Xiaobing Zou

A pulsed power generator for x-pinch experiments

Abstract A;500 kV0400 kA0100 ns pulsed power generator~PPG-I! for x-pinch experiments was designed and constructed atTsinghua University. It is composed of a Marx generator, a combined pulse forming line~PFL!, a gas-filled V0N fielddistortion switch, a transfer line, and a copper-sulphate resistive load for testing.The PPG-I implements a novel designin lines that four pieces of waterline with impedance 5V in parallel constitute a combined PFL with 1.25V, andincorporate each other by a common self-break V0N switch on a matched 1.25V transfer line. At the peak chargingvoltage of the PFL, the V0N switch breaks down in multi-channel discharge mode, and electric energy is fed into thetesting load through the 1.25V transfer line. This article presents the design and test of the PPG-I generator. Keywords: Multi-channel discharge; Pulse forming line; Pulsed power; Rise time; X-pinch 1. INTRODUCTION Between the output electrodes of a pulsed power generator,two or more wires crossing and touching at a single pointconstitute an “x” shape load. The cross position explodesfirst and then pinches axially to form the so-called x-pinchplasma,bythemagneticpressureaspulsedpowergeneratordelivers a suitable current to the “x” wires ~Kalantar H Shelkovenko

X-Ray Backlighting of Developments of

A pulsed-power generator (PPG-I) with a current of 400 kA in amplitude and 100 ns in pulsewidth was constructed. The development of X-pinches and the early stages of wire-array Z-pinches were investigated by X-ray backlighting using the X-pinch driven by PPG-I as X-ray source. The plasma explosion and implosion near the cross point of the X-shape wires can be clearly seen from a series of backlighting images taken at different times of X-pinch discharge. To study the early behavior of the plasma in wire-array Z-pinches, an X-pinch installed in the place of a current-return rod was used as the X-ray source to backlight a wire-array Z-pinch installed in the center between the anode and the cathode. The plasma formation, the interwire plasma merging, and the instabilities were also clearly observed, which can provide a better understanding of the physics of Z-pinches and the basic experimental data to validate simulation models.

X

The glow discharges in the gaps geometrically similar to that used in glow discharge cleaning of the International Thermonuclear Experimental Reactor (ITER) were numerically simulated based on a two-dimensional fluid model in which the linear dimensions of gap A are two times that of gap B and the pressure of gap A is half that of gap B. Under an applied voltage of 1000 V, the physical parameters at the corresponding point pz in these two gaps were compared. It was found that the electric potential U(pz), the reduced field E(pz)/p and the electron temperature Te(pz) are equal in values for these two gaps, but the electron density ne(pz) and ion density ni(pz) for gap B are four times that of gap A. All these parameter ratios are the same as that defined by similarity law, which confirmed that the similarity law is valid for the glow discharges in non-plane-parallel gaps.

-pinches and Wire-Array

The circuit simulation of nonuniform transmission lines that were under consideration for the next generation of petawatt-class Z-pinch drivers was performed. It was confirmed that the power-transport efficiency of the designed radial transformer with an exponential impedance profile is higher than that with Gaussian profiles. Based on Fouriers theorem and the principle of superposition, it was found that there are considerable low-frequency components for the input voltage of a half-sine wave with an angular frequency of 14 Mrad/s. Compared with the exponential transformer, the Gaussian transformers amplify the low-frequency components to a lower extent, which makes the difference in power efficiency between the two types of transformers. The radial transformer also serves as a passive high-pass filter, and the Gaussian transformer may give better performance than the exponential transformer does if the signal transport rather than the power transport is concerned.

Z

The evolution of an exploding titanium wire embedded in 10 kPa air was studied with Mach–Zehnder interferometry. The exploding wire is characterized by a central dense core as well as the surrounding plasma and a rapidly expanding gas shell. Based on the fringe shifts, the electron density of the plasma and the increased density in the gas shell were calculated to be about 1.94×1018/cm3 and 4.78×10−4 g/cm3, respectively. The expanding speed of the gas shell is about 2.28 km/s. A thermal expansion lag of the dense vapor core and two-phase vaporization of the wire were observed. Due to the wire exploding in “under heat” mode, the first phase of the vaporization is by the Joule heating and the second one by the plasma heating.

-pinches Using an

A flashover switch inserted between the cathode and wire end was used to enhance the deposition energy into an exploding wire in vacuum. It was found that this flashover switch can play two roles. The first one is to reduce the rise time of the current pulse, the second and more important one to greatly improve the surface radial electric field and suppress the electronic emission that is responsible for triggering the surface breakdown of the exploding wire. Only when the flashover switch is on the cathode, the second one can take effect and result in a much higher deposition energy. In this case, the deposition energy could increase by ∼three times (3.4 eV/atom–12 eV/atom) under the negative current pulse and by ∼two times (5.7 eV/atom–13 eV/atom) under the positive one, and fully vaporized wire core was observed with laser interferometry.

X

In order to investigate the influence of electrode radius on the characteristics of cathode fall thickness, experiments of low-pressure (20 Pa ≤ p ≤ 30 Pa) abnormal glow discharge were carried out between parallel-plane electrodes in different radii keeping gap distance unchanged. Axial distributions of light intensity were obtained from the discharge images captured using a Charge Coupled Device camera. The assumption that the position of the negative glow peak coincides with the edge of cathode fall layer was verified based on a two-dimensional model, and the cathode fall thicknesses, dc, were calculated from the axial distributions of light intensity. It was observed that the position of peak emission shifts closer to the cathode as current or pressure grows. The dependence of cathode fall thickness on the gas pressure and normalized current J/p2 was presented, and it was found that for discharges between electrodes in large radius the curves of pdc against J/p2 were superimposed on each other, however...

-pinch

The intersection of Paschens curves for argon with a same gap length but a different electrode radius was observed. While the breakdown voltage increases with the increase of the nonuniformity in the electric field of the gap at lower pressures, it decreases at higher pressures. The reason for the intersection of Paschens curves was given based on the mean free path length of the electrons inversely proportional to the gas pressure and the electron impact ionization coefficient exponentially increasing with the electric field. The intersection of the Paschens curves was qualitatively reproduced by a numerical simulation.

Validity of the similarity law for the glow discharges in non-plane-parallel gaps

A new method was designed for calibrating a Rogowski coil of fast time response. The method is based on the cable pulser method except that the voltage signal pick-off output was moved to a position with a distance l from the load. If 2l/v is longer than the time duration of the forward voltage pulse u(f)(t), the reflected voltage pulse u(r)(t) could be separated from u(f)(t) and directly measured. Using the formula i(t)=[u(f)(t)-u(r)(t)]/50 to calculate the primary current of the Rogowski coil, the coil could be more accurately calibrated.

Comparison of Nonuniform Transmission Lines With Gaussian and Exponential Impedance Profiles for

A three-frame Mach–Zehnder interferometer (TFMZI) was developed for taking three pictures of the imploding plasma produced by one shot of fast pulsed discharge. TFMZI consists of a YAG laser, a beam splitter, and a Mach–Zehnder interferometer. By passing through the beam splitter the laser beam was split into three beams with an interbeam delay of 13 ns and each laser beam will give a snapshot of the implosion state. The measurements with the TFMZI were conducted on a small gas-puff z-pinch device and gave good results. In principle an upgraded four or (more) frame Mach–Zehnder interferometer could be constructed if much improvement was made on the TFMZI.