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.     

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.     

Studies of the distribution of the Z-pinch radiance during the implosion of wire arrays in the angara-5-1 facility

Results are presented from time-integrated measurements of soft X-ray emission from Z-pinches during the implosion of simple and nested wire arrays. The blackening density distribution obtained with the help of a pinhole camera is recalculated into the time-integrated Z-pinch radiance. It is found that, in the case of a simple wire array, up to 70% of the total SXR energy emitted during a discharge is radiated from the axial region, the rest of energy being radiated from plasma jets, whereas in the case of a nested wire array, more than 90% of the SXR energy is radiated from the axial region.     

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.     

Implosion dynamics of a megampere wire-array Z-pinch with an inner low-density foam shell at the Angara-5-1 facility

The implosion dynamics of a pinch with a highly inhomogeneous initial axial distribution of the load mass was studied experimentally. A cascade array consisting of a double nested tungsten wire array and a coaxial inner cylindrical shell located symmetrically with respect to the high-voltage electrodes was used as a load of the Angara-5-1 high-current generator. The cylindrical foam shell was half as long as the cathode? anode gap, and its diameter was equal to the diameter of the inner wire array. It is shown experimentally that two stages are typical of the implosion dynamics of such a load: the formation of two separate pinches formed as a result of implosion of the wire array near the cathode and anode and the subsequent implosion of the central part of the load containing the cylindrical foam shell. The conditions are determined at which the implosion of the central part of the pinch with the foam cylinder is preceded by intense irradiation of the foam with the soft X-ray (SXR) emission generated by the near-electrode pinches and converting it into the plasma state. Using such a load, which models the main elements of the scheme of a dynamic hohlraum for inertial confinement fusion, it is possible to increase the efficiency of interaction between the outer accelerated plasma sheath and the inner foam shell by preionizing the foam with the SXR emission of the near-electrode pinches.     

Comparison of the measured and calculated time profiles of the leakage current in the magnetically insulated transmission line of the angara-5-1 facility

One of the factors limiting the transmission of the electromagnetic pulse to the load in high-power electrophysical facilities is the current leakage in magnetically insulated transmission lines (MITLs). In this paper, the Angara-5-1 eight-module facility with an output power up to 6 TW is considered. The experimental and calculated time profiles of the leakage current for eight-module shots with a dynamic load (cylindrical arrays made of 40 tungsten wires) and single-module shots with a solid cylindrical metal load are compared. When interpreting the results, the contribution of vacuum electrons to the leakage current at the transition from the cylindrical to the conical section of the MITL is taken into account.     

Specific features of the structure of the Z-pinch emitting region formed during the implosion of a foam-wire load at the ANGARA-5-1 facility

Results are presented from experimental studies of the structure of the compressed plasma of a Z-pinch produced during the implosion of a foam-wire load at the current of up to 3 MA. The foam-wire load consisted of two nested cylindrical cascades, one of which was a solid or hollow cylinder made of low-density agar-agar foam, while the other was a wire array. The wall thickness of a hollow foam cylinder was 100–200 μm. The images of the pinch and its spectrum obtained with the help of multiframe X-ray cameras and a grazing incidence spectrograph with a spatial resolution were analyzed. Data on the spatial structure of the emitting regions and the soft X-ray (SXR) spectrum of the Z-pinch in the final stage of compression of a foam-wire load were obtained. The implosion modes characterized by the formation of hot regions during implosion of such loads were revealed. The characteristic scale lengths of the hot regions were determined. It is shown that the energy distribution of SXR photons in the energy range from 80 eV to 1 keV forms the spatial structure of Z-pinch images recorded during the implosion of foam-wire loads. It is revealed that the spectral density of SXR emission in the photon energy range of 300–600 eV from hot Z-pinch regions exceeds the spectral density of radiation from the neighboring Z-pinch regions by more than one order of magnitude. Groups of lines related to the absorption and emission of radiation by atoms and multicharged ions of carbon and oxygen in the outer foam cascade of a foam-wire load were recorded for the first time by analyzing the spatial distribution of the SXR spectra of multicharged ions of the Z-pinch. The groups of absorption lines of ions (C III, O III, O IV, and O VI) corresponding to absorption of SXR photons in the Z-pinch of a tungsten wire array, which served as the inner cascade of a foam-wire load, were identified. The plasma electron temperature measured from the charge composition of carbon and oxygen ions in the outer agar-agar foam cascade was 10–40 eV. During the implosion of foam-wire loads at currents of up to 3 MA, SXR pulses (hν > 100 eV) with a duration of 10 ns and peak power of 3 TW were detected. It is shown that the temporal profile of single-peak and double-peak SXR pulses can be controlled by varying the parameters of the outer and inner cascades of the foam-wire load.     

Measurements of the azimuthal magnetic field within imploding multiwire arrays in the Angara-5-1 facility

Results are presented from measurements of the azimuthal magnetic fields within imploding multiwire tungsten arrays in the Angara-5-1 facility at currents of 2.5–4 MA. It is shown that the penetration of the magnetic field into the axial region of the wire array lags behind the discharge current pulse. The current of a precursor produced at the array axis prior to the implosion of the bulk array mass is measured. It is found that the magnetic field in the initial stage of implosion is azimuthally nonuniform. The mass distribution inside the array is calculated from the measured magnetic field.     

Measurements of the axial magnetic field during the implosion of wire arrays in the Angara-5-1 facility

Results are presented from measurements of the axial magnetic field during the implosion of tungsten wire arrays in the Angara-5-1 facility at currents of 2.5–4.5 MA. The azimuthal structure of the plasma produced from the wires is examined using the effect of the compression of the axial magnetic flux by this plasma. It is shown that the plasma starts to penetrate into the axial region of the wire array at the very beginning of implosion. A mechanism other than the formation of a closed current-carrying shell is proposed for describing the transfer of the external axial magnetic field to the central region of the array.     

Generation and anisotropy of neutron emission from a condensed Z-pinch

The paper presents results of measurements of neutron emission generated in the constriction of a fast Z-pinch at the S-300 facility (2 MA, 100 ns). An increased energy concentration was achieved by using a combined load the central part of which was a microporous deuterated polyethylene neck with a mass density of 100 mg/cm³ and diameter of 1–1.5 mm. The neck was placed between two 5-mm-diameter agar-agar cylinders. The characteristics of neutron emission in two axial and two radial directions were measured by the time-of-flight method. The neutron spectrum was recovered from the measured neutron signals by the Monte Carlo method. In all experiments, the spatiotemporal characteristics of plasma in the Z-pinch constriction were measured by means of the diagnostic complex of the S-300 facility, which includes frame photography in the optical, VUV, and soft X-ray (SXR) spectral regions; optical streak imaging; SXR detection; and time-integrated SXR photography. The formation of hot dense plasma in the Z-pinch constriction was accompanied by the generation of hard X-ray (with photon energies E > 30 keV), SXR (with photon energies E > 1 keV and duration of 2–4 ns), and neutron emission. Anisotropy of the neutron energy distribution in the axial direction was revealed. The mean neutron energies measured in four directions at angles of 0° (above the anode), 90°, 180° (under the cathode), and 270° with respect to the load axis were found to be of 2.1 ± 0.1, 2.5 ± 0.1, 2.6 ± 0.2, and 2.4 ± 0.1 MeV, respectively. For a 1-mm-diameter neck, the maximum integral neutron yield was 6 × 10⁹ neutrons. The anisotropy of neutron emission for a Z-pinch with a power-law distribution of high-energy ions is calculated.     

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 study of the ion flux parameters and anode plasma dynamics in the Angara-5-1 facility

Results are presented from experimental studies of the anode plasma dynamics and measurements of the ion flux ejected along the axis of a high-current Z-pinch. Pinch discharges were formed by the implosion of tungsten wire arrays in the Angara-5-1 facility. It is shown that the ion energy spectrum depends on the mass and configuration of wire arrays, as well as on the diameter of the anode aperture. The shape of the ion spectrum indicates that the plasma propagates in the form of a compact plasmoid. Shadow and X-ray images of the plasma show that the axial velocity of the plasma outflowing through the anode aperture is comparable with the velocity of radial plasma compression and, for tungsten ions, can reach a value corresponding to an energy of 100 keV. The experimental data indicate that the ion energy spectrum mainly forms due to the electrodynamical acceleration of the plasma and cumulative jets. A possible mechanism for the production of compact plasma formations in the course of electrodynamic plasma acceleration is discussed.     

Study of the formation,stability, and X-ray emission of the Z-pinch formed during implosion of fiber arrays at the Angara-5-1 facility

Results from experimental studies on the implosion of arrays made of kapron fibers coated with different metals (Al, In, Sn, and Bi) are presented. It is shown that the power, total energy, and spectrum of radiation emitted by the imploding array depend on the number of metallized fibers and the mass of the metal layer deposited on them but are independent of the metal characteristics (density, atomic number, etc.). Analysis of frame X-ray images shows that the Z-pinches formed in the implosion of metallized kapron fiber arrays are more stable than those formed in wire arrays and that MHD perturbations in them develop at a slower growth rate. Due to the lower rate of plasma production from kapron fibers, the plasma formed at the periphery of the array forms a layer that plays the role of a hohlraum wall partially trapping soft X-ray emission of the Z-pinch formed in the implosion of the material of the deposited metal layer. The closure of the anode aperture doubles the energy of radiation emitted in the radial direction.     

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.     

Studies of the implosion of cylindrical fiber arrays on the Angara-5-1 facility

Results are presented from experimental studies of the implosion of cylindrical kapron fiber arrays with addition of high- and medium-Z metal wires (tungsten, aluminum). The experiments were carried out on the Angara-5-1 facility at currents of 3–4 MA. The ablation rate in kapron fiber arrays is estimated and compared with that in tungsten wire arrays.     

High-power electromagnetic wave in a magnetically insulated transmission line of the ANGARA-5-1 facility

Studies of physical phenomena in magnetically insulated transmission lines (MITLs) of high-power pulsed current generators, analysis of operation of existing megavolt generators, and designing of new high-current generators with a power of up to ∼10 TW require creating an efficient numerical code for modeling the propagation of a high-power electromagnetic pulse in an MITL. This paper presents basic theoretical concepts of MITL operation in the framework of telegraph equations with allowance for electron leakage and variations in the electrode emissivity and analyzes propagation of an electromagnetic wave in the MITLs of the ANGARA-5-1 eight-module facility toward a dynamic load installed in the central unit with a matrix inductance.     

Study of the implosion of foam-wire loads at the Angara-5-1 facility

Results are presented from experiments on studying the compactness of compression of imploding nested foam-wire loads at currents of up to 4 MA at the Angara-5-1 facility. The degree of pinch compression was estimated from the dynamics of the spatial distribution of the current (magnetic field) and the shape of the soft X-ray pulse. The load consisted of nested cascades, one of which being a wire array and the other being a hollow or solid low-density cylinder made of agar-agar foam with a wall thickness of 100?C200 ??m. In some experiments, one of the cascades was made of C₂₀H₁₇O₆ solid-state organic acid foam. The radial distribution of the magnetic field inside the nested cascades of the imploding foam-wire load (both between the cascades and inside the inner cascade) was measured using tiny magnetic probes. The measured radial distributions of the magnetic field are compared with the magnetic field configuration calculated using a one-dimensional MHD code simulating the implosion of a nested foam-wire load. It is shown that the spatial structure of the current and magnetic field during the implosion of such a load is determined by the development of supersonic and subsonic magnetized plasma flows in its cascades. The specific features of pinch formation and methods for the compensation of the nonsimultaneous pinch compression between the anode and the cathode (the zipper effect) during the implosion of a nested foam-wire load are analyzed.     

Influence of the current growth rate on the polarity effect in a wire array in the Angara-5-1 facility

It is shown that, at a sufficiently high current growth rate, the initial stage of implosion of a wire array is significantly affected by the radial electric fields. Due to the specific electrode configuration of wire arrays, the magnitude of the oppositely directed radial electric fields in different wire segments can reach 5 MV/cm. It is found that the process of plasma formation proceeds in different ways in segments with oppositely directed initial radial electric fields. The influence of this effect (the so-called “polarity effect”) on the implosion of cylindrical tungsten wire arrays in the Angara-5-1 facility becomes significant when the load voltage grows at a sufficiently high rate.     

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.     

Laser radiation scattering from the wires and fibers of imploding arrays on the Angara-5-1 facility

A method is developed for measurements of laser radiation scattering by wires and fibers in different types of imploding arrays in the initial stage of plasma production at discharge currents per wire of up to 2 kA for aluminum arrays and up to 8 kA for tungsten arrays. The experiments were carried out on the Angara-5-1 facility at a current density in the wires of 10⁸ A/cm² and current growth rate of ～10¹³ A/s. It is found that the indicatrix of laser radiation reflected from the wires (fibers) in cylindrical and conical arrays is modified at currents of 0.1–10 kA per wire (fiber). The experimental data on the reflection and scattering of laser radiation from wires and fibers are compared with the results of numerical simulations of their electric explosion in vacuum. It is proposed that the change in the reflection indicatrix of laser radiation is caused by the onset of thermal instabilities. The typical size of density and temperature inhomogeneities on the wire surface is in a range of 10–20 μm, which probably results in a transition from specular to diffuse reflection of laser radiation. A simultaneous abrupt (over 2–3 ns) reduction in the reflection intensity from several wires of an array indicates a homogeneous distribution of the discharge current over the irradiated wires. This closes the issue of the quality of the contact between the wires and the electrodes. The obtained experimental information is of considerable importance for the development of numerical codes for simulations of the implosion of wire arrays and the refinement of the wire parameters in the initial stage of plasma production.