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.     

K-shell radiation from double-shell argon plasma liners

Experiments on the generation of argon K-shell radiation during the implosion of double-shell plasma liners are described. The optimum liner length with respect to the maximum K-shell radiation yield is determined. At a liner current of ～1.4 MA, the conversion efficiency of the generator electric energy into the K-shell radiation energy attains 8–9%. The spectrum of the argon K-shell radiation is measured by a set of photoeission X-ray diodes with different filters (including an argon gas filter). Based on the measurements of the emission power in different spectral intervals and calculations by the collision-radiative model, the ion density and electron temperature of the pinch plasma are estimated.     

Sharpening of the front of the current through a cylindrical foil liner

Results are presented from experiments on the electromagnetic implosion of aluminum foil liners at the MIG generator with a current rise time of ≈80 ns. Plasma with a density of 10¹⁷ cm^–3 was preliminarily injected into the liner region by using a set of radial plasma guns. The Lorentz force J × B causes plasma acceleration in the radial direction. Since the magnetic field pressure is inversely proportional to the radius squared, the plasma displacement is maximum near the liner surface. As a result, plasma motion becomes two-dimensional, a gap appears between the plasma and the liner, and the generator current is switched over to the liner. The plasma velocity at the liner surface is close to the local Alfvén velocity, while the time during which the current is switched over to the liner is nearly equal to the ratio of the liner length to the Alfvén velocity. The proposed scheme allows one to decrease the rise time of the current through the liner to several nano-seconds and, as a result, to reduce the initial liner radius and improve the stability of liner implosion.     

Study of the current-sheath formation during the implosion of multishell gas puffs

Results are presented from experimental studies of the dynamics of large-diameter multishell gas puffs imploded by microsecond megampere current pulses. The experiments were conducted on the GIT-12 generator in the regime of microsecond implosion (t _imp = 1.1–1.2 μs, I ₀ = 3.4–3.7 MA). The influence of the load configuration on the dynamics of current losses and gas-puff radiative characteristics was studied. The correlation between the radial compression ratio (the ratio between the initial and final Z-pinch radii) and the magnitude of the current flowing at the plasma periphery was investigated. The experiments show that, in a multishell gas puff, large-scale instabilities insignificantly affect the gas-puff implosion even over microsecond time intervals and that a compact dense pinch with a relatively high average electron temperature (400–600 eV) forms at the Z-pinch axis. The diameter of the plasma column radiating in the K-shell lines of neon is about 3–4 mm, the K-shell radiation yield being 5–11 kJ/cm. In the final stage of implosion, only a small portion of the current flows through the high-temperature central region of the pinch plasma, whereas the major part of the generator current flows through the residual peripheral plasma.     

Experiments on the implosion of heterogeneous wire arrays on the S-300 facility

Results are presented from experiments on the implosion of simple and nested wire arrays of different mass and material composition (W and/or Al). The experiments were performed on the S-300 facility (a high-current pulsed power generator with a voltage pulse amplitude of 700 kV, current amplitude of 2.5–3.5 MA, and pulse duration of 100 ns) at the Kurchatov Institute (Moscow). The imploding arrays were recorded using five-frame laser shadowgraphy, three-frame image-tube photography, an optical streak camera, X-ray pinhole cameras with different filters, X-ray polychromator, and X-ray spectrometer on the basis of a convex mica crystal. Laser probing measurements indicate that the current-carrying structure undergoes a fast (over a time shorter than 10 ns) global rearrangement, which manifests itself as the emergence of transparent regions. This effect is presumably related to the grouping of the wires, which carry currents of a few tens of kiloamperes, or to the current filamentation in their common plasma corona. The radiation of liners of different chemical composition in the final compressed state has been investigated. Electric measurements performed in experiments with nested arrays (e.g., with an aluminum outer liner and a tungsten inner liner) indicate that the inner array, which is still at rest, intercepts the electric current from the outer array when the latter penetrates through it. The effect of the “fall” of the outer liner through the inner one in the course of magnetic implosion has been revealed for the first time by analyzing X-ray emission spectra.     

Dynamics of the implosion of a tungsten wire array onto the central aluminum wire

Results are presented from the studies of the magnetic implosion of a tungsten wire liner onto an aluminum wire at currents of 2.0–2.6 MA. The experiments were carried out in the S-300 high-power pulsed facility at the Russian Research Centre Kurchatov Institute. The liner is composed of 50 wires 6 μm in diameter and 1 cm in length, which are equally spaced on a circle 1 cm in diameter. An aluminum wire 120 μm in diameter is positioned at the array axis. The liner implosion was accompanied by the generation of VUV and soft X-ray emission. The parameters of the pinch plasma produced during the liner implosion onto the aluminum wire were determined from the time-resolved spectral measurements by a five-channel polychromator. The ion and electron densities turned out to be equal to n _i≈4×10¹⁹ cm⁻³ and n _e≈4×10²⁰ cm⁻³, respectively, and the electron temperature was T _e≈40 eV. The radiation energy measured in the range 50–600 eV was 2–10 kJ. The sources of soft X-ray emission in hydrogen-and helium-like aluminum lines were the bright spots and local objects (clouds) formed in the plasma corona at an electron temperature of 200–500 eV and electron density of 10²¹–10²² cm⁻³. The possibility of both the generation of an axial magnetic field during the liner implosion and the conversion of the energy of this field into soft X-ray emission is discussed. __________ Translated from Fizika Plazmy, Vol. 28, No. 6, 2002, pp. 514–521. Original Russian Text Copyright ? 2002 by Bakshaev, Blinov, Dan'ko, Ivanov, Klír, Korolev, Kravárik, Krása, Kubeš, Tumanov, Chernenko, Chesnokov, Shashkov, Juha.     

Mediating N2O emissions from municipal solid waste landfills: Impacts of landfill operating conditions on community structure of ammonia-oxidizing bacteria in cover soils

Chemolithotrophic ammonia-oxidizing bacteria (AOB) can produce N₂O, a highly potent greenhouse gas. Denaturing gradient gel electrophoresis (DGGE) analyses of the ammonia monooxygenase structural gene (amoA) and 16S rDNA gene were used to investigate the AOB community structure in the cover soils of municipal solid waste (MSW) landfills under three operating conditions: (a) MSW with soil cover, (b) MSW with soil cover, irrigation piping and vegetation, and (c) MSW covered with high-density polyethylene (HDPE) liner, soil cover, irrigation piping and vegetation. AOB species in MSW cover soils were significantly distinguished by the operation of HDPE liner isolation. The community structures of the Nitrosomonas europaea-like AOB species dominated in soils without HDPE liner isolation, whether vegetation and irrigation with landfill leachate existed or not, whereas Nitrospira-like AOB species dominated in soils with HDPE liner isolation. Lower N₂O flux from the soils with HDPE liner isolation would be partially related to these special community structures.     

Interferometric measurements of the plasma density at the Z-pinch periphery in the angara-5-1 facility

Knowledge of spatial mass distribution is important for understanding the physics of implosion of megaampere-current wire arrays. The paper presents results from studying the electron density distribution at the periphery of a tungsten wire array near the instant of maximum compression by using laser interferometry at λ=0.69 µm. It is found that, at the instant of maximum compression (～100 ns after the beginning of the discharge), the estimated maximum local electron density inside the wire array reaches ～10¹⁸ cm^?3 at a distance of 0.3–3 mm from the initial wire positions. Assuming the average tungsten ion charge to be 10, the local linear mass density in this region turns out to be 3 µg/cm, which amounts to about 10% of the total linear mass density of the liner. A fraction of the generator current flows through this plasma. The duration of the soft X-ray pulse is 5–8 ns, which indicates the achievement of a fairly high compression ratio.     

Transportation of an electromagnetic pulse to the load in the Angara-5-1 facility

One of the main problems in Z-pinch experiments is to transport power and energy from the generator to the load. As the pulse produced in a double forming line propagates to the load along a water-vacuum insulator, its power and energy decrease due to current leakage in the plasma shortening the gap and during the establishment of magnetic self-insulation in regions with a zero magnetic field. Only a fraction of the delivered energy is spent on the load implosion, whereas the rest of the energy goes on creating the magnetic field around the load. In this work, an analysis is made of what is the fraction of the generator energy that reaches the liner, what fraction is radiated, and what are losses of energy and current in different stages of transporting the electromagnetic pulse to the load of the Angara-5-1 facility.     

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.     

X-ray backlighting of the axial region of a multiwire liner plasma in the angara-5-1 facility

Results are presented from experiments on the X-ray backlighting of the axial region of an imploding high-current multiwire liner. Backlighting was performed with the use of an X-pinch serving as a source of soft X-ray emission, which was recorded by pin diodes. The use of several filters with different passbands in front of the pin diodes allowed the interpretation of the results of measurements in experiments with cascade composite liners. The sensitivity of the diagnostics was ≈125 µg/cm² for a plasma of high-Z elements (W) and ≈220 µg/cm² for a plasma of low-Z elements (C, O, N) at a photon energy of the probing radiation of 1.0–1.5 keV. An advantage of the method is its high time resolution (≈1 ns) and the possibility of the separation in time of the emission bursts from Z-and X-pinches on the liner axis. The method does not impose restrictions on the pulse duration of the backlighting radiation source.     

A low-cost integrator for preparative ultracentrifuges

An integrator is described for the measurement of the time integral ∫₀^t rpm²dt in preparative ultracentrifuge where linearity exists either (a) between tachometer generator ac voltage amplitude and rpm (e.g., Sorvall RC2-B) or dc voltage and rpm, or (b) between the square-wave frequency from the tachometer generator and rpm (e.g., Beckman L2-65B). The construction and the precision levels of an integrator for Sorvall RC2-B preparative ultracentrifuge in the range 0–10,000 rpm and for Beckman L2-65B preparative ultracentrifuge in the range 0–40,000 rpm are described.     

Investigation of burst generation by the electrically coupled cyberchron network in the snail Helisoma using a single-electrode voltage clamp

This paper describes the results of investigating burst generation by the cyberchron network in the snail Helisoma. The cyberchron network is composed of approximately 20 electrically coupled neurons and controls the feeding behavior of the snail. The electrical coupling between network members has made it particularly difficult to distinguish between the importance and involvement of single-cell and network properties in burst generation by this system. The present investigations utilized the new single-electrode voltage clamp to examine the membrane properties and network interactions of the cyberchron neurons: (1) A slow outward current is activated by moderately large depolarizing commands (?40 to 0 mV) and does not undergo inactivation decay (i.e., decline in magnitude) during a command potential step maintained for 10 sec or more. The lack of inactivation of the outward current in cyberchron neurons appears to be due to the dominating role of a Ca-dependent K current. (2) There are two functionally distinct classes of cyberchrons—current generator cyberchrons and follower cyberchrons. (3) Primary current generator cyberchrons have membrane properties similar to endogenous bursting neurons (e.g., persistent inward Ca current and negative resistance region in I–V plot) and appear to provide the main driving and timing current for the rest of the network. (4) The vast majority of cyberchrons are secondary current generator cyberchrons with membrane properties which exhibit inward-going rectification and appear to burst as a result of regenerative excitation with one another and the primary current generator cyberchrons. (5) The second class of cyberchrons are driven by the electrical synaptic input from the current generator cyberchrons, do not exhibit inward-going rectification, and are called follower cyberchrons. (6) Burst termination is due to activation of a slow outward tail current in most cyberchrons during the burst (probably Ca-activated K current) which causes a hyperpolarization in individual cyberchrons, terminating the burst. (7) Decay of the outward tail current causes the cyberchrons to depolarize, which activates the persistent inward Ca current in the primary current generator cyberchrons, starting the burst cycle anew.     

Evolution of a capillary discharge induced by a semiconductor current generator

Results are presented from the intermediate stage of work on creating a current generator in a circuit with an inductive energy storage and a semiconductor opening switch made of 40 SDL-800 diodes. A six-diode generator prototype has been created with a current pulse amplitude of ～4.5 kA and a leading edge duration of ～10–20 ns at an inductive load of 30–35 nH. The generator was used to study discharges in capillaries filled with argon or hydrogen. It is shown that, in a 2-mm-diameter capillary, the initial azimuthal asymmetry of a structure arising during the breakdown ceases as the discharge evolves, whereas in a 0.8-mm-diameter capillary, it is retained. Time-resolved spectroscopic studies of the plasma reveal the presence of line emission of highly ionized argon (ArVII and ArVIII) in the hottest phase of the discharge, which indicates that a temperature of 20–40 eV has been achieved.     

Measurements of neutron emission from a Z-pinch constriction

Results are presented from measurements of neutron emission generated during discharges with current amplitudes of up to 3 MA and a current rise time of ～100 ns through profiled loads 10 mm in height and 4–5 mm in diameter. The experiments were performed with the S-300 eight-module high-power generator. To enhance the effect of energy accumulation, a≤1-mm-diameter neck was made in the central region of the load. An agar-agar foam of mass density 0.1 g/cm³ with an additive of deuterated polyethylene was used as a plasma-forming material. The formation of a hot plasma in the Z-pinch constriction was accompanied by the emission of soft X-ray (E = 1–10 keV), hard X-ray (E ≥ 30 keV), and neutron pulses with a minimum pulse duration of ≤10 ns. The neutron energy measured by the time-of-flight technique in three directions relative to the load axis (0°, 90°, and 180°) was found to be 2.5 ± 0.3 MeV, which corresponds to the dd reaction. The total neutral yield during the development of one constriction with a characteristic size of 100 μm attained 10⁸ neutrons per pulse.     

Fast Inactivation in Shaker K+ Channels : Properties of Ionic and Gating Currents

Fast inactivating Shaker H4 potassium channels and nonconducting pore mutant Shaker H4 W434F channels have been used to correlate the installation and recovery of the fast inactivation of ionic current with changes in the kinetics of gating current known as “charge immobilization” (Armstrong, C.M., and F. Bezanilla. 1977. J. Gen. Physiol. 70:567–590.). Shaker H4 W434F gating currents are very similar to those of the conducting clone recorded in potassium-free solutions. This mutant channel allows the recording of the total gating charge return, even when returning from potentials that would largely inactivate conducting channels. As the depolarizing potential increased, the OFF gating currents decay phase at −90 mV return potential changed from a single fast component to at least two components, the slower requiring ∼200 ms for a full charge return. The charge immobilization onset and the ionic current decay have an identical time course. The recoveries of gating current (Shaker H4 W434F) and ionic current (Shaker H4) in 2 mM external potassium have at least two components. Both recoveries are similar at −120 and −90 mV. In contrast, at higher potentials (−70 and −50 mV), the gating charge recovers significantly more slowly than the ionic current. A model with a single inactivated state cannot account for all our data, which strongly support the existence of “parallel” inactivated states. In this model, a fraction of the charge can be recovered upon repolarization while the channel pore is occupied by the NH₂-terminus region.     

Two degrees of freedom quasi-static EMG-force at the wrist using a minimum number of electrodes

Surface electromyogram-controlled powered hand/wrist prostheses return partial upper-limb function to limb-absent persons. Typically, one degree of freedom (DoF) is controlled at a time, with mode switching between DoFs. Recent research has explored using large-channel EMG systems to provide simultaneous, independent and proportional (SIP) control of two joints—but such systems are not practical in current commercial prostheses. Thus, we investigated site selection of a minimum number of conventional EMG electrodes in an EMG-force task, targeting four sites for a two DoF controller. In a laboratory experiment with 10 able-bodied subjects and three limb-absent subjects, 16 electrodes were placed about the proximal forearm. Subjects produced 1-DoF and 2-DoF slowly force-varying contractions up to 30% maximum voluntary contraction (MVC). EMG standard deviation was related to forces via regularized regression. Backward stepwise selection was used to retain those progressively fewer electrodes that exhibited minimum error. For 1-DoF models using two retained electrodes (which mimics the current state of the art), subjects had average RMS errors of (depending on the DoF): 7.1–9.5% MVC for able-bodied and 13.7–17.1% MVC for limb-absent subjects. For 2-DoF models, subjects using four electrodes had errors on 1-DoF trials of 6.7–8.5% MVC for able-bodied and 11.9–14.0% MVC for limb-absent; and errors on 2-DoF trials of 9.9–11.2% MVC for able-bodied and 15.8–16.7% MVC for limb-absent subjects. For each model, retaining more electrodes did not statistically improve performance. The able-bodied results suggest that backward selection is a viable method for minimum error selection of as few as four electrode sites for these EMG-force tasks. Performance evaluation in a prosthesis control task is a necessary and logical next step for this site selection method.     

On the Structure and Intensity of Vortex in RF Inductively Coupled Argon Plasma

Numerical simulation of a specific technical RF inductively coupled argon plasma with three coils, discharge current in the range of J_coil = 100–250 A, and generator frequency 3 MHz is presented. The temperature, pressure, and velocity fields are obtained under different discharge currents and different flow rates of central gas. A reversed flow (vortex) is found between the injected cool gas and high-temperature plasma-forming gas. The formation mechanisms of such a vortex and the influence of the discharge current and flow rate of central gas on the vortex structure and intensity are studied. Special attention is paid to investigating two different kinds of vortex flow patterns—Benard and toroidal. A critical flow rate of central gas above which the flow pattern would transform from Benard to toroidal is determined and approximated as a function of the discharge current by theoretical calculations and numerical simulations. The maximum negative velocities along the axis in the vortex zone are also determined under different discharge currents and different flow rates of central gas.     

Determination of the parameters of the hot plasma formed during the implosion of wire arrays from time-resolved X-ray spectra of H- and He-like ions

A system for recording X-ray ion spectra by means of a spherical crystal with the subsequent transformation of the X-ray spectrum into an optical image recorded with the help of an optical streak camera is described. A computer code intended to recover the plasma parameters from the intensities of spectral lines of H- and He-like ions of some chemical elements (z = 6–29) is developed. Results of experiments on the determination of the parameters of hot plasma formed during the implosion of nested aluminum wire arrays at the S-300 high-current generator are presented.     

EUV spectroscopy of plasmas created in the final anode-cathode gap of the Z-Machine high-current pulsed generator (SNL)

The effect of short-circuit across the final anode-cathode gap of powerful pulsed current generators could hamper efficient power delivery to the Z-pinch plasma. To study this effect, a novel EUV diagnostics of plasmas created in the final section of the transmission line (the anode-cathode gap near the main load) of the Z-Machine high-current generator (Sandia National Laboratories, United States) was developed. The work included developing spectroscopic instruments, theoretical and experimental studies of EUV spectra of iron ions in well-diagnosed laser-produced plasmas, and a comparison of these spectra with those of plasmas created in the final anode-cathode gap of the transmission line. The EUV spectra of highly charged Fe ions in the spectral range λ ～ 20–800 Å were investigated. In experiments performed at Sandia National Laboratories, spectra of FeXIII-FeXVII ions were observed. A comparison of the measured and calculated spectra shows that the electron plasma temperature in the anode-cathode gap is T _e ～ 200 eV.