Abstract The Dense Plasma Focus is being considered as the basis of sophisticated technologies such as an energy producing fusion machine, space propulsion for interplanetary and deep space missions and nanomaterial fabrication. However, there are problems with ensuring a reliable and reproducible operation of the device. It does not operate well immediately after the device is exposed to atmosphere and re-evacuated. A series of training shots is required after which the operation stabilizes until the vacuum chamber is opened again. Our previous work on numerical simulation of the formation phase shows that plasma behaviour on hydrodynamic time scales is difficult to predict because of simultaneous presence of many time scales and scale lengths. A systematic understanding of the role of the training shots in the formation process therefore requires a new experimental approach. This paper presents the first experiments looking at the symmetry of the discharge during its formation phase. The new diagnostic uses 3 d-dot probes symmetrically placed outside the squirrel cage cathode looking at the insulator through gaps between cathode rods. First results are presented and discussed.

Astrophysical jets are plasma flows, which are observed to substantially maintain their transverse size while travelling distances orders-of-magnitude larger. They are found in many astrophysical contexts, spanning several decades in energy and size, suggesting operation of an underlying scale-invariant mechanism. Similar phenomena observed in laboratory plasmas are often studied as surrogate models for astrophysical jets under the conjecture that the scale-invariance of that as-yet-unconfirmed mechanism continues to hold down to laboratory spatial and energy scales. The plasma focus is one such laboratory plasma device which offers the advantage of diagnostic accessibility at a relatively modest resource cost. The present paper uses the plasma focus to address one of the intriguing aspects of the astrophysical jet phenomenon. Theoretical models of astrophysical jets require presence of a poloidal magnetic flux but there is no observational basis for assuming its existence. Indeed, there is a fundamental theoretical impossibility of existence of poloidal magnetic flux in the natural symmetry of the jet phenomena about its axis in the context of magnetohydrodynamics. The next best evidence in support of the poloidal magnetic flux hypothesis of such theoretical models would be to look for it in surrogate experimental simulations of astrophysical jets. In this context, this paper demonstrates a new diagnostic method for detection of poloidal magnetic flux emission from a plasma focus. The results indicate that poloidal magnetic flux continues to be emitted even after the disruption of the plasma focus pinch phase and shows evidence of its being decoupled from the externally supplied discharge current. This observation is interpreted along with previous knowledgebase in terms of a conjecture regarding the scale-invariant mechanism that might also be involved in astrophysical jet phenomena.

Recent experiments using 15 frame interferometry on PF-1000 facility in Warsaw confirm the association between neutron emission and spontaneously self-organized, relatively long lasting, finite plasma structures. A crucial aspect of this association is the simultaneous observation of an axial magnetic field, which can allow magnetic flux lines to densely cover closed surfaces creating "magnetic flux surfaces". Evolution of such 3-dimensional (3-D) magnetic field structures is necessarily accompanied by induced electric field that can provide a very long (theoretically infinite) acceleration path length along a trajectory enclosed within the magnetic structure leading to high ion kinetic energy, resulting in a high reaction rate. Associated charge and current densities can be related to electric scalar potential and magnetic vector potential measured outside the plasma. We report our first observations of these fields outside the plasma focus and discuss their general features. The reported technique is capable of unambiguous first-principles interpretation of signals in terms of quantities related to distributions of charge density and rate of change of azimuthal current density ("electromagnetic structure") in the plasma focus. It is non-intrusive and completely insensitive to non-axisymmetric aspects of plasma. Our first results show that axial magnetic field generated by azimuthal current density distribution symmetric about the axis exists before, during and after the pinch phase.

The existence of axial (poloidal) magnetic field in a plasma focus and its significance in plasma focus phenomenology has been extensively discussed in a recent review paper. The poloidal magnetic field is a part of the transient 3-dimensional magnetic field structures which arise spontaneously, accelerate ions and keep them moving in trajectories that repeatedly cross a dense and warm plasma target. This is the origin of the abnormally high fusion reaction rate of the plasma focus, which has been known since the 1960s but has begun to be understood quite recently. Further progress now depends on explorations of the global aspects of the evolution of poloidal magnetic field. However, well-known experimental difficulties involved in standard techniques of axial magnetic field measurement hamper such research efforts. Taking cognizance of this stalemate, the International Scientific Committee for Dense Magnetized Plasmas launched an initiative to address this state of affairs in an International Video Conference on 24th April 2020. This paper reports on the first experiments that resulted from that initiative

A recent comprehensive review paper has summarized the current experimental state of knowledge on the question of existence of a poloidal component of magnetic field in the dense plasma focus and indirect evidence indicating its prominent role in neuron emission. This Letter looks at the role of the plasma as a curved, azimuthally continuous conductor moving in a small but nonzero geomagnetic field with a component along the device axis. The motional electric field then has an azimuthal component arising from the radial velocity, outward-directed near the cathode throughout the discharge, and inward directed in the radial implosion phase near the anode. This would drive an azimuthal current in the plasma. The electromechanical work done against the associated magnetic force density converts the kinetic energy into the associated magnetic energy density. This would act as a source of poloidal magnetic flux emission from a dense plasma focus, which has been experimentally demonstrated recently.

The purpose of this short review article is to present the results of the influence of the ionizing X-ray pulses of plasma focus on living organisms, by performing experiments in vivo and analyzing changes in some vital characteristics of the objects, such as survival ability, efficiency of photosynthesis of unicellular organisms at moderate doses (up to 65 mSv); efficiency of enzyme production, change in protein content and change in fungi mass in the radiation dose range 7 mSv ÷ 45 Sv; changes in mammalian blood characteristics and appearance of signs of anemia due to the significant dose load.

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.

Now the chances to get the energy released by the transformation of nuclei (fusion and fission) are significantly larger than they were 10 or 20 years ago due to the development of the hybrid nuclear reactors. They can provide energy with abundant resource, safe, and clean at reasonable cost. The research in nuclear fusion shows that several of the present day plasma devises (both stationary and pulse) have the potential to become the background of a hybrid (fusion-fission) industrial facility for energy production. They will burn the high-level radioactive waste, thus closing the nuclear cycle and will hinder the spread of hazardous materials. A breakthrough in nuclear power is expected in the near future, whichever of the three technologies - fast neutron reactors, accelerators driven systems or fusion-fission hybrid reactors will prove to be the most technologically or economically viable base for hybrid reactors.

In order to increase the biosynthetic capacity of the Trichoderma reesei-M7 strain – a notorious cellulase enzyme producer – the experimental biological object to be studied was exposed to the impact of the highly energetic X-ray pulses, produced by the dense plasma focus device (DPF). The X-ray pulses thus produced can have exceptionally high density of the flow of X-photons. The pulsed mode of irradiation fundamentally differs from the continuous when considering its effect on biological objects – they can rapidly absorb great amounts of radiation in a glimpse, leading to a qualitatively different interaction with their matter. In the experiments done the survivability was determined, as well as the vegetative biomass, four types of cellulase activity and the quantity of total protein after the treatment of the spores suspension (2.105 CFU/ml) of Trichoderma reesei M7 with this pulsed X-radiation. The absorbed doses were in the range of 7 to 45000 mSv. It was noticed that the survivability of the strain, if compared with the control samples, drops, reaching 57% of the initial value (of the untreated spores) for the sample with 45010 mSv of absorbed dose. From the data presented below it can be concluded that three of the samples with high absorbed doses, namely with 11754, 19439 and 42917 mSv, demonstrated increases their cellulase activities. In the case of 42917 mSv the increment of the endoglucanase activity, compared to the control, reaches 41,1%, FPA rises with 30,3%, and the β-glucosidase activity nearly doubles – and increase of 92,7%.

Detection of small concentrations of atomic Tungsten is of crucial importance for optimizing the construction of the divertor in the modern Tokamak devices. In this contribution we report on the development of spectral method for diagnostics of neutral W atoms. The tungsten atoms are sputtered in a pulsed hollow cathode discharge. Atoms are detected by their fluorescence induced by the second harmonic of a pulsed dye laser. The laser pulse may be applied with a controlled delay after the discharge pulse and thus the decay of the atomic concentration in time may be recorded. The expected sensitivity of the method is below 10(8) atoms/cm(-3) and presently we are working on the absolute calibration of detected signals. The present status of the experimental setup, the data processing system and latest results will be reported.

This paper is a short review on the principles, experimental devices and projects of fusion-fission nuclear plants for energy production, burning of the high radioactive nuclear waste and production of fuel for the conventional fission reactors

Nowadays there is a revival of interest towards the plasma focus study due to some yet not studied problems of the phenomena and the growing number of applications. Quite important can be the application of the PF systems in the radiobiology. These include studies of the impact of radiation emitted by the PF discharge on live microorganisms, living cells or other biologic objects. We report the results of the influence of X-ray radiation of 3 kJ PF of Sofia University on 3 types of living organisms. 1. A study of cell viability Saccharomyces cerevisiae – (yeast) after irradiation. The samples with the cells were irradiated by total dose of the X-ray radiation 65 mSv (determined by the TLD detectors placed upon the metal foil near the sample). No difference of the viability between the control probe and the sample was observed after irradiation. Similar results have been obtained before that with another type of yeast Kluyveromyces marxiamus, where no change of the survival activity was found after irradiation through a thick foil. 2. The irradiation of the Chlamydomonas reinhardtii samples by the X-ray emission of the PF through 20 μm Al foil with a dose of 11 mSv produces a considerable change of the photosynthesis parameters. Therefore this result is similar to the results from the previous works where strong effects of enzyme activity were derived with low doses but with a high dose power ensured by the plasma focus radiation. 3. Bioconversion of cellulose-containing substrate to glucose represents an important area of the modern biotechnology. Enzymes for the degradation of the polysaccharide part of biomass have been produced, mostly by fungi belonging to the genus Trichoderma. Trichoderma reesei M7 shows exceptionally high resistibility in respect to high doses of absorbed hard X-radiation, (mainly characteristics W Kα1 and Kα2 lines obtained using W inserts on the PF anode) with the vitality of spores practically unaltered – experiments were conducted with doses as high as 32 Sv. The analysis of the endogluconase activity and the measurement of the residual biomass and proteins quantities have shown that for moderate doses absorbed (200÷1200mSv) the effect of the micromycette producer is clearly expressible. We are studying whether the further increment of the dose has some measurable change of the parameters in question. *blagoev@phys.uni-sofia.bg

This paper presents the first results for irradiation of tungsten, molybdenum and stainless steel samples with the 4 kJ plasma focus (PF) device at the University of Sofia. The samples were placed 4 cm above the anode of the PF machine and were exposed to a considerable number of shots. The working gas was deuterium with the pressure adjusted in the range of 1-3.3 mbar. Thus, the plasma streams and the fast-ion beam, which appear after the pinch phase, impinge the samples. The interaction of the pinch products with the targets causes substantial surface damage to the specimens. A mesh of partially melted cracks and re-crystallized regions are revealed on this surface and various chemical compounds are also present.

Bioconversion of cellulose-containing substrate to glucose represents an important area of modern biotechnology. Enzymes for the degradation of the polysaccharide part of biomass have been produced, mostly by fungi belonging to genus Trichoderma. Studies were carried out with the mutant strain Trichoderma reesei-M7, a cellulase producer. Spores of the enzyme producer were irradiated with different doses of characteristic X-ray radiation from metallic tungsten (mainly the W Kα1 and Kα2 lines) with a high dose rate. The latter is a specific property of the dense plasma focus (DPF) device, which has pulsed operation and thus gives short and highly energetic pulses of multiple types of rays and particles. In this case, we focused our study on the influence of hard X-rays. The doses of X-rays absorbed by the spores varied in the range of approximately 5-11,000 mSv measured with thermoluminescent dosimeters (TLD). The influence of the applied doses in combination with exceptionally high dose rates (in the order of tens of millisieverts per microsecond) on the activity of the produced endoglucanase, amount of biomass and extra-cellular protein, was studied in batch cultivation conditions. In the dose range of 200-1200 mSv, some enhancement of endoglucanase activity was obtained: around 18%-32%, despite the drop of the biomass amount, compared with the untreated material.

The goal of this contribution is to present time-resolved optical spectroscopy studies of laser ablation of the Mo target with similar to 3.5 ns, 0.4 J pulses delivered by the Nd-YAG laser system at 1.06 mu m. The sample was placed in a vacuum chamber under 5x10(-5) mbar pressure and irradiated, with power densities varied up to 22.7 GW cm(-2). The ion emission from the plasma plume was measured using an electrostatic ion energy analyzer (IEA) and ion collector, which allowed us to estimate the ion kinetic energy and charge independent of the applied power densities. The signal collected by the IEA indicated the presence of molybdenum ions up to eight-ion charge. Simultaneously after the ion emission, the optical spectra acquired within 2 mu s of exposure time were observed in the wavelength range from 200 to 1000 nm with a Mechelle 5000 spectrometer equipped with an iCCD (iStar) detector. The plasma electron temperature was estimated from a Boltzmann plot based on the registered spectra as well as from the ion measurements.

In the experiments with the 3 kJ plasma focus device in the Sofia University are measured the basic characteristics, namely the discharge current, the current derivative, the soft X-ray and the hard X-ray emission from the plasma. By a set of magnetic probes the velocity of the current sheath during the run-down axial phase is determined. In the last two thirds of the axial phase the current layer velocity is a constant.

A 3 kJ plasma focus device was used to study the influence of the soft X-ray on live microorganisms. When Saccharomyces cerevisiae – (yeast) was treated with a dose of 65 mSv of the X-ray radiation (14 shots), no difference in the fertility activity between the control probe and the sample was observed. Also no change in the survival enzyme activity was found after irradiation through a 100 μm Al foil of another type of yeast – Kluyveromyces marxiamus. The irradiation of the Chlamydomonas reinhardtii samples by the PF-X-ray emission through 20 μm Al foil with a dose of 11 mSv produces a considerable change of the photosynthesis parameters. This result is similar to the results of previous studies with plasma focus radiation where strong effects were derived with low doses but with a high dose power.

In the experiments with the 3 kJ plasma focus device at the Sofia University are measured the basic characteristics, namely the discharge current, the current derivative, the soft X-ray and the hard X-ray emission from the plasma. The average dose of the X-Ray emission is a few tenth of Sv per shot. A study of the influence of soft X-rays upon biologic objects has been started. PACS codes: 52.70.La, 87.53.-j 1 Basic Measurements with the Plasma Focus Device 1.1 Experimental setup In the Faculty of Physics at the University of Sofia a Mather’s type dense plasma focus device (DPF) is in regular operation from about one and a half year. In the experiments with this device it is possible to obtain the quantitative results of the typical processes in a small DPF machines. As every usual DPF, it consists of a capacitor bank, fast switch and vacuum chamber, containing the electrode system. The capacitor bank has capacitance of 20 μF, with the maximal DC voltage of 40 kV. The insulator is molten quartz; the main switch is a vacuum spark gap. Only a brief description of the device will be presented here. More details for constructions and principles of operation of different PF machines are given in [1]. A procedure for optimizing the discharge conditions was carried out, changing the charging voltage and the gas pressure. So far the experiments were conducted in air. Thus the voltage operating range is 15–18 kV, and the nominal pressure is in the range of 1.0–2.0 mBar. 184 1310–0157 c © 2011 Heron Press Ltd. S. Zapryanov, V. Yordanov, A. Blagoev Figure 1. Mather’s type electrode system; hollow anode, cathode rods and quartz insulator. We are recording the discharge current, the current derivative, the soft X-ray and the hard X-ray emissions from the plasma. The data, received by the diagnostic tools in the first several hundred shots fired so far, reveal the typical peculiarities known from the literature for devices of this energy range (3–5 kJ): The oscilloscope pictures give the moment of the initial breakdown of the gas, followed in a few microseconds by the occurrence of a pinch. A correlation between the observed one or more peaks of soft and hard X-rays and the peculiarities of the dI/dt signal is established, the latter corresponding to the same number of pinches with the same mutual distance in time. More than one pinch is a situation typical of a DPF, operating with relatively heavy gasses, such as Nitrogen or Argon. On the other side, when working with Hydrogen Figure 2. Photograph of a shot with the same electrode system. 185 Basic Characteristics of the Dense Plasma Focus Device (including Deuterium and Tritium) or Helium, as a rule, only a single or two pinches take place [1]. Figure 1 shows the electrode system of the device with removed discharge chamber. Figure 2 shows a photograph of the discharge taken with usual camera. The bright spots above the anode correspond to the two successive pinches of one discharge pulse. Hard X-radiation is recorded by a plastic scintillation detector with PMT 56 TVP (Phillips). From the signals of the mounted 4 PIN BPX 65 photo diodes we achieve useful information about the soft X-ray emission of the pinch plasma. In front of each photodiode metal filters with different thickness are placed (10, 20 and 30 μm Aluminium and 0.5 mm Lead, respectively), transmitting only radiation with energy above certain values. These signals are in direct accordance with the number of contractions and with the soft X-ray yield. They depend also on the operating pressure and voltage [2]. We are considering as Xray signals the sharp peaks with a half width between 100 and 200 nanoseconds appearing almost simultaneously with the hard X-ray signals. 1.2 Measurements The two scope graphs below are from the experiment with the mentioned device when air is the working gas (nitrogen is the main component). Figure 3 shows the signals from PIN diodes (blue and red trace) and signal from the scintillation detectorgreen trace. Figure 3. Signals from PIN detectors (10 μ and 20μ Al filters) a scintillation detector (green): three contractions are visible.

A dedicated gas discharge tube equipped with a hollow cathode made of tungsten is setup for sputtering of W atoms. A multi-pass White type absorption cell is designed to increase the sensitivity of the absorption method. The optical system consists of three externally mounted aluminium-coated spherical mirrors with 70 cm radius of curvature. Measurements of the cell relative transmittance in the UV and visible spectral regions are carried out. Time-resolved measurements of sputtered ground-state tungsten atoms' density in 2-pass configuration, as well as assessment of the diffusion coefficient of these atoms in argon at 350 K temperature are made.

The breakdown phase in the plasma focus (PF) discharge is modelled by the Monte Carlo-particle in cell method. A surface ionization wave is derived. This numerical result is qualitatively confirmed in an experiment, performed by a dedicated model device with a PF-like geometry.