interaction of the cumulative jet and the electron beam with the anode surface is observed.

Laser-optical measurements of the velocities of the plasma jets formed from different gases in a kilojoule-range plasma focus facility

The velocities of the plasma jets formed from Ne, N2, Ar, and Xe gases in plasma focus facilities were determined by means of laser-optical shadowgraphy of the shock waves generated at the jet leading edge. In spite of the almost tenfold ratio between the atomic weights of these gases, the outflow velocities of the plasma jets formed in experiments with these gases differ by less than twice, in the range of (0.7–1.1) × 107 cm/s under similar discharge conditions. The energies of the jet ions were found to vary from 0.7 keV for nitrogen to 4 keV for xenon.

Application of shock waves for the improvement of current-carrying properties of YBCO(123) and Bi(2223) HTSC tapes in magnetic fields

Shock waves with a leading-edge pressure of ∼1011 Pa, which were produced in a plasma focus setup, were used to increase the critical current density in YBCO(123) and Bi(2223) HTSC tapes. It was shown that the effect of chemically inactive high-temperature high-density plasma on the HTSC tapes leads to an irreversible increase in the critical current in high magnetic fields. The improvement of the current-carrying properties of the YBCO(123) HTSC tape is confirmed also by the results of scanning Hall magnetometry at 77 K. In particular, in a field of 8 T applied perpendicular to the c axis (H ⊥ c), the increase in the critical current after shock-wave treatment is ∼60%. In the case of the Bi(2223) tape, the critical current in a zero field in the sample portion subjected to shock-wave action was found to be twice as high as that in the untreated portion (100 and 50 A, respectively). The increase in the critical current can be related to a number of possible structural transformations of the superconducting core. First of all, an increase in the density of current-carrying core, which leads to an increase in weak bonds at grain boundaries, is possible. In this case, the formation of nanosized defects, which are responsible for an increase in the force of pinning of Abrikosov vortices, is also possible.

Magnetic and neutron measurements on the PF-400 plasma focus facility

Parasitic currents shunting up to one-half of the total discharge current were detected using magnetic probes on a Filippov-type plasma focus facility with a maximum total current of 1 MA and stored energy of 80 kJ. The measured time dependence of neutron emission from the discharge indicates that the parameters of the neutron pulse are closely related to those of the imploding current sheath.

Influence of conditions of shock-wave effect of plasma on the structure and current-carrying capacity of multilayer high-temperature superconducting tapes

The article presents the results of the investigation of the influence of shock-wave effects on the structure and the critical currents of multilayer high-temperature superconductors—HTS tapes produced by the EAS-E HTS (VAC). Shock-wave exposure was carried out using an installation of a plasma focus (PF) type. It was experimentally found that an increase in the critical current by 20% or more was achieved in its own magnetic field and in external magnetic fields in the range of 0.5–2.5 T. The increase depended on the conditions of the shock-wave treatment (the distance from the plasma source (PF anode) and the number of shock wave pulses). In magnetic fields of more than 3 T, the effect of an increase in the critical current was not observed. Microstructural studies revealed both a compression and destruction of the individual layers of HTS in the strike zone depending on the conditions of the impact. The most severe degradation of the structure and the critical current was shown on the tape samples treated at distances of 25–30 mm from the PF anode. The critical current increased and exceeded the initial values of untreated tapes (75–85 A) at distances of 35–65 mm. The phase composition of HTS layers by XRD changed little after shock-wave treatment. Depending on the number of shock pulses and distance from the anode in the area of treatment, the tape’s thickness was reduced owing to compression or was increased owing to swelling of the tape.

Hydrogen transport in a niobium-foil assembly under the action of high-temperature hydrogen plasma on a plasma focus setup

The process of hydrogen transport under the action of hydrogen-plasma pulses in the Plasma Focus (PF-4) setup in an assembly of niobium foils with thicknesses of 110 μm each is studied. It is established that the implanted hydrogen is transported to large depths in the Nb foils under the action of the hydrogen plasma. These depths considerably exceed the paths of hydrogen ions from the plasma as the maximum ion velocities are on the order of 108 cm/s. The highest concentration of hydrogen (up to 60 at %) is reached on the surface of the third Nb foil of the assembly. The discovered phenomenon can be explained by the action of shock waves of arising stresses upon the transport and redistribution of hydrogen to considerable depths.

Deuterium accumulation in an assembly of nickel foils irradiated by high-temperature deuterium plasma

The accumulation of deuterium in an assembly of nickel foils by the pulsed irradiation of a deuterium plasma is studied. It is established that implanted deuterium is transferred to a much greater depth than that corresponding to the projective range of deuterium plasma ions with a maximum velocity of 108 cm/s. The maximum concentration (up to 4 at % deuterium) is observed in the second Ni foil. The observed phenomenon can be explained by the action of shock waves and concomitant stresses on the transport and redistribution of deuterium to greater depths.