Plasma Focus Experiments Research Articles

Increase in the neutron yield from a dense plasma-focus experiment performed with a conical tip placed in the centre of the anode end

The paper describes the evolution of self-organized structures inside a pinched plasma column during the phase of the effective production of fusion neutrons, as observed in the mega-ampere plasma focus experiment performed with a conical tip placed in the centre of the anode face. In a comparison with the plane anode face configuration, the described anode shape facilitated transformations in the pinch column during the neutron production and increased the neutron yield several times. Simultaneously, it decreased the minimal diameter and the length of the pinched column, and it depressed the first neutron pulse. It also induced shorter pulses of X-rays and neutrons, which enabled the determination of a temporal difference between the emission of electron and deuteron beams. The fast electrons were produced mainly during a disruption of the pinch constriction, while the fast deuterons – during the formation and explosion of plasmoids. The paper also presents the temporal evolution of a current distribution in the plasmoid during the neutron production, as well as the appearance and stable positions of current filaments traces upon the surface of the conical anode tip.

Measurements of fast electron beams and soft X-ray emission from plasma-focus experiments

Abstract The paper reports results of the recent experimental studies of pulsed electron beams and soft X-rays in plasma-focus (PF) experiments carried out within a modified PF-360U facility at the NCBJ, Poland. Particular attention was focused on time-resolved measurements of the fast electron beams by means of two different magnetic analyzers, which could record electrons of energy ranging from about 41 keV to about 715 keV in several (6 or 8) measuring channels. For discharges performed with the pure deuterium filling, many strong electron signals were recorded in all the measuring channels. Those signals were well correlated with the first hard X-ray pulse detected by an external scintillation neutron-counter. In some of the analyzer channels, electron spikes (lasting about dozens of nanoseconds) and appearing in different instants after the current peculiarity (so-called current dip) were also recorded. For several discharges, fast ion beams, which were emitted along the z-axis and recorded with nuclear track detectors, were also investigated. Those measurements confirmed a multibeam character of the ion emission. The time-integrated soft X-ray images, which were taken side-on by means of a pinhole camera and sensitive X-ray films, showed the appearance of some filamentary structures and so-called hot spots. The application of small amounts of admixtures of different heavy noble gases, i.e. of argon (4.8% volumetric), krypton (1.6% volumetric), or xenon (0.8% volumetric), decreased intensity of the recorded electron beams, but increased intensity of the soft X-ray emission and showed more distinct and numerous hot spots. The recorded electron spikes have been explained as signals produced by quasi-mono-energetic microbeams emitted from tiny sources (probably plasma diodes), which can be formed near the observed hot spots.

Study of soft X-ray emission during wire array implosion under plasma focus conditions at the PF-3 facility

Results of measurements of soft X-ray emission with photon energies of <1 keV under conditions of a plasma focus (PF) experiment are presented. The experiments were carried out at the world’s largest PF device—the PF-3 Filippov-type facility (I ⩽ 3 MA, T/4 ≈ 15–20 µs, W0 ⩽ 3 MJ). X-ray emission from both a discharge in pure neon and with a tungsten wire array placed on the axis of the discharge chamber was detected. The wire array imploded under the action of the electric current intercepted from the plasma current sheath of the PF discharge in neon. The measured soft X-ray powers from a conventional PF discharge in gas and a PF discharge in the presence of a wire array were compared for the first time.

Recent measurements of soft X-ray emission from the DPF-1000U facility

Abstract Soft X-ray imaging is a very useful diagnostic technique in plasma-focus (PF) experiments. This paper reports results of four experimental sessions which were carried out at the DPF-1000U plasma-focus facility in 2013 and 2014. Over 200 discharges were performed at various experimental conditions. Measurements were taken using two X-ray pinhole cameras with a line of sight perpendicular to the z-axis, at different azimuthal angles (about 20° and 200°), and looking towards the centre of the PF-pinch column. They were equipped with diaphragms 1000 μm or 200–300 μm in diameter and coated with filters of 500 μm Al foil and 10 μm Be foil, respectively. Data on the neutron emission were collected with silver activation counters. For time-resolved measurements the use was made of four PIN diodes equipped with various filters and oriented towards the centre of the PF-column, in the direction perpendicular to the electrode axis. The recorded X-ray images revealed that when the additional gas-puff system is activated during the discharge, the stability of the discharge is improved. The data collected in these experiments confirmed the appearance of a filamentary fine structure in the PF discharges. In the past years the formation of such filaments was observed in many Z-pinch type experiments. Some of the recorded X-ray images have also revealed the appearance of the so-called hot-spots, i.e. small plasma regions of a very intense X-ray emission. Such a phenomenon was observed before in many PF experiments, e.g. in the MAJA-PF device, but it has not been investigated so far in a large facility such as the DPF-1000U. The time-resolved measurements provided the evidence of a time lapse between the X-ray emission from plasma regions located at different distance from the anode surface. The formation of distinct ‘hot-spots’ in different instants of the DPF-1000U discharge was also observed.

Absolute determination of the neutron source yield using melamine as a neutron detector

A new approach to absolute determination of the neutron source yield is presented. It bases on the application of melamine (C3H6N6) to neutron detection combined with Monte Carlo simulations of neutron transport. Melamine has the ability to detect neutrons via 14N(n, p)14C reaction and subsequent determination of 14C content. A cross section for this reaction is relatively high for thermal neutrons (1.827 b) and much lower for fast neutrons. A concentration of 14C nuclei created in the irradiated sample of melamine can be reliably measured with the aid of the accelerator mass spectrometry (AMS). The mass of melamine sufficient for this analysis is only 10 mg. Neutron detection is supported by Monte Carlo simulations of neutron transport carried out with the use of MCNP-4C code. These simulations are aimed at computing the probability of 14C creation in the melamine sample per the source neutron. The result of AMS measurements together with results of MCNP calculations enable us to determine the number of neutrons emitted from the source during the irradiation of melamine.The proposed method was applied for determining the neutron emission from a commercial 252Cf neutron source which was independently calibrated. The measured neutron emission agreed with the certified one within uncertainty limits. The relative expanded uncertainty (k=2) of the absolute neutron source yield determination was estimated at 2.6%.Apart from calibration of radionuclide neutron sources the proposed procedure could facilitate absolute yield measurements for more complex sources. Potential applications of this methodology as it is further developed include diagnostics of inertial confinement fusion and plasma-focus experiments, calibration of neutron measurement systems at tokamaks and accelerator-based neutron sources as well as characterization of neutron fields generated in large particle detectors during collisions of hadron beams.

Calibration methodology for proportional counters applied to yield measurements of a neutron burst

This paper introduces a methodology for the yield measurement of a neutron burst using neutron proportional counters. This methodology is to be applied when single neutron events cannot be resolved in time by nuclear standard electronics, or when a continuous current cannot be measured at the output of the counter. The methodology is based on the calibration of the counter in pulse mode, and the use of a statistical model to estimate the number of detected events from the accumulated charge resulting from the detection of the burst of neutrons. The model is developed and presented in full detail. For the measurement of fast neutron yields generated from plasma focus experiments using a moderated proportional counter, the implementation of the methodology is herein discussed. An experimental verification of the accuracy of the methodology is presented. An improvement of more than one order of magnitude in the accuracy of the detection system is obtained by using this methodology with respect to previous calibration methods.

Plasma Focus Studies in Serbia

The plasma focus experiment in Belgrade, Serbia started in the late eighties of the last century. The historical overview of the research activity on the Belgrade plasma focus device (BPFD) will be presented in this work. The special attention has been made to the present status and the future plans for the fundamental and applied research as a part of the project of the studies of rare nuclear and particle processes in nature. BPFD is intended to operate as optimized neutron source or hard X-ray source. Using Lee model code as a reference, several upgrades of BPFD must be made: better shielding against EMI pulse, rearrangement of capacitors bank so that higher repetition rate can be achieved and also faster digital acquisition system. BPFD can be used for neutron activation or production of short-living radioisotopes. These radioisotopes will have very low activity which can be analyzed in the underground Low-Background Laboratory for Nuclear Physics, Zemun. Also, we compared the obtained experimental data (neutron yield, total current waveform, working gas pressure) with the numerical simulation code (The Lee model code) to test our plasma focus machine. Comparison between neutron yield from our experimental data and neutron scaling laws and neutron yields derived from computation using the Lee Model code shows good matching, but for better verification of the code, more experimental data are needed.

Improvement of time-of-flight methods for reconstruction of neutron energy spectra from D(d,n)3He fusion reactions

This paper deals with time-of-flight diagnostics of relatively short neutron pulses from tens to hundreds of nanoseconds. The aim is to determine the time evolution of energy spectra. The diagnostic setup consists of several neutron detectors placed in one direction at different distances. This approach is improved by placing additional detectors in the opposite direction. How the reconstruction method works with two opposite directions is explained in this paper. Improvement is limited to the deuterium fusion reaction D(d,n)3He. However, the principles described here could be applied in various fusion experiments. The advantage of our approach is not only better reconstruction but also the partial elimination of the influence of scattered neutrons. The described method was applied in the processing of data from fusion experiments in the PF-1000 facility and provided time evolution of the neutron energy spectra with energy resolution of 50 keV. In these plasma-focus experiments, results indicated a non-thermal production mechanism of the predominant part of the neutrons produced.

Search for thermonuclear neutrons in a mega-ampere plasma focus

Plasma focus experiments were carried out at a modified PF-1000 where the cathode disc was added in front of the anode. Experimental results indicated a fraction of thermonuclear neutrons on the mega-ampere current level. In order to prove the thermonuclear mechanism, the time of neutron production and the neutron energy spectrum were measured by time-of-flight (TOF) diagnostics. Neutron TOF signals showed that the neutron production was a multiphase process and more than one mechanism occurred simultaneously. The occurrence of the thermonuclear mechanism was most evident during the plasma stagnation at low deuterium pressures. At low filling pressures, the narrow width of the neutron energy spectra demonstrated an ion temperature of about 1 keV. The possibility of thermonuclear neutrons was studied also after the stagnation, during the main neutron emission. In this case, the thermonuclear mechanism could be verified by calculating the number of deuterons that participate in the fusion process. For the bulk of thermonuclear plasmas, a significant fraction of plasma should participate in fusion. Finally, the basic consideration of the thermonuclear mechanism in Z-pinches showed the reasonableness of the MagLIF concept.

Fusion neutron detector for time-of-flight measurements in z-pinch and plasma focus experiments

We have developed and tested sensitive neutron detectors for neutron time-of-flight measurements in z-pinch and plasma focus experiments with neutron emission times in tens of nanoseconds and with neutron yields between 10(6) and 10(12) per one shot. The neutron detectors are composed of a BC-408 fast plastic scintillator and Hamamatsu H1949-51 photomultiplier tube (PMT). During the calibration procedure, a PMT delay was determined for various operating voltages. The temporal resolution of the neutron detector was measured for the most commonly used PMT voltage of 1.4 kV. At the PF-1000 plasma focus, a novel method of the acquisition of a pulse height distribution has been used. This pulse height analysis enabled to determine the single neutron sensitivity for various neutron energies and to calibrate the neutron detector for absolute neutron yields at about 2.45 MeV.

Time-resolved spectra of dense plasma focus using spectrometer, streak camera, and CCD combination

A time-resolving spectrographic instrument has been assembled with the primary components of a spectrometer, image-converting streak camera, and CCD recording camera, for the primary purpose of diagnosing highly dynamic plasmas. A collection lens defines the sampled region and couples light from the plasma into a step index, multimode fiber which leads to the spectrometer. The output spectrum is focused onto the photocathode of the streak camera, the output of which is proximity-coupled to the CCD. The spectrometer configuration is essentially Czerny-Turner, but off-the-shelf Nikon refraction lenses, rather than mirrors, are used for practicality and flexibility. Only recently assembled, the instrument requires significant refinement, but has now taken data on both bridge wire and dense plasma focus experiments.

Absolute measurements of fast neutrons using yttrium

Yttrium is presented as an absolute neutron detector for pulsed neutron sources. It has high sensitivity for detecting fast neutrons. Yttrium has the property of generating a monoenergetic secondary radiation in the form of a 909 keV gamma-ray caused by inelastic neutron interaction. It was calibrated numerically using MCNPX and does not need periodic recalibration. The total yttrium efficiency for detecting 2.45 MeV neutrons was determined to be f(n) approximately 4.1x10(-4) with an uncertainty of about 0.27%. The yttrium detector was employed in the NX2 plasma focus experiments and showed the neutron yield of the order of 10(8) neutrons per discharge.

Experimental Study on the Importance of Accurate Neutron Angular Distribution Measurement for Determination of Neutron Yield in a 2.5 kJ Mather type PF

Three activation counters were constructed by thin wall Geiger and were placed in different angels with respect to anode axis; 0°, 45°, 90°; where the Geigers have 20 cm distance from anode top of the 2.5 kJ SBUPF1 plasma focus device. The counters were calibrated by a 5 Ci Am–Be neutron source with source removal method. A computer program receives the Results of each plasma focus experiment and background count rates, estimates the angular distribution of neutron emission and calculates the neutron yield of each shot. The neutron yield of the device was measured about 6 × 107 neutrons per shot. The results, indicate that using three counters (instead of one counter in 0°) in different angles for determination of total neutron yield, gives a more accurate measurement (up to 28% is measured in sample shots), and this error is bigger in those shots that thermonuclear fusion mechanism have greater share in neutron production.

Measurements of neutron yield from PF-1000 device by activation method

Silver and indium-detectors have been used since the very beginning of Plasma Focus experiment to measure the total neutron yield and neutron angular distribution. This paper reports on application of other materials to measure neutron yield in Plasma Focus experiments. The obtained results are quite encouraging and the determined neutron emissions are comparable with those obtained by means of the silver activation counters. In order to measure a neutron yield from PF-1000 device an activation method has been applied. Several activation materials have been used including Hf, In, Cd, Ni, Ti and Au. The activity of produced nuclides has been measured after activation allowing to identify those nuclides and γ-lines. Next the respective nuclear reactions have been identified. The neutron yield from PF-1000 has been estimated based on measured activity assuming point, isotropic and mono-energetic (2.45 MeV) source. Each of the foil has been activated during “shots”. These obtained results have been compared with another neutron diagnostics.

Fast valve and nozzle for gas-puff operation of dense plasma focus

A simple and reliable valve and nozzle system for a very fast injection of gas has been designed and constructed for its use in gas-puff mode of dense plasma focus experiments. It delivers a very quick rise time: 55μs. The pressure measured in our setup at a distance of 15 mm from the nozzle output is about 0.285 mbar, with a plenum pressure of 3 bars (absolute). The time between the valve aperture and pressure front arrival is 360μs. This result comes up as an average of about a hundred measurements. The energy input is 95 J (270 V on a 3000μF capacitor bank). The typical dimensions of the valve are 52 mm in diameter and 80 mm in length. The entire volume of the valve is, then, very small. The relative low pressure and voltage operation are significant advantages of this development. The performance of the valve satisfactorily fulfills the objectives of gas-puff plasma focus operation.

X-ray polarization studies of plasma focus experiments with a single hot spots

In high current pulse discharges of the plasma focus (PF) type, inside the collapsing pinch column, there are formed local micro-regions of high-density and high-temperature plasma, so-called hot spots. Individual hot spots are separated in space and time. Each hot spot is characterized by its specific electron concentration and temperature, as well as by the emission of x-ray lines with different polarization. When numerous hot spots are produced it is impossible to determine local plasma parameters and to interpret the polarization effects. To eliminate this problem this study was devoted to the realization of PF-type discharges with single hot spot only. It has been achieved by a choice of the electrode configuration, which facilitated the formation of a single hot spot emitting intense x-ray lines. At the chosen experimental conditions it was possible to determine local plasma parameters and to demonstrate evident differences in the polarization of the observed x-ray lines.

Preliminary neutron experiments with the PF-1000 plasma-focus facility

The paper reports on first neutron-emission experiments carried out with the modernized PF-1000 facility. Current sheath dynamics was investigated by means of high-speed cameras. Based on smear pictures, which were taken through a narrow radial slit, the compression velocity was determined. The total neutron yield and angular distribution anisotropy were measured with four silver-activation counters placed at different angles to the electrode axis. It was found that for each initial D/sub 2/-filling pressure value, with an increase in the charging voltage (V/sub 0/), the neutron yield increases to some maximum and with further increase in V/sub 0/ the neutron emission decreases. The highest neutron yield registered so far has been 2/spl middot/10/sup 11/ per shot. The neutron emission anisotropy seems to be lower than that registered in other plasma-focus experiments. The neutron energy spectra, which were measured by means of a scintillating probe in the upstream direction, have been shifted toward lower energy (2.2-2.3 MeV).

Enzyme Activation and Inactivation Induced by Low Doses of Irradiation

Activation phenomenon has been observed with two sets of enzymes under the conditions of low dosage irradiation. Activation was registered for angiotensin-converting enzyme under in vitro γ-irradiation (0.662 MeV, pulse duration approx 10s) at dose levels of 1–3 Gy and under X-ray irradiation (approx 9 keV, pulseduration approx 10−9s) at dose levels of 2×10−5 Gy. An activation effect has also occurred for native and recombinant horseradish peroxidase and tobacco peroxidase under γ-irradiation. The phenomenon observed is rationalized in terms of a kinetic model suggesting the existence of at least one activated enzyme conformation induced by radiolysis. The activity oscillations registered in dense plasma focus experiments were rationalized using the same model with the corresponding kinetic equation converted into the form describing the decaying oscillations caused by exciting force. The model analysis is presented.

Electron beam effects on the spectroscopy of multiply charged ions in plasma focus experiments

Argon–hydrogen mixture plasma focus experiments performed at the Warsaw Institute of Plasma Physics and Laser Microfusion show detailed space resolved spectra for Ar K-shell satellite lines up to F-like Ar and K-alpha of Ar. These transitions originating from autoionizing levels are caused by collisions of ions with the energetic electron beams which are created by the constrictions of the plasma column due to the development of magnetohydrodynamic instabilities. A collisional-radiative model was constructed using a non-Maxwellian eletron energy distribution consisting of a thermal Maxwellian part plus a Gaussian part to represent the high-energy electron beam. The shapes of the observed satellite structures are consistent with the calculated spectrum for electron temperatures between 20 and 230 eV, and beam densities of about 10 -3 times the plasma electron density.

System for Time-Resolved Observation of Impulsive Soft X-Ray Sources and Its Application to Plasma Focus Experiment

A soft X-ray pinhole camera system for time-resolved, one-dimensional (1D) measurement of an impulsive source whose energy is in the range from 1 keV to 20 keV is developed. The system is based on the filtered pinhole camera. The pinhole image is divided into six channels in the direction of the electrode axis through slits and bundled fiber optics. Each channel is detected with a scintillator-photomultiplier combination. The temporal resolution of this system is 5.2 ns. The absolute sensitivity of the channel is ∼4.2×10-20 A/[photons/(mrad2·s)]. This system was successfully used to observe the time evolution of the soft X-ray source generated with a plasma focus device with a gas-puffed target.