The structural and morphological changes of the tungsten alloy specimen consisting of 95 % tungsten and 5 % medium-entropy alloy (VTaCrTi or VTaCrMo) are investigated after deuterium plasma irradiation. The heat flux factor of deuterium plasma pulses is comparable to transient events in ITER. The study shows that the use of medium-entropy alloys with tungsten (W-MEA) reduces the extent of surface crack generation significantly compared to pure tungsten or other tungsten alloys. The layer of decreased hardness is about 20-40 mu m, i.e., less than in pure tungsten or other tungsten alloys.

The presented paper is a continuation of the Tikhonov Regularisation, Minimum Fisher Information, Maximum Entropy, and Maximum Likelihood methods application to unfolding the neutron spectra emitted by the fusion devices from activation measurement. The aim of the analysis is the evaluation of the possible uncertainty improvement. The developed algorithms and methodology were validated with 14 MeV portable neutron generators before application to the analysis of the neutron spectrum emitted from tokamak. The analysis concerns plasma-target generator did not provide satisfactory uncertainty of the results. The modification of the irradiation foils’ set and the improvement in the activity measurement precision progress the results. The sealed tube portable neutron generator irradiated the Al, Fe, Ni, Zr, Au, Mg, and Nb foils. The HPGe spectrometer measured the induced activity with an uncertainty of about 10 %. The synthetic data analysis was made to establish the ratio between the error of the spectrum reconstruction and the discrepancy between the input data and activities reconstructed for this spectrum by the FISPACT-II inventory code. Two bin structures were considered. One was compatible with the FISPACT-II code, but the energy resolution was not regular. The second was characterised by constant bin spacing connected with the modification requirement before simulation and additional uncertainty of about 5 %. The best results were obtained by calculation of the mean value from all methods. The corresponding reconstruction uncertainty is equal to 11 %. The reconstructed spectrum has a dominant deuterium tritium peak slightly shifted to the higher energies due to the beam-target reaction kinematics. The evaluation of the impact of the deuterium-deuterium reaction due to the long-term operation was not reachable due to the measured high intensity from scattered neutrons.

Experimental and theoretical study of plasma processes affected by strong laser generated magnetic fields is reported. The PALS laser system operating at 3 omega (438.5 nm) delivered intensities up to 1 x 1016 W cm-2 on targets. By using a special target system consisting of a Cu foil connected to a sub-mm coil, magnetic fields up to the level of 10 T were generated. This is 1.4 times higher than the field generated with a 1.315 mm (1 omega) laser beam at a comparable intensity. We found that these fields were sufficiently strong to modify plasma blow-off from the foil which resulted in the changed expansion dynamics and increased energy of hot electrons (HE) by 20%-40% compared to the plasma unaffected by the magnetic field. To obtain complementary experimental data, a complex diagnostic system was used enabling the visualization of the plasma expansion process both in visible light (3-frame composite interferometry) and in the soft x-ray region (4-frame pinhole x-ray camera) together with measurements of the HE parameters using two-dimensional imaging of the K alpha line emission from the Cu target and electron spectroscopy. Experimental data obtained from the angular distribution of electron energy spectra were used for three-dimensional (3D3V) numerical PIC simulations using a modified EPOCH code. By including interactions between ions, protons, hot and thermal electrons in forward and backward propagating particles, the effects of the magnetic field on the flux of HE were visualized and compared with the experiment. The PIC simulation confirmed that the interaction of the HE flux with the magnetic field generated by the target-coil system leads to an increased flux energy. However, this increase is accompanied by increased complexity of the spatial structure and heterogeneity of the flux as well as its angular divergence.

Plasma focus devices generate discharges with dense hot plasma (HP) flows and fast ion streams (FIS) with energies up to several MeV, enabling their use to test materials for nuclear fusion applications. Tungsten plates were exposed to deuterium and to mixed deuterium-helium plasmas at distinct power flux densities at the large PF-1000U device. Elemental modifications as the induced deuterium retention and co-deposition of impurities at the target surfaces were studied by scanning electron microscopy and, particularly, by ion beam analysis (IBA). The results evidence that typically, deuterium retention is enhanced during the first discharges and may decreases afterwards due to surface erosion. Additionally, by adding helium to the working gas to generate the plasmas, deuterium retention increase. Similar results may be achieved by decreasing incident flux densities. The experiment highlights the role of the use of IBA to quantify elemental modifications imposed by irradiation of materials in plasma focus facilities.

CrNbTaVWx high-entropy alloys have been developed for plasma facing components to be applied in nuclear fusion reactors. The CrNbTaVWx (x = 1 and 1.7) compositions were prepared by ball milling and consolidated at 1600 degrees C under 90 MPa. To study the irradiation resistance of these materials, deuterium plasmas were used to irradiate the samples in the PF-1000U facility with 1 and 3 discharges. Structural changes before and after irradiation were analyzed by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. Nuclear reaction analysis was carried out with 1000 and 2300 keV 3He+ ion beams to evaluate the profile and amount of retained deuterium on the irradiated samples. After irradiation, the sample with higher W content revealed swelling and melting for all discharges, while in the case of CrNbTaVW only blisters were observed. The deuterium retention was higher for CrNbTaVW1.7 when compared with CrNbTaVW for 3 discharges applied.

The paper presents an overview of the design status of the Radial Neutron Camera (RNC), that, together with the Vertical Neutron Camera, will provide, through reconstruction techniques applied to the measured line-integrated neutron fluxes, the time resolved measurement of the ITER neutron and α-source profile (i.e. neutron emissivity, neutrons emitted per unit time and volume). The RNC is composed of two subsystems, the In-Port RNC and Ex-Port RNC located, respectively, inside and outside the Plug of Equatorial Port #01. The In-Port subsystem is in a more advanced design stage since it has recently undergone the Final Design Review in the ITER procurement process. The paper describes the diagnostic layout, the interfaces, the measurement capabilities and the main challenges in its realization. Prototyping and testing of neutron detectors and electronics components were carried out and led to the choice of the component solutions that can match the environmental and operational constraints in terms radiation hardness, high temperature and electromagnetic compatibility. The performance of the RNC in terms of neutron emissivity measurement capability was assessed through 1D and 2D reconstruction analysis. It is proven that the neutron emissivity can be reconstructed in real-time within the measurement requirements: 10% accuracy, 10 ms time resolution and a/10 (a = plasma minor radius) space resolution.

The neutron activation method provides information about the total neutron emission, yet in many applications, the energy distribution of the produced particles is also required. In this paper, codes based on the methods popular in tomography (Tikhonov Regularization, Minimum Fisher Information, Maximum Entropy and Maximum Likelihood Methods) have been developed for the reconstruction of the energy spectra of neutrons emitted by a Gas-Plasma Target Neutron Generator manufactured by a Gradel-Fusion Company. To perform the measurements by the neutron activation analysis the following samples were selected: Al, Zn, Fe, Ni, Zr and Au. Before the analysis of the experimental results, the codes were tested with synthetic data. The correlation between the reconstruction error and the FISPACT-II inventory code simulation's agreement with the measurement data have been determined. The spectrum was reconstructed by all methods separately, and the final result was defined as a mean value of them. The uncertainty of the reconstructed neutron spectra for the activities with an error higher than 10% has been estimated at the level of 22%.

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.

Joint experiments (JEs) on small tokamaks have been regularly performed between 2005 and 2015 under the framework of the International Atomic Energy Agency (IAEA) coordinated research projects (CRPs).This paper describes the background and the rationale for these experiments,how they were organized and executed,main areas of research covered during these experiments,main results,contributions to mainstream fusion research,and discusses lessons learned and outcomes from these activities.We underline several of the most important scientific outputs and also specific outputs in the education of young scientists and scientists from developing countries and their importance.

A calibration procedure is proposed for ITER Radial Neutron Camera that relies on embedded sources, reference ITER pulses and cross-calibration with ITER fission chambers and activation system coupled to Monte Carlo simulations of radiation transport. The proposed procedure would allow to measure the neutron emissivity profile and of the fusion power with 10 % accuracy and precision, a time resolution of 10 ms and a spatial resolution of a/10 for ITER entire life-time.

Results are presented from laboratory simulations of plasma jets emitted by young stellar objects carried out at the plasma focus facilities. The experiments were performed at three facilities: the PF-3, PF-1000U and KPF-4. The operation modes were realized enabling the formation of narrow plasma jets which can propagate over long distances. The main parameters of plasma jets and background plasma were determined. In order to control the ratio of a jet density to that of background plasma, some special operation modes with pulsed injection of the working gas were used.

The paper is aimed at optimization of parameters of a copper plasma jet produced at the DPF-1000U device, in which the inner electrode face was conically shaped. Preliminary information was obtained by numerical simulations of the plasma jet creation for different copper cones with the use of the two-dimensional magneto-hydrodynamic code KAROL. The simulations suggested that the cone height in the range of 4-7 cm should ensure a good plasma jet quality. The experimental data delivered by means of a 16-frame laser interferometer and a four-frame X-ray pinhole camera fully confirmed this conclusion. In the paper, we demonstrate the results for a 5 cm height cone. The eroded Cu plasma, swept up by the deuterium plasma sheath, was accelerated axially and compressed to very small diameter (3 mm) with an electron density of 7 x 10(18) cm(-3). The Cu plasma jet achieved a velocity of 5 x 10(7) cm/s and reached in the period of about 230 ns a distance (length) of 7 cm. The above results prove a successful adaptation of the plasma focus device to the metallic plasma jet generator.

A process of irradiating and ablating solid-state targets with hot plasma and fast ion streams in two Dense Plasma Focus (DPF) devices - PF-6 and PF-1000 was examined by applying a number of diagnostics of nanosecond time resolution. Materials perspective for use in chambers of the mainstream nuclear fusion facilities (mainly with inertial plasma confinement like NIF and Z-machine), intended both for the first wall and for constructions, have been irradiated in these simulators. Optical microscopy, SEM, Atomic Emission Spectroscopy, images in secondary electrons and in characteristic X-ray luminescence of different elements, and X-ray elemental analysis, gave results on damageability for a number of materials including low-activated ferritic and austenitic stainless steels, beta-alloy of Ti, as well as two types of W and a composite on its base. With an increase of the number of shots irradiating the surface, its morphology changes from weakly pronounced wave-like structures or ridges to strongly developed ones. At later stages, due to the action of the secondary plasma produced near the target materials they melted, yielding both blisters and a fracturing pattern: first along the grain and then "in-between" the grains creating an intergranular net of microcracks. At the highest values of power flux densities multiple bubbles appeared. Furthermore, in this last case the cracks were developed because of microstresses at the solidification of melt. Presence of deuterium within the irradiated ferritic steel surface nanolayers is explained by capture of deuterons in lattice defects of the types of impurity atoms, pores and oxycarbonitride particles existed in the material. (C) 2016 Elsevier B.V. All rights reserved.

The time-varying inductance of the plasma-electrode system in the large plasma focus (PF) device PF1000 is experimentally determined from voltage and current-time derivative signals. The signals were acquired in several shots performed at the same conditions (2.4 mbar of D 2 , 24 kV). Using these results, the temporal evolution of the voltage drop on the pinch column is also assessed. The maximum values of these voltage drops exceed 100 kV in all the shots.

The paper presents results of experimental studies of dense and high-temperature plasmas, which were produced by pulsed high-current discharges within a modernised PF-1000U facility operated at different initial gas conditions, and supplied from a condenser bank which delivered energy of about 350 kJ. The investigated discharges were performed at the initial deuterium filling under pressure of 1.6-2.0 hPa, with or without an additional puffing of pure deuterium (1 cm(3), under pressure 0.15 MPa, at instants 1.5-2 ms before the main discharge initiation). For a comparison discharges were also performed at the initial neon filling under pressure of 1.1-1.3 hPa, with or without the addition of deuterium puffing. The recorded discharge current waveforms, laser interferometric images, signals of hard x-rays and fusion neutrons, as well as time-integrated x-ray pinhole images and time-resolved x-ray signals were compared. From a ratio of the x-ray signals recorded behind beryllium filters of different thickness there were estimated values of a plasma electron temperature (T-e) in a region at the electrode outlets. For pure deuterium discharges an averaged T-e value amounted to 150-170 eV, while for neon discharges with the deuterium puffing it reached 330-880 eV (with accuracy of +/- 20%).

System Level Design and Performances of the ITER Radial Neutron Camera D. Marocco, B. Esposito, G. Brolatti, M. Cecconello, D. N. Dongiovanni, O. Ficker, J.Kotula, A. Mancini, J. Mlynar, F. Moro, F. Belli, D. Bocian, C. Centioli, S. Conroy, F. Crescenzi, N. Cruz, L. Di Pace, A. Hjalmarsson, R. Kantor, D. Marzullo, G. Mazzone, R. Miklaszewski, F. Pompili, M. Riva, R. C. Pereira, S. Podda, A. Zimbal, R. Barnsley, L. Bertalot, V. Krasilnikov, A. Loarte, S. D. Pinches, A. Polevoi and B. Brichard. ENEA C. R. Frascati, Dipartimento FSN, Frascati, Italy. Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden. 3Institute of Plasma Physics of the Czech Academy of Sciences, Prague, Czech Republic. Institute of Nuclear Physics Polish Academy of Sciences, Kraków, Poland. Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal. 6 Consorzio di Ricerca per l'Energia e le Applicazioni Tecnologiche dell'Elettromagnetismo, Napoli, Italy. Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland 8 Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. 9ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, 13067 St. Paul Lez Durance Cedex, France Fusion for Energy, Barcelona, Spain E-mail contact of main author: daniele.marocco@enea.it

This paper aims at a test of a plasma focus (PF) device as a metallic plasma jet generator. The experiment was carried out at the DPF-1000U device in which the inner electrode face was conically shaped. Deuterium (D-2) with the initial pressure of 0.9 Torr was used as a filling gas. In the experiment, a metallic plasma was ensured due to erosion of the inner electrode during a PF discharge. To create the metallic plasma jet, the eroded copper (Cu) plasma, swept by the deuterium plasma sheath, was accelerated axially and compressed to very small radius (about 1-2 mm). The Cu plasma jet achieved a velocity of 3 x 10(7) cm/s. To study processes of the plasma jet creation and propagation a 16-frame laser interferometer and a fourframe X-ray pinhole camera were used. Recorded images prove a successful adaptation of the PF device to the metallic plasma jet generator.

This paper presents results of experimental studies of tungsten samples of 99.95% purity, which were irradiated by intense plasma-ion streams. The behaviour of tungsten, and particularly its structural change induced by high plasma loads, is of great importance for fusion technology. The reported measurements were performed within a modified PF-1000U plasma-focus facility operated at the IFPiLM in Warsaw, Poland. The working gas was pure deuterium. In order to determine the main plasma parameters and to study the behaviour of impurities at different instants of the plasma discharge, the optical emission spectroscopy was used. The dependence of plasma parameters on the initial charging voltage (16, 19 and 21 kV) was studied. Detailed optical measurements were performed during interactions of a plasma stream with the tungsten samples placed at the z-axis of the facility, at a distance of 6 cm from the electrode outlets. The recorded spectra showed distinct WI and WII spectral lines. Investigation of a target surface morphology, after its irradiation by intense plasma streams, was performed by means of an optical microscope. The observations revealed that some amounts of the electrodes material (mainly copper) were deposited upon the irradiated sample surface. In all the cases, melted zones were observed upon the irradiated target surface, and in experiments performed at the highest charging voltage there were formed some cracks.

This work reports on recent experiments performed at the modernized PF-1000U plasma-focus facility. In contrast to earlier studies the main attention was focussed on measurements of the soft x-ray emission. Detailed time-integrated x-ray measurements, carried out using filtered pinhole cameras with sensitive x-ray films, are presented and analysed. The fine structure of the collapsing current sheath and dense pinch column is investigated. Observations of 'plasma filaments' are discussed and compared with those from the old POSEIDON facility. New results are time-integrated x-ray images of PF-1000U discharges with additional gas puffing, which in many cases show distinct plasma filaments and/ or 'hot spots' formed inside the dense pinch column. The formation of such 'hot-spots' is explained by necking and breaking of the plasma filaments. Results of time-resolved x-ray measurements, performed outside the experimental chamber by means of scintillation probes, and inside with PIN-diodes placed behind pinholes and absorption filters, are also presented Time-resolved measurements, carried out using an old XUV framing-camera and a new soft x-ray four-frame camera (SXRFFC), are also presented and discussed. Correlations of the time-integrated x-ray images (of plasma filaments and hot spots) with time-resolved x-ray signals are discussed. The hypothesis that plasma-current filaments appear in almost all PF-type discharges is supported by pictures of radial erosion tracks on the anode front-plate after many discharges.

Summary form only given. The paper reports on recent measurements of the X-ray emission from a large plasma-focus PF-1000U facility [1] and compares them with results from the old Poseidon device [2]. The most important results of time-integrated measurements, which were performed by means of an X-ray pinhole camera, are presented and analyzed. Attention is paid to studies of a fine structure of the collapsing current sheath and dense pinch column. Observations of plasma filaments and so-called “hot spots” are discussed. The first observation of plasma filaments inside the PF pinch column [3] was later confirmed by some small-scale PF experiments, but in the large PF-1000U facility plasma filaments have for the first time been recorded recently. Other new results are time-integrated X-ray images of PF-1000U discharges with the gas puffing, which in some cases show very distinct “hot spots” formed outside the dense pinch core. They are explained by local neckings of spiral current filaments which surround the central pinch column.The paper reports also on time-resolved X-ray measurements, which were carried out by means of scintillation detectors coupled with fast photomultipliers, and by means of PINdiodes shielded with different absorption filters. Particular attention is paid to correlations of time-integrated X-ray images and time-resolved X-ray signals recorded during the recent PF-1000U experiments. Using collimators, which look at different parts of the PF pinch column, it has been possible to observe some delayed X-ray signals. Comparing these signals with time-integrated X-ray images it was possible to determine X-ray sources, and identify “hot spots” which have been formed later than others. It enabled dynamics of the “hot spots” formation to be investigated. A ratio of X-ray signals behind different filters has been used to estimate an electron temperature. The reported X-ray measurements are of importance not only for learning about physical phenomena in PF discharges, but also for possible applications of intense X-ray pulses.