Correlation of the neutron yield anisotropy with the electrical characteristics of a plasma focus discharge

The anisotropy of the yield and energy of neutrons generated in a small-size plasma focus chamber with a total neutron yield of about 4 × 10⁹ DD neutrons per shot was investigated experimentally. The neutrons were recorded using scintillation detectors on a 3-m-long flight base. The measurements were performed at the angles 0° and 90° with respect to the chamber axis. The maximum neutron energy measured by the time-of-flight method at the angles 0° and 90° was found to be 2.8 and 2.5 MeV, respectively. The measured anisotropy of the neutron yield was in the range 1.15–1.88. The integral DD neutron yield of the source was measured using the activation method (by activating silver isotopes). It is found that the neutron yield and the yield anisotropy depend linearly on the discharge current jump ΔI at the instant of neutron generation.     

Study of the azimuthal magnetic fields and scaling laws at the KPF-4-Phoenix facility

Results are presented from magnetic probe measurements in the pinching region formed during the compression of the plasma current sheath (PCS) in a discharge in deuterium at the KPF-4-Phoenix plasma focus facility. The fine structure (shock front-magnetic piston) of the PCS and its time evolution in the course of plasma compression toward the facility axis was studied by means of magnetic probes. It is shown that the fraction of the current transported into the axial region by the PCS does not exceed 65% of the total discharge current. The integral neutron yield Y _n is well described by the formula Y _n ≈ (1.5–3) × 10¹⁰ I _p ⁴ , where I _p (in MA) is the pinch current flowing in the region r ≤ 22 mm.     

Study of the fine structure of the plasma current sheath and magnetic fields in the axial region of the PF-1000 facility

Results of measurements of magnetic fields in the plasma pinching region during the compression of the deuterium plasma current sheath (PCS) at the PF-1000 plasma focus facility are presented. The fine structure of the PCS (shock wave-magnetic piston) and its variations in the course of plasma compression toward the facility axis are studied using magnetic probes and laser interferometry. The radial distributions of the plasma density and current in the PCS are compared. It is shown that, in the shock wave region, the electron density of the compressed plasma is on the order of ～10¹⁸ cm^?3, whereas the PCS current is almost entirely concentrated in the magnetic piston region—a plasma layer with an electron density of less than 10¹⁵ cm^?3. Efficient transportation of the current by the PCS into the axial region of the facility in discharges with a high neutron yield (Y _n > 10¹¹ neutrons/shot) is detected. It is shown that the total neutron yield is well described by the dependence Y _n ≈ (1.5–3) × 10¹⁰ I _p ⁴ , where I _p is the pinch current (in MA) flowing within the region r ≤ 13 mm.     

Development and study of a portable plasma focus neutron source

The work is devoted to designing a compact pulsed neutron source on the basis of a plasma focus (PF) discharge. The main task was to study the physical processes accompanying a sub-kilojoule repetitive PF discharge. A device with a power supply energy of up to 600 J and pulse repetition rate of up to 10 Hz has been developed and put into operation. The dependence of the neutron yield as a function of the pulse repetition rate has been studied experimentally. A neutron flux of ～10⁸ neutrons/s has been obtained in the 3-s-long packet mode with a repetition rate of 10 Hz and discharge current of 80–90 kA.     

Quasi-adiabatic dynamics of ions in a bifurcated current sheet

The study is devoted to ion dynamics in bifurcated current sheets with a two-peak current-density distribution observed in the Earth’s magnetotail and solar wind. The ion motion is described by a Hamiltonian system with two degrees of freedom. The presence of a small parameter κ characterizing the ratio between the amplitudes of the normal and tangential magnetic field components allows one to separate variables into fast and slow ones and introduce the quasi-adiabatic invariant of motion I _z . Conservation of this invariant makes it possible to analytically describe the dynamics of charged particles. Deviations of the particle dynamics from the quasi-adiabatic one, which are caused by the nonconservation of the quasi-adiabatic invariant, are investigated. The jump of the invariant ΔI _z is shown to depend on the small parameter according to the power-law ΔI _z ～ κ ^h , where the exponent h varies between unity and 3/4, depending on the level of current sheet bifurcation. The obtained dependence of ΔI _z on κ coincides with analytic expressions in the limiting cases of nonbifurcated and completely bifurcated current sheets.     

Do spherical tokamaks have a thermonuclear future?

This work has been initiated by the publication of a review by B.V.Kuteev et al., ??Intense Fusion Neutron Sources?? [Plasma Physics Reports 36, 281 (2010)]. It is stated that the key thesis of the above review that a spherical tokamak can be recommended for research neutron sources and for demonstration hybrid systems as an alternative to expensive ??classical?? tokamaks of the JET and ITER type is inconsistent. The analysis of the experimental material obtained during the last 10 years in the course of studies on the existing spherical tokamaks shows that the TIN-ST fusion neutron source spherical tokamak proposed by the authors of the review and intended, according to the authors?? opinion, to replace ??monsters?? in view of its table-top dimensions (2 m³) and laboratory-level energetics cannot be transformed into any noticeable stationary megawatt-power neutron source competing with the existing classical tokamaks (in particular, with JET with its quasi-steady DT fusion power at a level of 5 MW). Namely, the maximum plasma current in the proposed tokamak will be not 3 MA, as the authors suppose erroneously, but, according to the present-day practice of spherical tokamaks, within 0.6?C0.7 MA, which will lead to a reduction on the neutron flux by two to three orders of magnitude from the expected 5 MW. The possibility of the maintenance of the stationary process itself even in such a ??weakened?? spherical tokamak is very doubtful. The experience of the largest existing devices of this type (such as NSTX and MAST) has shown that they are incapable of operating even in a quasi-steady operating mode, because the discharge in them is spontaneously interrupted about 1 s after the beginning of the current pulse, although its expected duration is of up to 5 s. The nature of this phenomenon is the subject of further study of the physics of spherical tokamaks. This work deals with a critical analysis of the available experimental data concerning such tokamaks and a discussion of the potential possibility of their use in thermonuclear research.     

Crow’s index and reproductive characteristic of the Republic of Tatarstan population

On the basis of 1212 questionnaires filled up by women of postreproductive age living in five districts of the Republic of Tatarstan (Arsky, Atninsky, Kukmorsky, Buinsky, and Drozhzhanovsky), the basic reproductive characteristics and Crow??s index and its components have been calculated. The rural population is characterized by expanded reproduction with a mean sibship size of 2.68; in district administrative centers, there is only simple reproduction. Crow??s index and its components for the rural Tatar population are I _m = 0.026, I _f = 0.172, I _tot = 0.202. Graphic analysis of the mutual positions of some populations studied has been carried out.     

Study of X-ray and neutron emission in experiments with Al wires in an MA plasma focus

Results are presented from experimental studies of the correlation between X-ray and neutron emissions generated in the implosion of a deuteron plasma shell onto an Al wire. The experiments were carried out on the PF-1000 facility at currents of 1.5–1.8 MA. An Al wire 80 μm in diameter and 7–9 cm in length was placed at the end of the inner electrode. During the implosion of the plasma shell, Al K-shell X-rays were first emitted at the dip of the current derivative. After the X-ray pulse, a relatively stable corona with a diameter of 2–3 mm and lifetime of a few hundred nanoseconds formed around the wire. The presence of the wire did not considerably reduce the total neutron yield (at most 10¹¹ neutrons per shot) in comparison to discharges without a wire. As a rule, the intensity of neutron emission was maximal a few tens of nanoseconds after the peak of X-ray emission. A detailed comparison of two shots with low and high neutron yields have shown that the neutron yield depends on the configuration and dynamics of the discharge. The possible influence of the self-generated axial component of the magnetic field on the development of the plasma focus and the acceleration of fast deuterons is discussed.     

Measurements of the parameters of a condensed deuterated Z-pinch on the angara-5-1 facility

Results are presented from measurements of the parameters of high-temperature plasma in the Z-pinch neck formed when a current of up to 3.5 MA flows through a low-density polymer load. To enhance the effect of energy concentration, a deuterated microporous polyethylene neck with a mass density of 100 mg/cm³ and diameter of 1–1.3 mm was placed in the central part of the load. During the discharge current pulse, short-lived local hot plasma spots with typical dimensions of about 200–300 μm formed in the neck region. Their formation was accompanied by the generation of soft X-ray pulses with photon energies of E > 0.8 keV and durations of 3–4 ns. The plasma electron temperature in the vicinity of the hot spot was measured from the vacuum UV emission spectra of the iron diagnostic admixture and was found to be about 200–400 eV. The appearance of hot plasma spots was also accompanied by neutron emission with the maximum yield of 3 × 10¹⁰ neutrons/shot. The neutron energy spectra were studied by means of the time-of-flight method and were found to be anisotropic with respect to the direction of the discharge current.     

Effect of the direction of a preionization current on X-ray emission from a capillary discharge plasma

The effect of the direction of a preionization current on the generation of 469-Å X-ray emission from the plasma of a fast capillary discharge in argon was studied experimentally in the SIGNAL facility (the discharge current I = 25–40 kA and the current rise rate dI/dt ～ 10¹² A/s). The experiments were performed with 3.1-mm-diameter 157-mm-long ceramic capillaries filled with argon at a pressure of 0.2–1.0 Torr.     

Research of the small plasma focus with an auxiliary electrode at deuterium filling

The research was provided on the small plasma focus device with the Mather-type electrodes in configuration with an auxiliary electrode placed in front of the anode. It works at the current maximum of 200 kA. The total neutron yield from D-D reaction reaches 10⁵–10⁷ per one shot. The hard X-rays and neutrons were detected with scintillation detectors and the soft X-rays with a PIN detector and four MCP frames. We present preliminary results obtained from different configurations of the anode (inner electrode) and auxiliary electrode and we compare these results with that obtained in configuration without auxiliary electrode. The auxiliary electrode decreased both the neutron yield and the time of hard X-ray and neutron production. Different electrode faces has influence on the energy distribution of generated neutrons.     

A source with a 1013 DT neutron yield on the basis of a spherical plasma focus chamber

Results from preliminary experimental research of neutron emission generated by a spherical plasma focus chamber filled with an equal-component deuterium-tritium mixture are presented. At a maximum current amplitude in the discharge chamber of ～1.5 MA, neutron pulses with a full width at half-maximum of 75–80 ns and an integral yield of ～1.3 × 10¹³ DT neutrons have been recorded.     

Neutron emission during the implosion of a wire array onto a deuterated fiber

Results are presented from Z-pinch experiments performed in the S-300 facility (Kurchatov Institute) at a maximum current of 2 MA and current rise time of 100 ns. The Z-pinch load was a 1-cm-long 1-cmdiameter cylindrical array made of 40 tungsten wires with a total mass of 160 μg, at the axis of which a 100-μm-diameter (CD₂) _n deuterated fiber was installed. Hard X-ray and neutron signals were recorded using five scintillation detectors oriented in one radial and two axial directions. The maximum neutron yield from the DD reaction reached 3 × 10⁹ neutrons per shot. The average neutron energy was determined from time-of-flight measurements and Monte Carlo simulations under the assumption that the neutron emission time was independent of the neutron energy. The average neutron energy in different experiments was found to vary within the range 2.5–2.7 MeV. The fact that the average neutron energy was higher than 2.45 MeV (the energy corresponding to the DD reaction) is attributed to the beam-target collisional mechanism for the acceleration of deuterons to 100–500 keV.     

Spatial Genetic Structure of Campanula sabatia, a Threatened Narrow Endemic Species of the Mediterranean Basin

Campanula sabatia is endemic to NW Italy (Liguria) and it was included in the European Red list of endangered species due to the heavy human pressure on its habitat. AFLP markers were used to detect the genetic diversity within and among ten populations (totalling 83 individuals) representing the range of the species. In spite of the limited distribution of this endemic taxon, high levels in percentage of polymorphic bands (PPB), gene diversity (H _S and H _T) and Shannon??s information index (I) were detected both at population (PPB?=?60?%, H _S?=?0.1853, I?=?0.2836) and at species level (PPB?=?100?%, H _T?=?0.2415, I?=?0.3871). The coefficient of genetic differentiation among population (G??_ST) was 0.1935, while the level of gene flow (N?? _m) was estimated to be 2.0832. AMOVA analysis identified a genetic variation within populations of 83?% of the total. Bayesian clustering methods assigned individuals to two geographical groups partly found within the same population, probably due to a high rate of genetic exchange among its populations. Conservation measures are suggested on the basis of the genetic diversity detected to ensure an effective protection for this endemic species.     

Experimental verification of the effect of different preionization systems on the implosion dynamics of an argon double gas puff

Experiments are reported on the implosion of argon double gas puffs in the GIT-12 current generator (T _fr=0.25 μs, I _m =2.3 MA). The gas-puff medium was preionized by different methods. The experimental data provide evidence for a strong effect of the initial conditions for the formation of the current-carrying shell on the implosion process. Emphasis is given to a discussion of the following issues: the enhanced scatter in both the emission power and X-ray yield in the Ar K-lines, the existence of a large number of current filaments, the uncertainty in the process by which the generator current is redistributed among a progressively smaller number of current filaments, and the redistribution of the generator current between the inner and outer gas-puff shells. __________ Translated from Fizika Plazmy, Vol. 27, No. 7, 2001, pp. 584–591. Original Russian Text Copyright ? 2001 by Rousskikh, Baksht, Labetsky, Shishlov, Fedyunin.     

The properties of high-current high-pressure discharges

The properties of high-current high-pressure gas discharges (I = 60 kA, p = 1 atm, τ_1/2 = 1.6 ms, r ～ 10 cm) just before the end of the discharge are investigated experimentally. It is shown that the anomalously high rate of gas cooling after the current is switched off is related to the turbulent hydrodynamic processes induced by the Rayleigh-Taylor instability at the discharge boundary in the stage of volume radiative cooling of the discharge channel. The turbulent thermal conductivity is estimated using experimental data on the recovery of the electric strength of the discharge gap.     

Saturation of the neutron yield from megajoule plasma focus facilities

A relation is investigated between the saturation of the neutron yield from megajoule plasma focus facilities and that of the total discharge current. An analytic formula for the neutron yield as a function of the facility energy is derived by simple calculations of the discharge circuit and is verified by computer simulations of the dynamics of the current sheath. The dependence obtained differs from the generally accepted one but agrees well with experimental data.     

Current scaling for the radiative characteristics of a micropinch discharge

The absolute VUV and soft X-ray (hν > 100 eV) yield from a micropinch discharge is measured for a fixed current of 150 kA. The current scaling in the range of 30–250 kA is found for a number of the discharge parameters: the VUV and soft X-ray yield, the electron temperature, the effective temperature of suprathermal electrons, and the energy of bremsstrahlung emission from thermal electrons. The experimental data are in good agreement with the simulations performed by using the model of radiative collapse in fast Z-pinches in plasmas of high-Zelements.     

Design of a 500-kJ Mather-type plasma focus device

In this article, design of a 500-kJ Mather-type plasma focus device to achieve 10¹¹ neutrons/shot is reported. One of its important characteristics is the triple-part anode design. The anode is surrounded by an alumina insulator. The second part of the anode is changeable in order to locate cylindrical, conical, or other shapes of pieces mounted on it. This geometry leads to the easier investigation of the neutron and X-ray emissions of the device. The third part of the anode is for changing the materials exposed to different kinds of radiations. The design parameters are considered by semiempirical and empirical formulas and are sketched by Solidworks software. Also, the peak current and neutron yield are estimated.     

Generation of K-shell radiation in a double shell plasma linear with a microsecond current generator

Experiments on the generation of K-shell radiation in a double-shell neon liner with a microsecond current generator (τ≈1 μs, I _max=380 kA) are described. The yield of neon K-shell radiation attains 50–80 J per pulse. For the given current amplitude, such a radiation yield could be expected at a rise time as low as τ≈100 ns. Such a high radiation efficiency may be attributed to the sharpening of the front of the inner-shell current pulse because of the detachment of the outer shell from the electrode. __________ Translated from Fizika Plazmy, Vol. 27, No. 11, 2001, pp. 1003–1008. Original Russian Text Copyright ? 2001 by Chaikovsky, Sorokin.