Neutron Emission Characteristics of Pinched Dense Magnetized Plasmas (original) (raw)
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IEEE Transactions on Plasma Science
Plasma in a pinch column, as produced by a plasma-focus discharge at the deuterium filling and the current intensity reaching 1 MA, was investigated at the total neutron yield reaching about 10 10 per discharge. The use was made of neutron diagnostics, laser interferometry, soft X-ray measurements, optical emission spectroscopy, magnetic probes, as well as electron and ion measurements with the temporal, spatial, and energetic resolutions. The detailed studies showed the ordered toroidal, helical, and plasmoidal structures which could contain currents with poloidal and toroidal components and their associated magnetic fields. Their spontaneous transformations were explained by changes in a topology of magnetic field lines due to magnetic reconnections. A nonthermal acceleration of fast electrons and ions (producing hard X-rays and fusion neutrons, respectively) corresponded to: 1) the formation of plasmoids in the pinch column and 2) a decay of pinch constrictions and secondary plasmoids during the evolution of instabilities. A filamentary structure of the current flow could explain the high energy density and fast transformations of the magnetic energy into kinetic energy of electron and ion beams (reaching energy of hundreds of kiloelectronvolt). This paper summarizes the results obtained with the PF-1000 facility in 2009-2017, and describes the internal transformations in a dense plasma column during the evolution of MHD instabilities.
Physics of Plasmas, 2017
The paper concerns important differences in the evolution of plasma column structures during the production of fusion neutrons in the first and subsequent neutron pulses, as observed for plasmafocus discharges performed with the deuterium filling. The first neutron pulse, of a more isotropic distribution, is usually produced during the formation of the first big plasmoid. The next neutron pulses can be generated by the fast deuterons moving dominantly in the downstream direction, at the instants of a disruption of the pinch constriction, when other plasmoids are formed during the constriction evolution. In both cases, the fusion neutrons are produced by a beam-target mechanism, and the acceleration of fast electron-and deuteron-beams can be interpreted by transformation and decay of the magnetic field associated with a filamentary structure of the current flow in the plasmoid.
Physics of Plasmas
This paper concerns the evolution of internal structures and the neutron production in plasma-focus discharges performed in the presence of a permanent magnet (placed inside the anode front) and within a residual magnetic field (after the removal of this magnet). The initial magnetic field generated by this magnet prevented: (i) the effective compression of a dense pinch column, (ii) the formation of plasma organized structures, and (iii) the evolution of plasma instabilities. The experimental results have shown an increase in the initial magnetic field due to a magnetic dynamo effect in the presence of the permanent magnet, as well as in a series of shots performed after its removal. It was observed that the appearance of plasmoidal structures is necessary for the emission of fusion neutrons. A characteristic quasicylindrical plasma layer of the radius corresponding to the plasma lobule tops, which might be identified with a ring region of the acceleration of fast deuterons, was also observed.
Journal of Fusion Energy
Dynamical discharge characteristics and their relation with the total neutron yield emitted from a 400 J plasma focus operating in deuterium gas are presented. The dynamical nature of the plasma focus is obtained merely from the analysis of the voltage and current electrical signals without considering any particular geometry for the plasma sheath. It is calculated that large neutron yields are obtained when plasma inductance, mechanical energy and plasma voltage at pinching time have larger values. In contrast, no correlations are found among neutron yields either with plasma propagation velocities or quantities at the beginning of the radial phase. There is also found that the current sheath geometry changes according to the gas pressure, having larger curvature for lower pressures. The calculations also provide estimations of sheath thicknesses at the detachment from the insulator in the range of 0.5–1 mm, being thicker for larger neutron yield.
Shock dynamics and neutron production in an explosive generator driven dense plasma focus
Applied Physics Letters, 1982
An explosive generator driven dense plasma focus is simulated numerically using a magnetohydrodynamic model which includes thermal conduction, resistive diffusion, and radiation in addition to the Lorentz force and shock hydrodynamics. It is shown that the dominant heating mechanism is shock heating. Neutron yield is shown to occur at the current minimum. The neutron yield is a result of the initial shock reflecting off the axis of symmetry, reflecting off the magnetic piston driving the focus, and reconverging on axis. Peak neutron production occurs when post-shock plasma converges on axis.
Correlation between pinch dynamics, neutron and X-ray emission from megajoule plasma focus device
Vacuum, 2004
The paper presents the results of recent experiments performed within the PF-1000 facility. The main goal of these investigations was to determine the neutron emission characteristics, which could enable an approximate settlement of the problem of the neutron emission mechanism. Three scintillation probes, located at different angles (7 m from the electrode outlet), were used to perform time-resolved measurements of the hard X-ray radiation and neutron emission. Time-resolved X-ray signals were measured by means of PIN diodes covered with different filters, and they were compared with other traces (voltage waveforms, dI/dt signals, and neutron-induced pulses) in order to determine their correlations.
Neutron emission from a fast plasma focus of 400 Joules
Applied Physics Letters, 2003
The neutron emission from a small and fast plasma focus operating in deuterium is presented. The system operates at low energy in the hundred of joules range (880 nF capacitor bank, 38 nH, 20-35 kV, 176-539 J, ~300 ns current rise time). The neutrons were measured by means of a silver activation counter, and the total neutron yield versus deuterium gas filling pressure was obtained. For discharges operating at 30 kV charging voltage, the maximum neutron yield was (1.06+/-0.13)×106 neutrons per shot at 9 mbar.
Important issues in high - current plasma experiments of the Z - pinch type
2012
The term “linear Z-pinch” is used to denote the effect of a radial compression of the plasma column by an azimuthal magnetic field generated by the current flowing through this column. Unfortunately, the Z-pinch column is prone to various magnetohydrodynamic (MHD) instabilities. In particular, a small local narrowing developes into the so-called sausage instability (m = 0), while a small bending leads to the strong “kink instability” (m = 1), as shown in Fig. 1. Linear or quasi-linear Z-pinch discharges are observed in nature, e.g. as intense lightnings, as well as in various laboratory experiments, as presented in Fig. 2. In fact a dense Z-pinch (DZP) was one of the earliest plasma-confinement schemes examined in the quest for controlled thermonuclear fusion. The research on controlled thermonuclear fusion started in early 1950’s with experiments involving Z-pinches in deuterium. These experiments were launched almost simultaneously with the effort to construct a thermonuclear weap...
Plasma Physics and Controlled Fusion, 2020
The differences in performance of a 1.9 kJ plasma-focus device PACO assembled with three different cathode configurations are experimentally qualified. In particular, the current sheath kinetics and the neutron yield operating with deuterium gas are systematically studied for the whole range of neutron-producing pressures, and the measurements are analyzed searching for relations between relevant physical magnitudes. The pinching time was found correlated with the dimensionless driver parameter, and this feature was found statistically independent of the cathode. The variation of the inductance jump associated with the radial collapse stage is used to estimate the effective pinch length, () 7.3 1.6 mm, and radius, () 3.6 2.1 mm. The maximum production in a single shot was registered for the smallest cathode radius, 41 mm, whereas the intermediate cathode radius, 45 mm, scored better in average. In all configurations, the neutrons per deuteron pair correlates fairly well with an estimation of the effective equilibrium temperature of the pinch, which suggests a prevalence of thermonuclear neutrons measured perpendicularly to the focus axis.