Xinlei Zhu

X-Ray Backlighting of the Initial Stage of Single- and Multiwire

The initial stage of single- and multiwire Z-pinch was investigated by X-ray backlighting using an X-pinch as the soft X-ray source. The experiments were carried out on the pulsed power generator PPG-I (400 kA/500 kV/100 ns), which was designed and constructed by Tsinghua University. The source X -pinch and object single- or multiwire Z-pinch are installed in the place of a current-return rod or the center between the anode and the cathode, respectively. The X-ray films with high resolution and high sensitivity were used to record the results. A resistive current probe and a Rogowski coil were used to monitor the current, and a step wedge filter was designed to measure the mass ablation rate of the thin wire. By a large number of imaging experiments, the time sequence of images for the coronal plasma formation, the interwire plasma merging, and the development of plasma instabilities of Z-pinch, as well as some important quantitative parameters such as mass ablation ratio and core expansion ratio, were obtained.

Z

Wire explosion is the initial stage of the development of wire array Z-pinch load driven by high pulsed current. Based on the pulsed power generator PPG-1 (400 kA/100 ns), the process of 10-μm-W wire explosion at tens of kiloamperes was investigated. The time sequence images of wire explosion were obtained by X-ray backlighting with X-pinch X-ray point source, and the mass density distribution of the exploding wire at a certain time was calibrated by method of grayscale contrast based on a particular step wedge filter. By image processing of the time sequence images of wire explosion, the evolution of mass density distribution of wire explosion was presented.

-Pinch

The evolution of single- and dual-wire Z-pinch of 8-μm W was investigated by X-ray backlighting using an X-pinch X-ray source. The experiments were carried out on the pulsed-power generator PPG-I (400 kA/500 kV/100 ns), which was designed and constructed by the Department of Electrical Engineering of Tsinghua University. To scale the mass density distributions at different explosion time, a mass step wedge including eight tungsten layers was fabricated and inserted between the object Z-pinch and the X-ray film. By a large number of imaging experiments, the physical images of coronal plasma formation and interwire plasma merging were obtained. Based on the X-ray photos of 8-μm W and mass step wedge, the mass density distributions at different explosion time were drawn, and the conductance curve of time dependence of 8-μm W was also calculated using waveform of voltage and current.

Measuring the Evolution of Mass Density Distribution of Wire Explosion

Based on an X-pinch X-ray point source, the electrical explosion of coated and non-coated wires with 25 μm diameter was backlit. The experiments were performed on the pulsed power device PPG-1 (500 kV/400 kA/100 ns) which was designed and constructed by the Department of Electrical Engineering of Tsinghua University. The source X-pinch was installed between the main output electrodes, while the object coated and non-coated wires were placed at the positions of the left and right current return rods, respectively. The backlighting images were recorded by X-ray films with high resolution and sensitivity. By a large number of backlighting experiments, the exploding physical images and expansion ratio curves of the coated and non-coated wires were obtained, and the results showed that coating can make the wire expand to be larger and more uniform.

Mass Density Evolution of Wire Explosion Observed Using X-Ray Backlight

In order to use two paralleled X-pinches as X-ray sources for the time-resolved backlighting of wire-array Z-pinch plasma, it is necessary to make these two X-pinches emit X-rays at different but roughly preset time instants. The timing of the X-ray burst from an X-pinch independence of the current, and the wire mass of the X-pinch was investigated. The currents flowing through two paralleled X-pinches were measured and it was found that the total current is almost equally divided between these two X-pinches no matter how different the wires for these two X-pinches are. The reason for the equal current division between two paralleled X-pinches was given based on the inductance calculation of the X-pinch circuit.

The development of coated and non-coated wire explosions observed by X-ray backlighting

The initial stage of single-wire and multi-wire Z-pinch were investigated by X-ray backlighting using an X-pinch as the soft X-ray source. The experiments were carried out on the pulsed power generator PPG-I (400kA/500kV/100ns) which was designed and constructed by Tsinghua University. The source X-pinch and object single-wire or multi-wire Z-pinch are installed in the place of a current-return rod or the center between the anode and the cathode, respectively. The X-ray films with high resolution and high sensitivity were used to record the results. A resistive current probe and a Rogowski coil were used to monitor the current, and a step wedge filter was designed to measure the mass ablation rate of the thin wire. By a large number of imaging experiments, the time sequence of images for the coronal plasma formation, the inter-wire plasma merging, and the development of plasma instabilities of Z-pinch as well as some important quantitative parameters like mass ablation rate and core expansion rate were obtained.

Current division between two paralleled X-pinches

The current and the voltage of an X-pinch were measured. The inductance of the X-pinch was assumed to be a constant and estimated by the calculation of the magnetic field based on the well-known Biot-Savarts Law. The voltage of the inductance was calculated with Ldi/dt and subtracted from the measured voltage of the X-pinch. Then, the resistance of the X-pinch was determined and the following results were obtained. At the start of the current the resistance of the exploding wires is several tens of Ohms, and then it falls down quickly to about 1Ω, which reflects a current transition from the overheated and highly resistive wire core to the highly conductive plasma. The resistance is only a little bit lower than the impedance. It seems that the material and diameter of the wires have no strong influence on the resistance.

X-ray backlighting of the initial stage of single-wire and multi-wire Z-pinch

Based on a compact (2m×1m×1.5m) pulsed current generator (~ 100 kA, 60ns), a table-top X-pinch device was constructed and tested. The load current was almost unchanged for X-pinches made using different wires (5μm, 8μm, 10μm and 13μm Wu wire> 13μm and 25μm Mo wire), which means that the impedance of the wires is much lower than the total impedance of the load section. When the above mentioned wires were used as two-wire load, X-ray pulses from X-pinch were always observed. As the mass of the two-wire load increases, the time delay of the x-ray emission relative to the beginning of the load current increases. As was expected, the X-ray pulse consists of single peak or two overlapping peaks of subnanosecond pulsewidth. The x-ray source is usually one point or two partly overlapping points, which is consistent with the measurement of x-ray pulse. The size of x-ray point source is ranging from 5 Dm to 50 Dm. Two X-ray pulses with a time interval on the order of 10 ns were often observed for a small mass load when the load current is high enough. The appearance of the second X-ray pulse is attributed to the second pinch of the plasma.

Timing of the x-ray burst from paralleled X-pinches

Based on a compact (2 m × 1 m × 1.5 m) pulsed-current generator (~100 kA and 60 ns), a tabletop X-pinch device was constructed and tested. The load current was almost unchanged for X-pinches made using different wires (5-, 8-, 10-, and 13-μm W wires and 13- and 25-μm Mo wires), which means that the impedance of the wires is much lower than the total impedance of the load section. When the aforementioned wires were used as a two-wire load, X-ray pulses from the X-pinch were always observed. As the mass of the two-wire load increases, the time delay of the X-ray emission relative to the beginning of the load current increases. As was expected, the X-ray pulse consists of a single peak or two overlapping peaks of subnanosecond pulsewidth. The X-ray source is usually one point or two partly overlapping points, which is consistent with the measurement of an X-ray pulse. The size of the X-ray point source ranges from 5 to 50 μm. Two X-ray pulses with a time interval on the order of 10 ns were often observed for a small-mass load when the load current is high enough. The appearance of the second X-ray pulse is attributed to the second pinch of the plasma.

X-ray emission from a table-top X-pinch device

The x-ray burst timings of X-pinches, TXB, made using eight different wires for different current were measured. The results showed that a higher current makes a shorter TXB for a given X-pinch wire. In other words, TXB scales linearly with the line mass density for a given current. Based on the snow-plow model for Z-pinch plasma, it was derived that for a given X-pinch wire the integral of the current over time from zero to TXB is constant, i.e., ∫0TXBi(t)⋅dt=const.. This theoretically derived relation was confirmed by our experiments.