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 research and development effort related to PF devices was started nearly half a century ago in Russia [3] and USA [15], and later also in Europe [14,17], Asia [10] and Africa as well. Large plasma focus devices were operated among others at the Kurchatov Institute, Moscow, Russia (since 1962), the Lebedev Institute, Moscow, Russia (since 1968), at Los Alamos, USA (since 1964), at CEA Limeil in France (1968–1979), in Frascati, Italy (1970–1982), and at the Institut fur Plasmaforschung (IPF) in Stuttgart, Germany (1970–1995). In 1997 the International Centre for Dense Magnetized Plasmas (ICDMP) was established in Warsaw, Poland, and the Fusion Laboratory has recently been founded in Australia [11]. Although many questions regarding the efficient operation of PF devices for fast ion, fast electron, X-ray and neutron emission had been solved, there remains still a lot to be done to properly understand and control processes taking place, especially at the beginning and the end of a discharge. The variation in the proportion of neutrons produced in thermonuclear reactions and beam-target interaction and their scaling with the input energy are still a subject of investigation. This paper concentrates on the neutron emission of PF devices. In particular, results on the detection of time-integrated and time-resolved fusion-produced (fast) neutrons using activation methods, nuclear track detectors, Diagnostics and scaling of fusion-produced neutrons in PF experiments Hellmut Schmidt

The total current Itotal waveform in a plasma focus discharge is the most commonly measured quantity, contrasting with the difficult measurement of Ipinch. However, yield laws should be scaled to focus pinch current Ipinch rather than the peak Itotal. This paper describes how Ipinch may be computed from the Itotal trace by fitting a computed current trace to the measured current trace using the Lee model. The method is applied to an experiment in which both the Itotal trace and the plasma sheath current trace were measured. The result shows good agreement between the values of computed and measured Ipinch.

Summary form given only. The plasma focus PF-1 (MX) device at IPPLM Warsaw is convenient for the study of mechanism of D-D reaction due to a relative high neutron yield and horizontal position of a discharge axis. Scintillation detectors could be placed at the distances between 7 and 85 m from the neutron source in both directions, i.e. upstream and downstream. This geometry gives unique opportunity to determine the time and velocity of produced neutrons and compare them with X-rays, fast electrons and visible, X-ray and shadow frames. The dominant part of neutrons is generated by beam-target mechanism of deuterons moving from the anode in the downstream direction'. small part of neutrons with the energy above 2.45 MeV is detected upstream as well. This contribution deals with a few problems. The first one is the possibility of the existence of thermonuclear mechanism and/or the beam-target mechanism upstream. The second discussed problem concerns to the correlation of neutron production with the necking of the instability and forming of a dense and hot structure at heal of the umbrella-shape current sheath. The third problem concerns to differences in temporal distribution of fast electrons (energies 50-400 keV) registered with Cherenkov detector downstream, upstream and side-on. The fourth question concerns to the temporal evolution of hard X-rays detected with scintillation detectors outside the discharge chamber (photon energies above 100 keV) and inside the chamber (photon energies above 7 keV). Then we discuss the possibility of heating of the dense structure by the fast deuterons producing at the necking. Finally we describe diagnostics which is being prepared for future study, i.e. scintillation detectors placed side-on for more detailed description of the radial distribution of neutrons energies and shadow laser diagnostics with a few beams for more detail visualization of dense plasma structures during neutron production.

The conventional tapered anode of the NX2 repetitive plasma focus device has been changed to two other shapes (straight and spherical) to investigate the effect of anode shape on neutron emission characteristics of the device. The newly designed beryllium counter and 3 He proportional counter are used for estimation of neutron yield and time resolved neutron emission characteristics are measured using two PMT based plastic scintillator detectors located at different radial distances from the anode axis. It is found that the straight anode mostly produces multiple neutron peaks, the spherical anode on the other hand essentially has a single neutron peak in its signal while the tapered anode has a mixed behavior. It is found that there is no significant difference in the average neutron yield (typically of the order of 10 7 neutrons per shot) for different anode shapes. However the pressure at which the maximum neutron yield occurs depends on the anode shape. The anode with spherical tip shows the most stable neutron emission with consistently good neutron yield over a much wider pressure range than the other anode shapes.

In the plasma focus discharge with the total neutron yield 10 10 – 10 11 about 70% of neutrons was generated by deuterons moving with the dominant velocity component in the downstream direction. About 10% of fast deuterons moved with dominant velocity component in the direction upstream and minimally 20% had the dominant side-on velocity component. We evaluated energy distribution of deuterons producing neutrons and we discussed a possibility of neutron production from the dense and hot structure, which forms at the beginning of the stagnation phase and transforms to the sphere at the heel of the umbrella-shape current sheath and finally we discussed the possibility of heating of this dense structure by the fast deuterons.

The NX2 plasma focus has been operated in repetitive mode to investigate the effect of repetition rate on neutron emission stability of th e device. For novel applications, requiring short pulses of neutrons, the stability of the shot to shot neutron yield may also be important. The stability of neutron emission for a range of re petitive rates from 1/5 to 3 Hz has been investigated. The shot to shot yield was estimated using a beryllium activation counter which detects neutrons with energy >1 MeV. The shot to shot variation for 50 focus shots were analyzed for a given repetition rate. Shot to shot variation in neutron yield is high with typical standard deviation approximately equal to average y ield which is similar to the one found in single shot foci. It was, however, found that there was no significant change in average yield over 50 shots without gas change.

One outstanding property of pinch plasmas (Z-pinch, plasma focus, capillary discharge) is the generation of intensive very high energy electron and ion beams caused by high-amplitude electromagnetic fields. High-temperature dense magnetized plasmas are produced in high-current electrical discharges of various geometries by fast magnetic and shock compression. If the discharge is operated in deuterium gas, fusion reactions take place. It has been found from neutron emission characteristics that most of the fusion reactions are from the beam target and not from thermonuclear processes. Ion trajectories within and outside the plasma can be described by a generalized gyrating particle model (GPM). The results of space-, time-, and energy-resolved measurements of fusion neutrons (2.45 MeV) and protons (3 MeV), obtained from a 500-kJ plasma focus, are presented. Results are interpreted based on the GPM. Index Terms-Neutron sources, neutron spectroscopy, plasma focus, plasma measurements, plasma pinch.

Substantial progress has been made in various diagnostics for investigating results of experiments during the last years performed with the PF1000 device at IPPLM, Warsaw, Poland. In addition to standard diagnostics of the electrical characteristics of up to 1 MJ discharges in a Mather type plasma focus geometry, such as high speed photography, X-ray, fast electron beam and time-integrated neutron measurements, there have been made among others at least three quite successful efforts recently: 1) Setting up of a neutron time-of-flight line with up to five scintillation detectors including optical fibre based data collection equipment, 2) Use of a Mechelle spectrometer with CCD registration and possibility to take time-resolved spectra with resolutions down to 100 ns and 3) Setting up and using pinhole cameras equipped with solid state nuclear track detectors for the detection of fusion generated protons. Correlations of emission events as well as plasma and electrical current dynamics are investigated. Neutron emission characteristics and fusion products production mechanisms are discussed considering a generalized beam target model, called Gyrating Particle Model.

A fast-neutron beryllium activation counter has been constructed for neutron measurements on a high repetition rate deuterium plasma focus. Beryllium activation is especially suitable for measurements of DD neutron yields. The cross section for the relevant reaction, Be9(n,α)He6, results in a maximum sensitivity at the characteristic energy of the DD neutrons (∼2.5MeV) and practically no sensitivity to neutrons with energies <1MeV. The short half-life (0.8s) for the decay of He6 enabled the shot-to-shot neutron yield from the plasma focus to be measured for repetition rates from 0.2to3Hz (and for a range of deuterium gas pressures). With careful analysis, the shot-to-shot yield can be measured up to a maximum repetition rate of 3Hz, beyond which the pileup of counts from the previous shots reduces the accuracy of the measurements to an unacceptable level. This new beryllium activation counter has been cross-checked against an indium activation counter to obtain absolute neutron yields. At a charging voltage of 12.5kV (bank energy of 2.2kJ), the average neutron yield was found to be (7.9±0.7)×107 per shot (standard deviation of 4×107). It was found that activation of the plasma focus construction materials (especially aluminum) must be taken into account.

In the PF 1000 plasma-focus device, deuterium is used as a filling gas for the study of fast neutrons (originated from D-D fusion reactions) and X-rays. The X-ray signals have two peaks. The first peak corresponds to the time of the minimum diameter of the pinch phase, as recorded by the visible frames. The second peak has its maximum 150 to 200 ns later. The electrons with energy above a few hundreds of kiloelectronvolts are registered mostly at the first peak in both axial directions. Upstream and downstream electrons differ in their intensity (ratio 3: 1), temporal profile, and time of their maximum. The energy of the neutrons and the time of their generation are determined by the time-of-flight method using six or seven scintillation detectors positioned in the axial direction. Each neutron pulse has a dominant portion of beam-target origin with downstream energies up to 3.2 MeV and the final portion of the neutrons with energies in the range of 2.2 to 2.7 MeV. The evolution of the neutron pulses correlates with the visible frames. The first pulse correlates with the fast downstream motion of the intense radiating axis layer of the pinch and with the forming and existence of the radiating ball-shaped structure at the bottom of the dilating plasma sheath. The second neutron pulse correlates with the exploding of the plasma after the second pinching, and with the forming and existence of the structure of the dense plasma at the bottom of the dilating current sheath, which is similar to the first pulse.

The spatial distribution of DD-fusion neutrons emitted by the Plasma Focus Device PF 1000 at the Institute of Plasma Physics and Laser Microfusion, Warsaw was studied experimentally. The measurements of neutrons with thermoluminescent (TL) dosimeters and fast-neutron moderators show a distinct emission anisotropy owing to the neutron scattering on the discharge chamber. The measured anisotropy is compared with the computational prediction of the emission anisotropy induced by the discharge-chamber structure. Introduction The principle of a Dense Plasma Focus Device operation lies in the conversion of energy collected in a capacitor bank into electromagnetic acceleration and compression of short-lived plasma which becomes a source of X-rays, charged particles and nuclear fusion netrons, when operated in deuerium. The measurement of the neutron emission is affected by the neutron interaction with the discharge chamber, surrounding equipments and laboratory walls etc. [1]. The goal of our contribution is the observation of the anisotropy of the neutron emission by the Plasma Focus 1000 facility operated at optimal conditions of deuterium pressure and electrode configuration. Exper imental ar rangement The plasma was generated in the PF-1000 device discharging energy of 450-500 kJ in the deuterium with pressure of 3.5 Torr. The recorded maximum neutron yield is ... 3.5·10 34th EPS Conference on Plasma Phys. Warsaw, 2 6 July 2007 ECA Vol.31F, P-5.083 (2007)

The implosion of a deuterium plasma toward an Al wire was performed at the current amplitudes of 1.5–1.8 MA. The Al wire of 120 µm diameter and 4–5 cm length was placed on the top of the inner electrode. Pulses of XUV radiation and soft X-rays were detected at the maximum compression of the plasma focus column. They contained lines of Al VI - Al Xm ions. The presence of the Al wire on the axis of plasma focus column caused a decrease in the hard X-ray production and neutron yield to about 30–50%. The neutron pulse was characterized by the FWHM ≈ 200 ns and the yield up to 5×1010. The maximum of the neutron production occurred up to 200 ns later than the maximum of the soft X-rays.

A new method for determining the dimension of a hot spot in hot dense plasmas is presented. A time-integrated pin-hole camera was used and the experimental densitograms from the pin-hole pictures of hot spots were computationally simulated using a specific source intensity distribution function. The method was used in plasma focus DPF-78. The best estimate for the hot spot size is the effective size, the FWHM of the intensity distribution function. The effective radius of hot spots in DPF-78 varies in the range 30 - .

Abstract Space- and time-resolved studies of high-energy (> 50 keV) deuteron beams emitted from 3 different Mather-type plasma focus facilities of energy capacity ranging from about 10 kJ to above 200 kJ are presented. In deuteron beams space-resolved with a pinhole camera a spike structure is observed. In quasi-monoenergetic, e.g. 305±20 keV deuterons chosen from a Thomson-spectrograph image there appear up to 3 separate pulses of 8–10 ns FWHM, separated by 30–40 ns intervals. Some pulses demonstrate a fine spike structure with FWHM

The fine structure of a soft (0.8–3 keV) X-ray emitting zone in a 280 kJ neutron optimized plasma focus is investigated. Distinct continuous filaments are registered during the pinch phase. Time-resolved X-ray measurements show that the filamentation exists to ≈ 20–50 ns after the maximum compression. A comparison of these observations with those performed previously with a 56 kJ plasma focus device is also presented.

The structure of the final minimum-energy state of the focus plasma was studied, using Taylor's relaxation theory (1974). A superposition of the reversed pinch field and the field of eddies was obtained. Similar structures could be observed experimentally.