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.

The results of studies of the plasma-current sheath structure on the PF-1000 facility in the stage close to the instant of pinch formation are presented. The measurements were performed using various modifications of the calibrated magnetic probes. Studies of the influence of the probe shape and dimensions on the measurements accuracy were done. The current flowing in the converging sheath at a distance of 40 mm from the axis of the facility electrodes was measured. In the optimal operating modes, this current is equal to the total discharge current, which indicates the high efficiency of current transportation toward the axis. In such shots a compact high-quality sheath forms with shock wave in front of the magnetic piston. It is shown that the neutron yield depends on the current compressed onto the axis. This dependence agrees well with the known scaling, Y n ∼ I 4 . The use of the total discharge current in constructing the current scaling, especially for facilities with a large stored energy, is unjustified.

Z-pinch experiments with deuterium gas puffs have been carried out on the GIT-12 generator at 3 MA currents. Recently, a novel configuration of a deuterium gas-puff z-pinch was used to accelerate deuterons and to generate fast neutrons. In order to form a homogeneous, uniformly conducting layer at a large initial radius, an inner deuterium gas puff was surrounded by an outer hollow cylindrical plasma shell. The plasma shell consisting of hydrogen and carbon ions was formed at the diameter of 350 mm by 48 plasma guns. A linear mass of the plasma shell was about 5 µg cm −1 whereas a total linear mass of deuterium gas in single or double shell gas puffs was about 100 µg cm −1 . The implosion lasted 700 ns and seemed to be stable up to a 5 mm radius. During stagnation, m = 0 instabilities became more pronounced. When a disruption of necks occurred, the plasma impedance reached 0.4 Ω and high energy (>2 MeV) bremsstrahlung radiation together with high energy deuterons were produced. Maximum neutron energies of 33 MeV were observed by axial time-of-flight detectors. The observed neutron spectra could be explained by a suprathermal distribution of deuterons with a high energy tail. Neutron yields reached 3.6 × 10 12 at a 2.7 MA current. A high neutron production efficiency of 6 × 10 7 neutrons per one joule of plasma energy resulted from the generation of high energy deuterons and from their magnetization inside plasmas.

The magnetic field distribution substantially affects mechanisms for the generation of radiation in Z-pinches. Investigation of the axial component of the magnetic field is one of the important problems in plasma focus studies. The measurements of the Bz-component of the magnetic field on the PF-1000 facility were done with the multichannel absolutely calibrated probe both at the stage of plasma-current sheath radial compression and in the dense-pinch stage. In the compression stage, the axial component of the magnetic field reaches several kG that comprises ∼ 10% of the azimuthal component. The presence of the Bz field is a powerful argument in favor of the existence of closed magnetic configurations, which play an important role in the generation of neutrons.

No abstract

In this paper, the possible evolution of a pinched plasma column is presented from the results of temporally resolved measurements using a magnetic probe, interferometry and neutron diagnostics performed on the plasma focus PF-1000 device with deuterium as the filling gas. Together with the discharge axial current of about 1.5 MA a toroidal current component of the order of 100 kA was estimated in the toroidal, helical and plasmoidal structures formed within the dense plasma column. The mass inside these structures increases due to injection of the plasma from the neighborhood regions with a higher pinching pressure. This injected plasma increases the intensity of the internal magnetic field, probably through turbulent motion and the magnetic dynamo effect. The neutrons from the D-D fusion reaction, produced during the formation and decay of plasmoidal structures and constrictions, are accompanied by changes in the axial component of the magnetic field. Then, the transformation and decay of internal closed currents can contribute to the acceleration of high-energy electrons and ions.

No abstract

In this paper the results of temporally resolved measurements using calibrated azimuthal and axial magnetic probes are presented, together with interferometry and neutron diagnostics performed on the PF-1000 (IPPLM, Warsaw, 2 MA) device with a deuterium filling and 10 11 neutron yield. The probes located in the anode front at three different radial positions allow determination of the dominant part of the discharge current flows behind the imploding dense plasma layer. The current sheath is composed of both the axial and azimuthal components of the magnetic field. After reaching the minimum diameter, the current sheath continues in a radial motion to the axis and then penetrates into the dense plasma column. At the final phase of stagnation, the dominant current passes through the dense column. The probes located on the axis of the anode front registered an increase and a decrease in the pulse of the axial component of the magnetic field in correlation with the formation and decay of the dense plasmoidal structure. The estimated values of the axial component of the magnetic field at the center of the plasmoids in the first neutron pulse and close before its decay and dominant neutron production can reach 2 and 10 T; it is 10-30% of the value of the azimuthal magnetic field of the dense column boundary.