Electron and ion beam dynamics of the PF-1000 facility were investigated for the first time at its upper energy limit (≈1 MJ) in relation to neutron emission, the pinch's plasma ('target') characteristics and some other parameters with the help of a number of diagnostics with ns temporal resolution. Special attention was paid to the temporal and the spatial cross correlations of different phenomena. Results of these experiments are in favour of a neutron emission model based on ion beam-plasma interaction with three important features: (1) the plasma target is hot and confined during a few 'inertial confinement times'; (2) the ions of the main part of the beam are magnetized and entrapped around the pinch plasma target for a period longer than the characteristic time of the plasma inductive storage system and (3) ion-ion collisions (both fusion collisions, due to head-on impacts and Coulomb collisions) are responsible for neutron emission. Analysis has shown that one of the ways for achieving a future improvement in the neutron yield of the PF-1000 facility may by changing the geometry of the device. It may ensure an increase in both the discharge current and the initial working gas pressure, eventually resulting in the neutron yield boost.

The paper presents a comparison of the main characteristics and experimental results obtained in plasma focus (PF) experiments in the POSEIDON 500 kJ facility in Stuttgart and the PF-360 kJ device in Świerk. Parameters of various electrodes and insulators are given, and studies on the evolution of the discharges are summarized. Selected data on X-ray, ion and neutron emission are given. Also presented are recent experimental results — a maximum neutron yield of up to 2.5 × 1011 for 500 kJ/80 kV runs with a new ceramic insulator in POSEIDON and an average neutron yield of 1.2 × 1011 for operation at 171 kJ/36 kV in PF-360. Particular attention is paid to the neutron scaling and the saturation effects observed at higher energy and current levels. Proposals are made for new experimental studies which can facilitate further progress in PF research.

Angular-and energy-distributions of ions emitted from different plasma-focus (PF) devices of energy capacity ranging from 3.6 kJ to about 200 kJ are presented. Also compared are space-and timeresolved ion signals obtained from different PF facilities. Comparative studies show some similarity features in the ion emission characteristics.

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This paper is a sequel to the 1998 review paper “Scientific status of the Dense Plasma Focus” with 16 authors belonging to 16 nations, whose initiative led to the establishment of the International Center for Dense Magnetized Plasmas (ICDMP) in the year 2000. Its focus is on understanding the principal defining characteristic features of the plasma focus in the light of the developments that have taken place in the last 20 years, in terms of new facilities, diagnostics, models, and insights. Although it is too soon to proclaim with certainty what the plasma focus phenomenon is, the results available to date conclusively indicate what it is demonstrably not. The review looks at the experimental data, cross-correlated across multiple diagnostics and multiple devices, to delineate the contours of an emerging narrative that is fascinatingly different from the standard narrative, which has guided the consensus in the plasma focus community for several decades, without invalidating it. It raises a question mark over the Fundamental Premise of Controlled Fusion Research, namely, that any fusion reaction having the character of a beam-target process must necessarily be more inefficient than a thermonuclear process with a confined thermal plasma at a suitably high temperature. Open questions that need attention of researchers are highlighted. A future course of action is suggested that individual plasma focus laboratories could adopt in order to positively influence the future growth of research in this field, to the general benefit of not only the controlled fusion research community but also the world at large.

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