A. Yu. Shashkov

Study of the Dynamics of the Electrode Plasma in a High-Current Magnetically Insulated Transmission Line

A series of experiments was carried out in the S-300 facility (3 MA, 0.15 Θ, 100 ns) to study the behavior of a section of a magnetically insulated transmission line (MITL) at current densities of up to 500 MA/cm2 and linear current densities of up to 6 MA/cm (i.e., at parameters close to those expected in a fast Z-pinch fusion reactor projected in Sandia National Laboratories). The surface explosion of the ohmically heated MITL electrode is accompanied by the formation of a plasma layer on its surface. This can deteriorate of the transmission properties of the line because the vacuum gap is short-circuited by the plasma produced. The parameters of the electrode plasma and its effect on the MITL transmission properties were investigated experimentally. Possible consequences of the above effects are evaluated, and MHD simulations of the electrode explosion and the subsequent spread of the plasma layer are performed. It is shown that the time during which an MITL segment preserves its transmission properties conforms to the requirements of the conceptual fusion reactor.

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.

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 ne=1022 cm−3, electron temperature Te=0.8–1.5 keV, and ion temperature Ti=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.

Diagnostic arrangement on S-300 facility

A set of experiments on neutron and x-ray generation was performed on the S-300 facility for the last few months. The S-300 facility is a pulse generator with electric current achieving 3.5 MA, voltage 400–500 kV, and rise time ∼100 ns. (Chernenko et al., Proc. of the 11th International Conference of Power Particle Beams, Prague, 1966, p. 154). It is designed as van eight-module machine transmitting electromagnetic power to the low-inductive load through the vacuum line with self-magnetic isolation. Diagnostic techniques and methods on S-300 used are described in this article. Ten-channel polychromator is used for the soft x-ray and vacuum ultraviolet bands (50–500 eV) measurements with spectral resolution of 5%–20% and time resolution less than 2.5 ns. Radiated power measurements in the region of 0.1–10 keV are performed by vacuum x-ray diodes and semiconductor detectors equipped with different filters. Curved crystal x-ray spectrography is intended for plasma density, electron, and ion temperature evalu...

Spiky structure of coherent emission from optically thick media

The spiky structure of the spontaneous emission from praseodymium ions in the LaF3 lattice on the 3P0-3H6 transition under coherent pulsed excitation of the adjacent 3H4-3P0 transition has been studied experimentally and theoretically. The experimental results are in good agreement with the theoretical conclusions based on analysis of the emission dynamics of an excited ensemble of three-level systems in the mean field approximation. The generation of a spiky structure is interpreted as the emission of a cnoidal wave pulse. The recorded random pattern of time intervals between the spikes reflects the sensitivity of the cnoidal wave period to fluctuations in the system. The developed theory has been found to be related to models of deterministic chaotic dynamics.

Superradiant emission of LaF3:Pr3+ in resonator

Experimental data concerning superradiant emission from the LaF3 medium doped with impurity praseodymium ions are presented in the cases of a free medium and when the medium is placed into an optical resonator. The spike structure of superradiance is registered and studied. When the medium is placed inside a cavity, a new channel of energy removal by superradiance related to the cavity mode appears; the old noncavity channels are preserved. The duration of superradiance in the cavity channel is decreased, and regular modulation arises. These peculiarities of superradiance induced by the presence of an optical resonator are explained.

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.

The experimental investigations of imploding plasma as a source of hard x-ray

The stability of magnetic compression of an imploding produced by a gas jet with annual supersonic nozzle is studied the aim to achieve a high X-ray output. A stable regime is identified. (AIP)

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.

Anomalies in the concentration quenching of luminescence in doped Y2SiO5:Pr3+ nanocrystals

In Y2SiO5:Pr3+ nanocrystals, an ordered phase is observed in the 1D2 luminescence decay curves of Pr3+ ions at their anomalously low concentration (0.5 at %). This effect is caused by the predominant accumulation of activator ions near the nanocrystal surface, which provides relaxation of the elastic strains arising as a result of the misfit between the ionic radii of Pr3+ and Y3+. The concentration quenching of Pr3+ luminescence is due to cooperative cross relaxation.