Hybrid X-pinches

Results from experimental studies of a hybrid X-pinch with an initial configuration in the form of a high-current diode with conical tungsten electrodes spaced by 1?C2 mm and connected to one another with 20- to 100-??m-diameter wires are presented. The experiments were carried out at four facilities with a current amplitude from 200 to 1000 kA and front duration from 45 to 200 ns. It is shown that, in spite of their simpler configuration, hybrid X-pinches with a short rise time of the current pulse (50?C100 ns) are highly competitive with standard X-pinches in the generated soft X-ray power and the formation of a single hot spot in them is much more stable, while hard X-ray emission is almost absent. The possibility of using hybrid X-pinches as soft X-ray sources for point projection X-ray imaging of plasma objects is considered.     

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.     

Diagnostics of soft X-ray emission from the plasma of a fast capillary discharge

Results are presented from time-resolved measurements of the soft X-ray emission in the 10-to 40-eV spectral range from the plasma of a pulsed capillary discharge in argon at current pulse amplitudes of up to 26 kA and a current rise time of ～10¹² A/s. The experiments were carried out with 0.3-cm-diameter 15.7-cm-long ceramic capillaries filled with argon at a pressure of 0.25–0.5 Torr in the SIGNAL electrophysical facility. The experimental data are interpreted via computer simulations of the magnetohydrodynamics and level-by-level ion kinetics of an argon plasma. The results obtained indicate that soft X-ray laser pulses with a photon energy of 26.4 eV and duration of 1–2 ns are generated ≈33 ns after the beginning of the discharge current pulse.     

Study of the effect of preliminary wire explosion on X-ray generation during wire array Z-pinch implosion

Results are presented from experimental studies and numerical simulations of the effect of preliminary wire explosion on the parameters of X-ray emission generated during wire array Z-pinch implosion. The wire array implosion was driven by a current pulse with an amplitude of 0.5 MA and a rise time of 0.5 μs, while the preliminary wire explosion was produced by a current pulse with an amplitude of 0.5–1 kA per wire, a rise time of 100 ns, and a full width at half maximum of ～200 ns. The experiments showed that the current prepulse significantly impaired the parameters of X-ray pulses. In particular, along with a decrease in the amplitude and an increase in the duration of the X-ray pulse, its spiky structure became more pronounced. The results of numerical simulations with the use of a one-dimensional radiative MHD code are in good agreement with the parameters of Z-pinch emission in experiments with and without a current prepulse.     

Studies of plasma dynamics in megampere X-pinches

Results are presented from experimental studies of the dynamics and parameters of the plasma generated by megampere (up to 2.3 MA) currents flowing through X-pinches formed from crossed wires made of various materials. The experiments were performed with different numbers of wires and different wire diameters. The X-ray yield in neon-like molybdenum lines (in the 2.5- to 3-keV photon energy range) from a <20 μm hot spot was larger than 10 J, and the soft X-ray power emitted from this spot reached 120 GW. The hot-spot lifetime estimated by comparing the measured and calculated emission intensities and line profiles of helium-like Fe and Cr ions was ～10 ps. Hard X-ray emission in the photon energy range ≥800 keV was also detected.     

Dynamics of an Al wire corona of a megaampere Z-pinch

It is shown that the development of instabilities in a Z-pinch plasma formed by loading a relatively thick Al wire (an initial diameter of 120 μm and a maximum discharge current of 2–3 MA) is slowed down due to the high plasma density in the wire corona. A cylindrically symmetric, regular, and stable corona surrounding the wire contains a helical formation with a dense, cold, and magnetized plasma. X-ray pulses with a photon energy of several keV and an FWHM duration of 10–20 ns are generated by a few imploded neck structures in the pinch phase of the corona evolution (70–100 ns after the current onset). The main part of X radiation emitted by individual bright spots in the photon energy range 1.5–2.4 keV (up to 40 J at a peak power of 4 GW) consists of the continuum and the bound-bound transition radiation from H-and He-like Al ions. A possible scenario for the axial magnetic field evolution during an X-ray pulse is outlined. __________ Translated from Fizika Plazmy, Vol. 28, No. 4, 2002, pp. 329–336. Original Russian Text Copyright ? 2002 by Kubeš, Renner, Krousky, Kravárik, Bakshaev, Blinov, Chernenko, Gordeev, Dan’ko, Korolev, Shashkov.     

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.     

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.     

Specific features of X-ray generation by plasma focus chambers with deuterium and deuterium–tritium fillings

The process of hard X-ray (HXR) generation in plasma focus (PF) chambers was studied experimentally. The radiation was recorded using scintillation detectors with a high time resolution and thermoluminescent detectors in combination with the method of absorbing filters. Time-resolved analysis of the processes of neutron and X-ray generation in PFs is performed. The spectra of HXR emission from PF chambers with deuterium and deuterium–tritium fillings are determined. In experiments with PF chambers filled with a deuterium–tritium mixture, in addition to the HXR pulse with photon energies of up to 200–300 keV, a γ-ray pulse with photon energies of up to 2.5–3.0 MeV is recorded, and a mechanism of its generation is proposed.     

Dynamics of the optical emission from a high-voltage diffuse discharge in a rod-plane electrode system in atmospheric-pressure air

Results are presented from experimental investigations of the dynamics of optical emission from a nanosecond diffuse discharge in a rod-plane electrode system. A study was made of discharges in a 10-cm-long interelectrode gap in atmospheric-pressure air (the cathode being a 1-cm-diameter rod with a bullet-shaped end). The voltage across the discharge gap was 220 kV and the voltage pulse duration was 180 ns, the voltage rise time being 10 ns. In experiments, the discharges were observed to evolve through two stages: the bridging stage and the conduction stage. The bridging stage begins with intense optical emission from the cathode region, the onset of the emission being delayed with respect to the beginning of the voltage pulse. Simultaneously with the onset of optical emission, a displacement current corresponding to the motion of charged particles begins to be generated in the cathode region. The duration of this current corresponds to the time the emission front takes to bridge the gap. As the emission front reaches the anode region, the current increases abruptly, indicating the beginning of the conduction stage. It was found that the time delay of optical emission relative to the beginning of the voltage pulse largely governs the discharge parameters: as the time delay becomes longer, the emission front velocity in the bridging stage increases from 0.6 to 1.5 cm/ns, the probability of realizing a multichannel structure of the discharge becomes higher, and the discharge current and the intensity of X-ray emission from the discharge grow.     

Output power variations in a plasma relativistic microwave amplifier during a 500-ns relativistic electron beam current pulse

A relativistic plasma microwave amplifier with a gain of about 30 dB and an output power of about 60–100 MW in the frequency range from 2.4 to 3.2 GHz is studied experimentally. The total duration of the output microwave pulse is equal to the duration of the current pulse of the driving relativistic electron beam (500 ns); however, the maximum output power is observed only within 200 ns. It is shown that variations in the output microwave power during the current pulse of the annular relativistic electron beam are caused by variations in the beam radius and thickness. Analysis of the experimental data and results of numerical simulations show that the thickness of the electron beam is determined by the density of the cathode emission current.     

Studies of exposure of rabbits to electromagnetic pulsed fields

Dutch rabbits were acutely exposed to electromagnetic pulsed (EMP) fields (pulse duration 0.4μs, field strengths of 1–2 kV/cm and pulse repetition rates in the range of 10 to 38 Hz) for periods of up to two hours. The dependent variables investigated were pentobarbital-induced sleeping time and serum chemistry (including serum triglycerides, creatine phosphokinase (CPK) isoenzymes, and sodium and potassium). Core temperature measured immediately pre-exposure and postexposure revealed no exposure-related alterations. Over the range of field strengths and pulse durations investigated no consistent, statistically significant alterations were found in the end-points investigated.     

Electrohydrodynamic effect resulting from high-frequency barrier discharge in gas

The formation of an electrohydrodynamic flow in atmospheric air by using a high-frequency barrier discharge distributed over the dielectric surface is investigated. The influence of variations in parameters of a fully solid-state pulse generator (with a peak voltage of 0–12 kV, a tunable repetition rate of 10–25 kHz, and a pulse duration of 7 μs) on the current of plasma ion emitter and velocity characteristics of airflow is considered.     

Experimental investigation of X-ray lasing in a capillary argon discharge

Results are presented from experiments on the laser generation of X-ray radiation at the wavelength λ=469 ? (ε=26.4 eV) on the 3p(J=0)−3s(J=1) transition of Ne-like Ar ions. Experiments were carried out on the SIGNAL electrophysical facility with a 3.1-mm-diameter 157-mm-long Al₂O₃ ceramic capillary filled with argon at a pressure of 0.2–1.0 Torr. The discharge current amplitude was I ∼ 25–40 kA, the current rise rate being dI/dt ∼ 10¹² A/s. By a vacuum X-ray diode tuned to detect X-ray photons with energies in the range 10–40 eV, laser pulses with a duration of t ₁ ∼ 1 ns and maximum energy of E _1,max ∼ 1 μJ were recorded. The pulses were generated 35 ns after the discharge current was switched on. The line spectra in the wavelength range of 150–500 ? showed the bright λ=469 ? line. The angular divergence of the generated X-ray laser beam was estimated to be Δϑ ∼ 2 mrad. Original Russian Text ? O.N. Gilev, V.I. Afonin, V.I. Ostashev, V.Yu. Politov, A.M. Gafarov, A.L. Zapysov, A.V. Andriyash, é.P. Magda, L.N. Shamraev, A.A. Safronov, A.V. Komissarov, N.A. Khavronin, N.A. Pkhaĭko, L.V. Antonova, L.N. Shushlebin, 2006, published in Fizika Plazmy, 2006, Vol. 32, No. 2, pp. 160–165.     

Experimental verification of the technique for calculating a plasma opening switch and its matching to the load

A technique for calculating a plasma opening switch in an external magnetic field and its matching to a load the impedance of which increases with time was verified experimentally. The experiments were performed in the RS-20 facility both in the absence of a load and with various inductive loads. The amplitude of the voltage pulse at the input of the plasma opening switch was 0.36–0.84 MV, the current amplitude was 280–320 kA, and the pulse duration was ～2 μs, whereas the corresponding parameters of the output pulse were 0.8–3.2 MV, 0–240 kA, and ～100 ns.     

Ultra-wideband electromagnetic pulses: Lack of effects on heart rate and blood pressure during two-minute exposures of rats

Exposure to fast-rise-time ultra-wideband (UWB) electromagnetic pulses has been postulated to result in effects on biological tissue (including the cardiovascular system). In the current study, 10 anesthetized Sprague-Dawley rats were exposed to pulses produced by a Sandia UWB pulse generator (average values of exposures over three different pulse repetition rates: rise time, 174–218 ps; peak E field, 87–104 kV/m; pulse duration, 0.97–0.99 ns). Exposures to 50, 500 and 1000 pulses/s resulted in no significant changes in heart rate or mean arterial blood pressure measured every 30 s during 2 min of exposure and for 2 min after the exposure. The results suggest that acute UWB whole-body exposure under these conditions does not have an immediate detrimental effect on these cardiovascular system variables in anesthetized rats. Bioelectromagnetics 19:330–333, 1998. Published 1998 Wiley-Liss, Inc.     

Study of hydrodynamic instabilities of a Z-pinch during a high-current explosion of a thin wire

Results are presented from laboratory and numerical experiments on the influence of the core and associated hydrodynamic instabilities on the high-current implosion of a plasma of exploding metal wires. The experimental investigation of the discharge structure was carried out using the multiframe X-ray backlighting technique with high temporal and spatial resolution (<1 ns and 1 μm, respectively); X-pinches were used as small-sized radiation sources. The implosion of a dense Z-pinch was modeled by the free-point method with the use of a two-dimensional radiative MHD code. The onset of instabilities at the corona-core boundary was modeled by the NUTCY Eulerian code. The results show that hydrodynamic processes in the core are primarily responsible for the formation of small bright regions observed in X-rays. After the reflection of a shock wave from the axis, the rapid onset of hydrodynamic instabilities can occur at the corona-core boundary. __________ Translated from Fizika Plazmy, Vol. 26, No. 9, 2000, pp. 797–810. Original Russian Text Copyright ? 2000 by Gus’kov, Ivanenkov, Mingaleev, Nikishin, Pikuz, Rozanov, Stepniewski, Tishkin, Hammer, Shelkovenko.     

Study of the magnetic fields and soft X-ray emission generated in the implosion of double wire arrays

Results are presented from measurements of the azimuthal magnetic field generated during the implosion of double (nested) tungsten wire arrays in the Angara-5-1 facility at currents of ～3 MA. It is found that the inner array affects the current distribution in the interarray space and that there is an optimal mass (an optimal number of wires) of the inner array at which the full width at half-maximum of the soft X-ray pulse (in the photon energy range of >100 eV) is minimal. On the average, double wire arrays provide a better reproductibility, higher power, and shorter duration of the soft X-ray pulse in comparison to single arrays.     

Feasibility of stabilizing a vacuum-diode X-ray source with a laser-plasma cathode

Results are presented from experimental studies of discharge instabilities and the energy and temporal characteristics of a vacuum-diode X-ray source with a laser plasma cathode over a wide range of energies, intensities, and durations of the plasma-forming laser pulse. It is experimentally shown that the vacuum-discharge dynamics and radiation processes in different discharge stages substantially depend on the parameters of the laser radiation. The shortest recorded pulse duration (10 ns) of Ti K-line radiation (4.5 keV) with a total photon number of 10¹¹ is achieved when the laser plasma cathode is produced by a laser pulse with a duration of 27 ps and an intensity of 10¹³ W/cm². It is found that the contrast of characteristic emission against the bremsstrahlung background is maximum when discharge instabilities are suppressed and the accelerating voltage is three to four times higher than the threshold voltage for line excitation.     

Kinetics of homomeric GluR6 glutamate receptor channels.

We studied the kinetics of the unedited version of rat GluR6 glutamate (glu) receptor channels, GluR6Q, in outside-out patches using a system for submillisecond solution exchange. Half-maximum activation of the channels was reached with approximately 0.5 microM glu. The maximum slope of the double-logarithmic plot of the peak current versus glu was approximately 1.3, indicating that at least two binding steps are necessary to open the channels. Currents in response to a pulse of 10 microM glu had a short rise time (10-90% of peak current) of approximately 220 microseconds at approximately 20 degrees C. The rise time increased with falling glu concentration, reaching approximately 6.0 ms with 10 microM glu. In the continued presence of glu, the channels desensitized, and this desensitization can be described with a single time constant of approximately 7.0 ms for a pulse of 10 microM glu. The steady-state current in response to a long pulse of 10 microM glu was below 1/280th of the peak current. The time constant of desensitization was found to be independent of concentration between 30.0 and 0.3 microM glu, but to be increased for lower concentrations. After a short pulse of 1 ms duration and 10 or 0.3 microM glu, currents decayed with a time constant of approximately 2.5 ms. Recovery from desensitization after a pulse took approximately 5 s, and the half-time of recovery was approximately 2.2 s. Continuous application of low concentrations of glutamate reduced the peak currents in response to a pulse of 10 microM glu markedly. Fifty percent response reduction was observed in the continuous presence of approximately 0.3 microM glu. Our results for homomeric GluR6 agree with a cyclical reaction scheme developed for completely desensitizing, glu-activated channels on crayfish muscles.