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.     

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.     

Scaling of a fast spherical discharge

The influence of the discharge cavity dimensions on the properties of the spherical plasma formed in a fast discharge was studied experimentally. The passage of a current pulse with an amplitude of 30–40 kA and a rise rate of ~10¹² A/s (a fast discharge) through a spherical ceramic (Al₂O₃) cavity with an inner diameter of 11 mm filled with argon at a pressure of 80 Pa results in the formation of a 1- to 2-mm-diameter spherical plasma with an electron temperature of several tens of electronvolts and a density of 10¹⁸–10¹⁹ cm^–3. It is shown that an increase in the inner diameter of the discharge cavity from 11 to 21 mm leads to the fourfold increase in the formation time of the spherical plasma and a decrease in the average ion charge. A decrease in the cavity diameter to 7 mm makes the spherical plasma unstable.     

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.     

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.     

Long-lived Ar-Hg plasma in the afterglow of a high-current pulsed discharge

High-density (n > 10¹² cm^?3) argon-mercury plasma produced by a short (t ～ 20 μs) high-power pulsed discharge in argon with an admixture of mercury vapor at a discharge current of ～50 A, an argon pressure of ～4 mm Hg, and a mercury vapor pressure of ～10^?3 mm Hg was studied using optical spectroscopy and radio physics methods. It is found that the lifetime of this plasma after the end of the discharge pulse is up to 10^?2 s. It is shown that such an abnormally long lifetime of such an afterglow plasma, as compared to the plasma of an argon discharge without an admixture of mercury vapor, is related to the long residence time of atoms and ions of both argon and mercury in highly excited states due to chemi-ionization processes involving long-lived metastable argon ions. It is suggested that dissociative recombination of highly excited molecular ions of argon play an important role in the transfer of excitation to argon atoms and ions that are close to autoionization states.     

Formation of hot spots in the plasma of a Z-pinch produced from low-density deuterated polyethylene

Results are presented from experimental studies of the plasma formation dynamics in a Z-pinch produced from a cylindrical microporous agar-agar load. The experiments were performed on the S-300 facility at a current of 2 MA and current rise time of 100 ns. To enhance the energy concentration, a deuterated polyethylene neck with a mass density of 50–75 μg/cm³ and diameter of 1–2 mm was made in the central part of the load. The spatiotemporal characteristics of the Z-pinch were studied using an optical streak camera and fast frame photography in the optical and soft X-ray spectral ranges. X-ray emission was detected using semiconductor and vacuum diodes, and neutron emission was studied by means of the time-of-flight method. It is found that, in the course of continuous plasma production, hot spots with a diameter of 100 μm form in the pinch plasma. The hot spots emit short soft X-ray pulses with a duration of 2–4 ns, as well as neutron pulses with an average neutron energy of about 2.45 MeV. The maximum neutron yield was found to be 4.5 × 10⁹ neutrons per shot. The scenario of hot spot formation is adequately described by two-dimensional MHD simulations.     

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.     

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.     

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.     

The dynamics of and radiation from a copper-wire corona in a megaampere Z-pinch

Evolution of the extreme ultraviolet (XUV) and soft X-ray (SXR) emission in the 50-to 2000-eV photon energy range from a plasma corona formed by loading a relatively thick Cu wire (with an initial diameter of 120 µm) was observed in a Z-pinch discharge with a maximum current of 2 MA and current rise time of 100 ns. A diagnostic complex consisting of a five-channel SXR polychromator, a four-frame X-ray pinhole camera, and a mica crystal spectrograph shows that double-humped emission pulses in the XUV and SXR spectral ranges are generated 70–130 ns after the onset of the discharge current. The total energy of the pulses is 5 kJ, and the maximum power is 60 GW. A part of the observed kiloelectronvolt X-ray emission from three to five spots with diameters of 1–2 mm consists of the Cu K-and L-shell lines.     

Determination of plasma density from data on the ion current to cylindrical and planar probes

To improve probe methods of plasma diagnostics, special probe measurements were performed and numerical models describing ion transport to a probe with allowance for collisions were developed. The current–voltage characteristics of cylindrical and planar probes were measured in an RF capacitive discharge in argon at a frequency of 81 MHz and plasma densities of 10¹⁰–10¹¹ cm^–3, typical of modern RF reactors. 1D and 2D numerical models based on the particle-in-cell method with Monte Carlo collisions for simulating ion motion and the Boltzmann equilibrium for electrons are developed to describe current collection by a probe. The models were used to find the plasma density from the ion part of the current–voltage characteristic, study the effect of ion collisions, and verify simplified approaches to determining the plasma density. A 1D hydrodynamic model of the ion current to a cylindrical probe with allowance for ion collisions is proposed. For a planar probe, a method to determine the plasma density from the averaged numerical results is developed. A comparative analysis of different approaches to calculating the plasma density from the ion current to a probe is performed.     

Study of the plasma in a preformed Z-pinch constriction

The development of a preformed constriction in cylindrical agar-agar loads at currents of up to 3 MA is studied experimentally. The loads 3–5 mm in diameter have a mass density of 0.1 g/cm³ and are filled with different materials. Due to the implosion of the constriction to a minimum size of 40–70 μm, a hot dense plasma (with the electron density n _e=10²² cm⁻³, electron temperature T _e=0.8–1.5 keV, and ion temperature T _i=3–12 keV) is produced. It is found that the ion temperature substantially exceeds the electron temperature. The lifetime of the high-temperature plasma determined from the FWHM of a soft X radiation (SXR) pulse is shorter than 5 ns, the radiation power of photons with energies of ≥1 keV is higher than 0.5×10¹⁰ W, and their total energy attains 50 J. High-speed photography in the VUV, SXR, and optical spectral regions indicates the protracted generation of the high-temperature plasma. Calculations by the two-dimensional ideal MHD model of the Z-pinch show that the most important consequence of the protracted plasma generation in the constriction region is that the current is intercepted by a freshly produced plasma. In the course of plasma generation, the current near the axis inside the region of radius 50 μm is at most one-half of the total current. After the plasma generation comes to an end, almost the entire current is concentrated in this region for several nanoseconds; this process is accompanied by a sharp increase in the plasma temperature. __________ Translated from Fizika Plazmy, Vol. 27, No. 12, 2001, pp. 1101–1110. Original Russian Text Copyright ? 2001 by Bakshaev, Blinov, Vikhrev, Gordeev, Dan’ko, Korolev, Medovshchikov, Nedoseev, Smirnova, Tumanov, Chernenko, Shashkov.     

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.     

Implosion dynamics of condensed Z-pinch at the Angara-5-1 facility

The implosion dynamics of a condensed Z-pinch at load currents of up to 3.5 MA and a current rise time of 100 ns was studied experimentally at the Angara-5-1 facility. To increase the energy density, 1- to 3-mm-diameter cylinders made of a deuterated polyethylene?agar-agar mixture or microporous deuterated polyethylene with a mass density of 0.03–0.5 g/cm³ were installed in the central region of the loads. The plasma spatiotemporal characteristics were studied using the diagnostic complex of the Angara-5-1 facility, including electron-optical streak and frame imaging, time-integrated X-ray imaging, soft X-ray (SXR) measurements, and vacuum UV spectroscopy. Most information on the plasma dynamics was obtained using a ten-frame X-ray camera (Е > 100 eV) with an exposure of 4 ns. SXR pulses were recorded using photoemissive vacuum X-ray detectors. The energy characteristics of neutron emission were measured using the time-offlight method with the help of scintillation detectors arranged along and across the pinch axis. The neutron yield was measured by activation detectors. The experimental results indicate that the plasma dynamics depends weakly on the load density. As a rule, two stages of plasma implosion were observed. The formation of hot plasma spots in the initial stage of plasma expansion from the pinch axis was accompanied by short pulses of SXR and neutron emission. The neutron yield reached (0.4–3) × 10¹⁰ neutrons/shot and was almost independent of the load density due to specific features of Z-pinch dynamics.     

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.     

Experimental and theoretical studies of the physical processes occurring in thin plane targets irradiated by intense X-ray pulses

Results are presented from experimental and theoretical studies of the interaction of intense X-ray pulses with different types of plane targets, including low-density (～10 mg/cm³) ones, in the Angara-5-1 facility. It is found experimentally that a dense low-temperature plasma forms on the target surface before the arrival of the main heating X-ray pulse. It is demonstrated that the contrast of the X-ray pulse can be increased by placing a thin organic film between the target and the discharge gap. The expansion velocity of the plasma created on the target surface irradiated by Z-pinch-produced X rays was found to be (3–4) × 10⁶ cm/s. A comparison between the simulation and experimental results confirms the validity of the physical-mathematical model used.     

Spectroscopic and probe measurements of the electron temperature in the plasma of a pulse-periodic microwave discharge in argon

A pulse-periodic 2.45-GHz electron-cyclotron resonance plasma source on the basis of a permanent- magnet mirror trap has been constructed and tested. Variations in the discharge parameters and the electron temperature of argon plasma have been investigated in the argon pressure range of 1 × 10^–4 to 4 × 10^–3 Torr at a net pulsed input microwave power of up to 600 W. The plasma electron temperature in the above ranges of gas pressures and input powers has been measured by a Langmuir probe and determined using optical emission spectroscopy (OES) from the intensity ratios of spectral lines. The OES results agree qualitatively and quantitatively with the data obtained using the double probe.     

Diagnostics of MHD instability in the Globus-M spherical tokamak

The paper describes a diagnostic system for studying MHD plasma perturbations in the Globus-M spherical tokamak (a major radius of 0.36 m, a minor radius of 0.24 m, and an aspect ratio of 1.5). The system includes a poloidal and a toroidal array consisting of 28 and 16 Mirnov probes, respectively, as well as a 32-channel proportional soft X-ray detector. Methods are described for calculating the poloidal and toroidal numbers of the dominant helical perturbations by using data from probe measurements. Results are presented of processing the experimental data from some tokamak discharges with a plasma current of 150–250 kA, an average electron density of up to 10²⁰ m^?3, and a toroidal magnetic field of 0.4 T. Specific features of MHD perturbations and their influence on the parameters of the plasma column in different stages of a discharge are briefly discussed.