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.     

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.     

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.     

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.     

Neutron emission during the implosion of a wire array onto a deuterated fiber

Results are presented from Z-pinch experiments performed in the S-300 facility (Kurchatov Institute) at a maximum current of 2 MA and current rise time of 100 ns. The Z-pinch load was a 1-cm-long 1-cmdiameter cylindrical array made of 40 tungsten wires with a total mass of 160 μg, at the axis of which a 100-μm-diameter (CD₂) _n deuterated fiber was installed. Hard X-ray and neutron signals were recorded using five scintillation detectors oriented in one radial and two axial directions. The maximum neutron yield from the DD reaction reached 3 × 10⁹ neutrons per shot. The average neutron energy was determined from time-of-flight measurements and Monte Carlo simulations under the assumption that the neutron emission time was independent of the neutron energy. The average neutron energy in different experiments was found to vary within the range 2.5–2.7 MeV. The fact that the average neutron energy was higher than 2.45 MeV (the energy corresponding to the DD reaction) is attributed to the beam-target collisional mechanism for the acceleration of deuterons to 100–500 keV.     

High-energy neutron spectroscopy with thick silicon detectors

The high-energy neutron component of the space radiation environment in thick structures such as the International Space Station contributes to the total radiation dose received by an astronaut. Detector design constraints such as size and mass have limited the energy range of neutron spectrum measurements in orbit to about 12 MeV in Space Shuttle studies. We present a new method for high-energy neutron spectroscopy using small silicon detectors that can extend these measurements to more than 500 MeV. The methodology is based on measurement of the detector response function for high-energy neutrons and inversion of this response function with measured deposition data to deduce neutron energy spectra. We also present the results of an initial shielding study performed with the thick silicon detector system for high-energy neutrons incident on polyethylene.     

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.     

Collective ion acceleration during the decay of a high-current Z-pinch

A study is made of the Z-pinch plasma expansion after the current is switched off. Measurements were carried out in experiments on the implosion of tungsten wire arrays in the Angara-5-1 facility. It is found experimentally that, at a distance of 2 m from the pinch, the ion velocity in the expanding Z-pinch plasma is about (2.5–4.0) × 10⁷ cm/s, which substantially exceeds the thermal velocity of tungsten ions. A model describing the plasma expansion process is proposed that is based on the ambipolar acceleration mechanism. The results of numerical simulations are compared with the experimental data.     

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.     

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.     

High-energy ion generation by short laser pulses

This paper reviews the many recent advances at the Center for Ultrafast Optical Science (CUOS) at the University of Michigan in multi-MeV ion beam generation from the interaction of short laser pulses focused onto thin foil targets at intensities ranging from 10¹⁷ to 10¹⁹ W/cm². Ion beam characteristics were studied by changing the laser intensity, laser wavelength, target material, and by depositing a well-absorbed coating. We manipulated the proton beam divergence using shaped targets and observed nuclear transformation induced by high-energy protons and deuterons. Qualitative theoretical approaches and fully relativistic two-dimensional particle-in-cell simulations modeled energetic ion generation. Comparison with experiments sheds light on ion energy spectra for multi-species plasma, the dependences of ion-energy on preplasma scale length and solid density plasma thickness, and laser-triggered isotope yield. Theoretical predictions are also made with the aim of studying ion generation for high-power lasers with the energies expected in the near future, and for the relativistic intensity table-top laser, a prototype of which is already in operation at CUOS in the limits of several-cycle pulse duration and a single-wavelength spot size.     

Generation of superthermal electrons in a micropinch discharge

Results are presented from experimental studies of the formation of the superthermal electron component in a micropinch discharge plasma. Radiative collapse in a Z-pinch is found to affect the energy of the accelerated electrons. That the radiative collapse has been reached may be inferred from the energy of the emitted hard X-ray photons.     

Correlation of the neutron yield anisotropy with the electrical characteristics of a plasma focus discharge

The anisotropy of the yield and energy of neutrons generated in a small-size plasma focus chamber with a total neutron yield of about 4 × 10⁹ DD neutrons per shot was investigated experimentally. The neutrons were recorded using scintillation detectors on a 3-m-long flight base. The measurements were performed at the angles 0° and 90° with respect to the chamber axis. The maximum neutron energy measured by the time-of-flight method at the angles 0° and 90° was found to be 2.8 and 2.5 MeV, respectively. The measured anisotropy of the neutron yield was in the range 1.15–1.88. The integral DD neutron yield of the source was measured using the activation method (by activating silver isotopes). It is found that the neutron yield and the yield anisotropy depend linearly on the discharge current jump ΔI at the instant of neutron generation.     

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.     

Spatiotemporal behavior of X-ray emission above 20 keV from a Z-pinch produced by wire-array implosion

Results are presented from experimental studies of hard X-ray (HXR) emission in the photon energy range above 20 keV from dense radiating Z-pinch plasmas. The work is aimed at revealing the nature of fast-electron (electron beam) generation during the implosion of cylindrical and conical wire arrays in the Angara-5-1 facility at currents of up to 3 MA. It is found that the plasma implosion zippering caused by the inclination of wires affects the parameters of the HXR pulse emitted during the implosion of a conical array. It is shown that HXR emission correlates well with the decay of the plasma column near the cathode in the stagnation phase. HXR images of the pinch are produced by the bremsstrahlung of fast electrons generated during plasma column decay and interacting with plasma ions and the anode target. It is found that the use of conical arrays makes it possible to control the direction of plasma implosion zippering and the spatiotemporal and energy parameters of the pinch X-ray emission, in particular the X-ray yield. For wire array with diameters of 12 mm and linear masses of 200–400 μg/cm, the current of the fast electron beam is 20 kA and its energy is 60 J, which is about 1/500 of the energy of the main soft X-ray pulse.     

Electromagnetic plasma acceleration in an inverse Z-pinch geometry

Results are presented from experimental studies of the electromagnetic acceleration of a hydrogen or deuterium plasma in an inverse Z-pinch geometry. The acceleration dynamics of the plasma shell was simulated in a zero-dimensional model and was measured with magnetic probes. The ion energy spectrum in the plasma flow was determined with the help of ion collectors by the time-of-flight technique.     

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.     

Increasing the Specific Energy Concentration of High-Current Z-Pinch Plasma by Using Transient Compression Modes

Plasma Physics Reports - The results of experiments on increasing the specific energy density of high-current Z-pinch plasma by using various compression modes are presented. The experiments were...     

Theory of the anomalous resistivity of a randomly inhomogeneous plasma

The effect of random density inhomogeneities on the anomalous plasma resistivity caused by a current-driven ion acoustic instability is considered. It is shown that, under certain conditions, dissipation due to the plasma inhomogeneity can be more efficient than that due to nonlinear effects. The scenario under consideration can occur in the low-density corona of a high-density Z-pinch.     

Influence of asynchronous actuation of the Angara-5-1 facility modules on the penetration of the magnetic field inside a wire array

The influence of asynchronous actuation of the Angara-5-1 facility modules on the implosion symmetry of the wire array plasma in the Z-pinch mode is studied. It is shown that an increase in the r.m.s. jitter in the start times of the modules leads to an appreciable azimuthal asymmetry of magnetic field penetration inside the wire array and, as a consequence, to an increase in the duration of the soft X-ray pulse, a decrease in its power, and a shift of the Z-pinch with respect to the array axis. Necessary conditions for axisymmetric pinch implosion are determined. Experimental data on the magnitude and azimuthal distribution of the current (magnetic field) inside the wire array were obtained from magnetic probe measurements. The position of the Z-pinch with respect to the wire array axis was determined from two-dimensional X-ray images and radial optical streak images.