Inertial electrostatic confinement and nuclear fusion in the interelectrode plasma of a nanosecond vacuum discharge. I: Experiment

Properties of an aerosol substance with a high power density in the interelectrode space of a nano- second vacuum discharge are studied. The possibilities of emission and/or trapping of fast ions and hard X-rays by ensembles of clusters and microparticles are analyzed. The possibility of simultaneous partial trapping (diffusion) of X-rays and complete trapping of fast ions by a cluster ensemble is demonstrated experimentally. Due to such trapping, the aerosol ensemble transforms into a “dusty” microreactor that can be used to investigate a certain class of nuclear processes, including collisional DD microfusion. Operating regimes of such a microreactor and their reproducibility were studied. On the whole, the generation efficiency of hard X-rays and neutrons in the proposed vacuum discharge with a hollow cathode can be higher by two orders of magnitude than that in a system “high-power laser pulse-cluster cloud.” Multiply repeated nuclear fusion accompanied by pulsating DD neutron emission was reproducibly detected in experiment. Ion acceleration mechanisms in the interelectrode space and the fundamental role of the virtual cathode in observed nuclear fusion processes are discussed.     

Distribution of matter in the current-carrying plasma and dense core of the discharge channel formed upon electrical wire explosion

Distribution of matter in the discharge channel formed upon a nanosecond electrical explosion of a single wire in air and vacuum was studied experimentally. Simultaneous use of optical, UV, and X-ray diagnostics made it possible to distinguish qualitatively different regions of the discharge channel, such as the current-carrying layers and the region occupied by a weakly conducting cold plasma. Several series of experiments with 25-μm-diameter 12-mm-long wires made of different materials were performed. The charging voltage and the current amplitude were varied in the ranges of U ₀ = 10–20 kV and I _max ∼ 5–10 kA, respectively. Explosion regimes with a current pause and with and without current interruption, as well as with wire preheating in air and vacuum, were studied. Shadow and schlieren images of the discharge channel were obtained using optical probing at the second harmonic of a YAG: Nd⁺³ laser (λ = 0.532 μm, τ ∼ 10 ns). In the experiments carried out in vacuum, X-ray images of the discharge channel were also obtained using an X-pinch as a point source of probing radiation and UV images were recorded using a four-frame MCP camera.     

Runaway electron beams in the gas discharge for UV nitrogen laser excitation

The review of the methods for obtaining the runaway electron beams in the gas discharge is performed. The new method is offered, using which the beam is first formed in a narrow gap (∼1 mm) between the cathode and the grid and then it is accelerated by the field of the plasma column of the anomalous self-sustained discharge in the main gap (10–20 mm long). The electron beams with an energy of about 10 keV and current density of 10³ A/cm² at a molecular nitrogen pressure of up to 100 Torr have been obtained experimentally. The results of research of the UV nitrogen laser with an excitation via runaway electron beam and radiation of energy of ∼1 mJ are given. The UV nitrogen laser generation with the energy of ∼1 mJ has been obtained by the runaway electron beams.     

Filling the vacuum chamber of a technological system with homogeneous plasma using a stationary glow discharge

Experimental study of a glow discharge with electrostatic confinement of electrons is carried out in the vacuum chamber volume V ≈ 0.12 m³ of a technological system “Bulat-6” in argon pressure range 0.005–5 Pa. The chamber is used as a hollow cathode of the discharge with the inner surface area S ≈ 1.5 m². It is equipped with two feedthroughs, which make it possible to immerse in the discharge plasma interchangeable anodes with surface area S _a ranging from ∼0.001 to ∼0.1 m², as well as floating electrodes isolated from both the chamber and the anode. Dependences of the cathode fall U _c = 0.4−3 kV on the pressure p at a constant discharge current in the range I = 0.2−2 A proved that aperture of the electron escape out of the electrostatic trap is equal to the sum S _o = S _a + S _f of the anode surface S _a and the floating electrode surface S _f . The sum S _o defines the lower limit p _o of the pressure range, in which U _c is independent of p. At p < p _o the cathode fall U _c grows up dramatically, when the pressure decreases, and the pressure p tends to the limit p ^ex, which is in fact the discharge extinction pressure. At p ≈ p ^ex electrons emitted by the cathode and the first generation of fast electrons produced in the cathode sheath spend almost all their energy up to 3 keV on heating the anode and the floating electrode up to 600–800°C and higher. In this case the gas in the chamber is being ionized by the next generations of electrons produced in the cathode sheath, their energy being one order of magnitude lower. When S _a < (2m/M)^1/2 S, where m is the electron mass and M is the ion mass, the anode may be additionally heated by plasma electrons accelerated by the anode fall of potential U _a up to 0.5 kV.     

Picosecond runaway electron beams in air

Experimental data on the generation of picosecond runaway electron beams in an air gap with an inhomogeneous electric field at a cathode voltage of up to 500 kV are presented. The methods and equipment developed for these experiments made it possible to measure the beam characteristics with a time resolution of better than 10⁻¹¹ s, determine the voltage range and the beam formation time in the breakdown delay stage, and demonstrate the influence of the state of the cathode surface on the stability of runaway electron generation. It is demonstrated that the critical electron runaway field in air agrees with the classical concepts and that the accelerated beam can be compressed to ∼20 ps. It is unlikely that, under these conditions, the beam duration is limited due to the transition of field emission from the cathode to a microexplosion of inhomogeneities. The maximum energy acquired by runaway electrons in the course of acceleration does not exceed the value corresponding to the electrode voltage.     

Accumulation of inorganic polyphosphates in <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> under nitrogen deprivation: Stimulation by magnesium ions and peculiarities of localization

The yeast Saccharomyces cerevisiae was shown to have a high potential as a phosphate-accumulating organism under growth suppression by nitrogen limitation. The cells took up over 40% of phosphate from the medium containing 30 mM glucose and 5 mM potassium phosphate and over 80% of phosphate on addition of 5 mM magnesium sulfate. The major part of accumulated Pi was reserved as polyphosphates. The content of polyphosphates was ∼57 and ∼75% of the phosphate accumulated by the cells in the absence and presence of magnesium ions, respectively. The content of long-chain polyphosphates increased in the presence of magnesium ions, 5-fold for polymers with the average length of ∼45 phosphate residues, 3.7-fold for polymers with the average chain length of ∼75 residues, and more than 10-fold for polymers with the average chain length of ∼200 residues. On the contrary, the content of polyphosphates with the average chain length of ∼15 phosphate residues decreased threefold. According to the data of electron and confocal microscopy and X-ray microanalysis, the accumulated polyphosphates were localized in the cytoplasm and vacuoles. The cytoplasm of the cells accumulating polyphosphates in the presence of magnesium ions had numerous small phosphorus-containing inclusions; some of them were associated with large electron-transparent inclusions and the cytoplasmic membrane.     

Beams of fast neutral atoms and molecules in low-pressure gas-discharge plasma

Fast neutral atom and molecule beams have been studied, the beams being produced in a vacuum chamber at nitrogen, argon, or helium pressure of 0.1–10 Pa due to charge-exchange collisions of ions accelerated in the sheath between the glow discharge plasma and a negative grid immersed therein. From a flat grid, two broad beams of molecules with continuous distribution of their energy from zero up to e(U + U _c ) (where U is voltage between the grid and the vacuum chamber and U _c is cathode fall of the discharge) are propagating in opposite directions. The beam propagating from the concave surface of a 0.2-m-diameter grid is focused within a 10-mm-diameter spot on the target surface. When a 0.2-m-diameter 0.2-m-high cylindrical grid covered by end disks and composed of parallel 1.5-mm-diameter knitting needles spaced by 4.5 mm is immersed in the plasma, the accelerated ions pass through the gaps between the needles, turn inside the grid into fast atoms or molecules, and escape from the grid through the gaps on its opposite side. The Doppler shift of spectral lines allows for measuring the fast atom energy, which corresponds to the potential difference between the plasma inside the chamber and the plasma produced as a result of charge-exchange collisions inside the cylindrical grid.     

Optical studies of plasma inhomogeneities in a high-current pulsed magnetron discharge

Results are presented for experimental studies of the plasma glow in a high-current pulsed magnetron discharge by using a high-speed optical frame camera. It is found that the discharge plasma is inhomogeneous in the azimuthal direction. The plasma bunches rotate with a linear velocity of ∼1 cm/μs in the direction of electron Hall drift, and their number is proportional to the discharge current. Plasma inhomogeneities in the form of plasma jets propagate in the form of plasma jets from the cathode region toward the anode. It is shown analytically that the formation of inhomogeneities is caused by the necessity to transfer high-density electron current across the magnetic field.     

Ecton processes in the generation of pulsed runaway electron beams in a gas discharge

As was shown earlier for pulsed discharges that occur in electric fields rising with extremely high rates (10¹⁸ V/(cm s)) during the pulse rise time, the electron current in a vacuum discharge is lower than the current of runaway electrons in an atmospheric air discharge in a 1-cm-long gap. In this paper, this is explained by that the field emission current from cathode microprotrusions in a gas discharge is enhanced due to gas ionization. This hastens the initiation of explosive electron emission, which occurs within 10^–11 s at a current density of up to 10¹⁰ A/cm². Thereafter, a first-type cathode spot starts forming. The temperature of the cathode spot decreases due to heat conduction, and the explosive emission current ceases. Thus, the runaway electron current pulse is similar in nature to the ecton phenomenon in a vacuum discharge.     

Experimental study of the plasma effect on the generation of microwave radiation in systems with a virtual cathode

Results are presented from experimental studies of the plasma effect on the generation of microwave radiation in systems with a virtual cathode. Using a triode with a virtual cathode as an example, it is shown that the cathode and anode plasmas reduce the generation efficiency; in particular, the power of the generated microwave radiation decreases and the radiation frequency and the microwave pulse duration change appreciably. It is demonstrated that, at high microwave powers, the power radiated into free space can be reduced by the plasma generated at the surface of the output window. This plasma appears due to discharges developing on the window surface under the combined action of bremsstrahlung, UV radiation, electrons and ions arriving from the beam formation zone, and the microwave electric field.     

Simulation of oscillatory processes in a beam-plasma system with a virtual cathode in gas-filled interaction space

Physical processes occurring in an intense electron beam with a virtual cathode in an interaction space filled with neutral gas are studied in a two-dimensional model. A mathematical model is proposed for investigating complicated self-consistent processes of neutral gas ionization by the beam electrons and the dynamics of an electron beam and heavy positive ions in the common space charge field with allowance for the two-dimensional motion of charged particles. Three characteristic dynamic regimes of the system are revealed: complete suppression of oscillations of the virtual cathode as a result of neutralizing its space charge by positive ions; the pulsed generation regime, in which the ions dynamics repeatedly suppresses and restores the virtual cathode oscillations; and the continuous generation regime with an anomalously high level of noisy oscillations.     

Environmental controls on microbial colonization of high Arctic cryptoendolithic habitats

Sandstone outcrops around Eureka, Ellesmere Island, Nunavut (80°N) in the Canadian high Arctic are host to abundant cryptoendolithic microbial communities. Continuous measurements over 2 years (2002–2004) of climate and environmental parameters showed that cryptoendolithic habitats experience warmer temperatures and wetter conditions than the exposed rock surface. Subsurface temperature fluctuations were moderated by the thermal capacity of the rock substrate and varied as a function of depth, aspect, and albedo. Rain, snow or snowmelt substantially increased subsurface moisture levels, which persist for significant time periods after initial precipitation events. These conditions produced a habitat amenable to colonization by cyanobacteria, fungi and algae. The dominance of one microbial community over another varied between sites, however these differences existed in habitats with similar temperature conditions. Greater diversity of microorganisms at this Arctic location compared to similar habitats in the Antarctic Dry Valleys is explained by warmer temperatures during summer months that lead to longer periods for both active (∼3,700 h year⁻¹) and ideal (∼2,500 h year⁻¹) calculated metabolic activities as well as abundant moisture in the form of liquid water.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Diagnostics of plasma produced by femtosecond laser pulse impact upon a target with an internal nanostructure

X-ray diagnostics of the interaction of femtosecond laser pulses with intensities of 10¹⁶–10¹⁸ W/cm² with CO₂ clusters and frozen nanosize water particles is carried out. The stage of cluster expansion and the formation of a plasma channel, which governs the parameters of the formed X-ray radiation source and accelerated ion flows, is studied. The measurements are based on recording spatially resolved X-ray spectra of H- and He-like oxygen ions. Utilization of Rydberg transitions for spectra diagnostics makes it possible to determine plasma parameters on a time scale of t ∼ 10 ps after the beginning of a femtosecond pulse. The role of the rear edge of the laser pulse in sustaining the plasma temperature at a level of ∼100 eV in the stage of a nonadiabatic cluster expansion is shown. The analysis of the profiles and relative intensities of spectral lines allows one to determine the temperature and density of plasma electrons and distinguish the populations of “thermal” ions and ions that are accelerated up to energies of a few tens of kiloelectronvolts. It is shown that the use of solid clusters made of frozen nanoscale water droplets as targets leads to a substantial increase in the number of fast He-like ions. In this case, however, the efficiency of acceleration of H-like ions does not increase, because the time of their ionization in plasma exceeds the time of cluster expansion.     

Visualization of plasma-induced processes by a projection system with a Cu-laser-based brightness amplifier

A novel method for visualization of the process of interaction of high-power energy fluxes with various surfaces is proposed. The possibility of the dynamic visualization of a surface covered with a ∼3-cm-thick plasma layer with a linear density of ∼10¹⁶ cm⁻² is demonstrated experimentally. A scheme of intracavity shadowgraphy of phase objects with the use of a laser projection microscope is developed. Shadow images illustrating the development of the plasma torch of an erosion capillary discharge in air are presented.     

The effects of tree establishment on water and salt dynamics in naturally salt-affected grasslands

Plants, by influencing water fluxes across the ecosystem–vadose zone–aquifer continuum, can leave an imprint on salt accumulation and distribution patterns. We explored how the conversion of native grasslands to oak plantations affected the abundance and distribution of salts on soils and groundwater through changes in the water balance in naturally salt-affected landscapes of Hortobagy (Hungary), a region where artificial drainage performed ∼150 years ago lowered the water table (from −2 to −5 m) decoupling it from the surface ecosystem. Paired soil sampling and detailed soil conductivity transects revealed consistently different salt distribution patterns between grasslands and plantations, with shallow salinity losses and deep salinity gains accompanying tree establishment. Salts accumulated in the upper soil layers during pre-drainage times have remained in drained grasslands but have been flushed away under tree plantations (65 and 83% loss of chloride and sodium, respectively, in the 0 to −0.5 m depth range) as a result of a five- to 25-fold increase in infiltration rates detected under plantations. At greater depth, closer to the current water table level, the salt balance was reversed, with tree plantations gaining 2.5 kg sodium chloride m⁻² down to 6 m depth, resulting from groundwater uptake and salt exclusion by tree roots in the capillary fringe. Diurnal water table fluctuations, detected in a plantation stand but not in the neighbouring grasslands, together with salt mass balances suggest that trees consumed ∼380 mm groundwater per year, re-establishing the discharge regime and leading to higher salt accumulation rates than those interrupted by regional drainage practices more than a century ago. The strong influences of vegetation changes on water dynamics can have cascading consequences on salt accumulation and distribution, and a broad ecohydrological perspective that explicitly considers vegetation–groundwater links is needed to anticipate and manage them.     

Study of the possibility to control ion generation and transport in a high-current vacuum spark

The possibilities of optimizing a high-current vacuum spark as a source of metal ions are discussed. The influence of the shape and size of the electrodes on both the depth to which the hot plasma region is immersed in the surrounding cold matter and the plasma state in the hot spot, which is the source of multicharged ions, is demonstrated. Methods for optimization of the design of the discharge device for increasing the ion yield from a high-current vacuum spark are considered.     

Separation of the heavy and light ion components in a plasma flow propagating in a curvilinear magnetic field

The propagation of a metal plasma jet in a transport system with a curvilinear magnetic field was studied experimentally. The jet was generated by a pulsed vacuum arc discharge with a composite (W + Fe) cathode. Spatial separation of ions of the cathode material was observed at the exit from the system. The ions of the lighter element (Fe) were concentrated in the inner part of the cathode plasma jet deflected by the magnetic field. The jet is also found to be deflected along the binormal to the magnetic field lines due to plasma drift in the crossed magnetic and electric fields. The degree of mass separation of elements is shown to increase with increasing jet deflection along the binormal. The maximum value of the mass separation efficiency reaches 45, the effective value being 7.7.     

Glow discharge with electrostatic confinement of electrons in a chamber bombarded by fast electrons

A metal substrate is immersed in plasma of glow discharge with electrostatic confinement of electrons inside the vacuum chamber volume V ≈ 0.12 m³ filled with argon or nitrogen at pressures 0.005–5 Pa, and dependence of discharge characteristics on negative substrate potential is studied. Emitted by the substrate secondary electrons bombard the chamber walls and it results in electron emission growth of the chamber walls and rise of gas ionization intensity inside the chamber. Increase of voltage U between the chamber and the substrate up to 10 kV at a constant discharge current I _d in the anode circuit results in a manifold rise of current I in the substrate circuit and decrease of discharge voltage U _d between the anode and the chamber from hundreds to tens of volts. At pressure p < 0.05 Pa nonuniformity of plasma density does not exceed ∼10%. Using the Child-Langmuir law, as well as measurement results of sheath width d between homogeneous plasma and a lengthy flat substrate dependent on voltage U ion current density j _i on the substrate surface and ion-electron emission coefficient γ _i are calculated. After the current in circuit of a substrate made of the same material is measured, the γ _i values may be used to evaluate the average dose of ion implantation. The rate of dose rise at a constant high voltage U is by an order of magnitude higher than in known systems equipped with generators of square-wave high-voltage pulses. Application to the substrate of 10-ms-wide sinusoidal high-voltage pulses, which follow each other with 100-Hz frequency, results in synchronous oscillations of voltage U and ion current I _i in the substrate circuit. In this case variation of the sheath width d due to oscillations of U and Ii is insignificant and d does not exceed several centimeters thus enabling substrate treatment in a comparatively small vacuum chamber.     

Epitaxial synthesis of diamond layers on a monocrystalline diamond substrate in a torch microwave plasmatron

The epitaxial growth of a diamond single-crystal film in a torch microwave discharge excited by a magnetron of a domestic microwave oven with the power of ≤1 kW in an argon-hydrogen-methane mixture with a high concentration of methane (up to 25% with respect to hydrogen) at atmospheric pressure on a sub-strate of a synthetic diamond single crystal (HPHP) with the orientation (100) and 4 × 4 mm in size is obtained. A discharge with the torch diameter of ∼2 mm and the concentration of the microwave power absorbed in the torch volume of >10³ W/cm³ is shown to be effective for epitaxial enlargement of a single crystal of synthetic diamond. The structure of the deposited film with the thickness up to 10 μm with high-quality morphology is investigated with an optical microscope as well as using the methods of the Raman scattering and scanning electron microscopy.     

Simultaneous synthesis of luciferase and human nerve growth factor in caterpillars infected with a recombinant baculovirus

Human nerve growth factor (hNGF) gene was proliferated with human leucocyte DNA as template by PCR. Then a fusion gene coding hNGF and luciferase (Luc) cDNAs was inserted into transfer vector pSXIVVI⁺X3/3 with the control of Syn XIV promoter. Luc and hNGF were simultaneously synthesized in Spodoptera larvae upon infection with a recombinant baculovirus, TnNPV-Luc-NGF-OCC⁺. Densitometric scanning of SDS-PAGE revealed that ∼18% of the total Coomassie blue-stainable protein of the infected larvae was represented by Luc protein, while the hNGF level was ∼8%. Both proteins were similar to their authentic counterparts in terms of immunoreactivity. Received 16 August 1998/ Accepted in revised form 14 October 1998