A. Bartov

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.

Transportation Properties of a High-Current Magnetically Insulated Transmission Line and Dynamics of the Electrode Plasma

Results are presented from experimental studies of a section of a magnetically insulated transmission line (MITL) with a current density of up to 500 MA/cm2 and linear current density of up to 7 MA/cm (the parameters close to those in a fast-Z-pinch-driven fusion reactor projected at Sandia Laboratories). The experiments were performed in the S-300 facility (3 MA, 0.15 Ω, 100 ns). At high linear current densities, the surface of the ohmically heated MITL electrode can explode and a plasma layer can form near the electrode surface. As a result, the MITL can lose its transmission properties due to the shunting of the vacuum gap by the plasma produced. In this series of experiments, the dynamics of the electrode plasma and the dependence of the transmission properties of the MITL on the material and cleanness of the electrode surface were studied. It is shown experimentally that, when the current with a linear density of up to 7 MA/cm begins to flow along a model MITL, the input and output currents differ by less than 10% over a time interval of up to 230 ns for nickel electrodes and up to 350 ns for a line with a gold central electrode. No effect of the oil film present on the electrode surface on the loss of the transmission properties of the line was observed. It is also shown that electron losses insignificantly contribute to the total current balance. The experimental results are compared with calculations of the electrode explosion and the subsequent expansion of the plasma layer. A conclusion is made that the life-time of the model MITL satisfies the requirements imposed on the transmission lines intended for use in the projected thermonuclear reactor.

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)

Experimental Modeling on the Recyclable Magnetically Insulated Transporting Lines Aimed at the IFE Reactor

At the Kurchatov Institute, the series of IFE experiments is carried out on the S-300 high-current generator (3 MA, 100 ns, 0.15 Ohm). In this talk, the experimental results on the S-300 machine are presented, related to the study of operation of co-axial magnetically self-insulated transporting line, by the linear current flow density on the inner electrode surface up to j ≈ 7 MA/cm. The specific parameters of this current-carrying line fairly correspond to those of the Sandia Laboratories’ conceptual project of IFE reactor based on the fast Z-pinch [1]. The near-electrode plasma dynamics and its effect on the current transport were studied. The cathode material effect, as well as that of gases and oil layer absorbed by its surface, on the near-electrode plasma dynamics and the current pulse transport along the short MITL segment ware studied too. With respect to the whole project, our conclusion is quite optimistic, to wit, we have found the idea of Recyclable Transporting Lines being adequate to the IFE reactor requirements.

Pulsed power experiments at the Kurchatov Institute aimed at ICF

A series of experiments has been carried out at the linear current flow densities up to 7 MA/cm, aimed at physical modeling of magnetically insulated transmission line of the Inertial Fusion Energy reactor based on the fast Z-pinch. The goals of these experiments were as follows: a) the study of the near-electrode plasma and its effect on the energy transfer; b) determination of the critical MITL parameters to foresee possible restrictions on the efficiency of IFE reactor. The loads were fabricated on the base of foam as profiled cylinders of (3–5) mm in diameter and with 30 mg/cm3 density, with the neck in its central part of about 1 mm in diameter. The goal of this series of experiments was the investigation of plasma dynamics in the Z-pinch neck and the mechanism of neutron generation accompanying the current-driven implosion. The prospects of application plasma opening switches as output cascades of pulsed power generators of megajoule range is studied on base of RS-20 machine. By using the programmed fill the diode gap by plasma, the suppression of pre-pulse has been achieved and shortening the pulse from 40 μs to 100 ns has been obtained.

Experiments and Simulations on the Plasma Dynamics in Vacuum Transporting Lines Aimed at the Z‐Pinch IFE Reactor

A series of experiments has been carried out on the S‐300 pulsed power machine (3 MA, 0.15 Ohm, 100 ns), devoted to the study of a section of magnetically insulated vacuum transporting line (MITL), by the current flow density up to 500 MA/cm2, by the linear current flow density being up to 6 MA/cm. These parameters fairly correspond to those of the Sandia Laboratories’ conceptual project of IFE reactor based on the fast Z‐pinch. The goals of experiments were as follows: 1) study of the near‐electrode plasma and its effect on the energy transfer; 2) testing the 2‐temperature MHD code NPINCH modeling the behavior of near‐electrode plasmas. As a result, the conditions of plasma formation and reconnection of the MITL gap have been determined.

Experiments aimed at the “Baikal” program

On the S-300 pulsed power generator (4.5 MA, 70 ns, 0.15 Ohm), within the frames of ICF program based on fast high-current Z-pinches, experiments are being carried out studying promising schemes of output units. In particular, a nanosecond-range plasma flow switch is being investigated aimed at sharpening the pulse. As a result, the switching rate as high as 2.5 MA / 2.5 ns has been achieved. The numerical simulation of such a device has been carried out. The results of experiments on the extrinsic magnetic field influence on high-impedance plasma opening switch (POS) operation are reported. It has been demonstrated that the POS output voltage could be increased by the factor of 1.5 compared to the POS free of an extrinsic field. Also the possibility of the expansion of POS conduction phase duration up to 40 µs has been presented. The development of more new laser and nuclear diagnostic methods will definitely contribute to attain new data which could help in understanding the POS operation physics.