A novel flux compression/dynamic transformer technique for high-voltage pulse generation

Monitoring the velocity of the insulator-metallic phase transition in aluminium powder under shock loading

“…The use of the Hugoniot data for aluminium powder [8,9] and Mylar [10], together with the flyer velocity, indicates that the possible conditions attained at the intersection between the flyer and the powder are: a pressure of 45 kbar, a particle velocity of 1.99 km s −1 and a shock velocity of 2.9 km s −1 . However, earlier work [3] using less complex sensors has demonstrated that the recorded velocity of the phase transition of about 5 km s −1 is well above the shock velocity of these theoretical predictions.…”

“…The plane electric gun [3] consists of a 26.9 µF capacitor charged to 20 kV and then discharged through a 100 nH transmission system that includes the exploding aluminium foil (thickness 25.4 µm, length 15 mm, width 15 mm) to which the flyer (a 250 µm Mylar-polyethylene package) is thermally bonded. The flyer is accelerated to about 3 km s −1 in the 3.2 mm barrel between its initial position and the probe tip embedded in the aluminium powder.…”

“…Although under normal conditions the thin oxide coating on the particles causes them to behave as an insulator, it is known that a high pressure wave can destroy this coating and thereby convert the powder into a metallic conductor. In the present study a plane electric gun [3,6] was used to accelerate a 250 µm thick Mylar foil, with the impact of this on the aluminium powder generating shock waves within the powder.…”

“…The insulator-metallic phase transition in aluminium powder under shock loading has been successfully exploited in several magnetic cumulation experiments, using either magnetic fluxcompression generators with high-explosive charges [1,2] or electromagnetically accelerated foils [3] to produce the driving shock wave. Although details are available of the complex electromagnetic phenomena involved in the insulator-metallic phase transition [4,5], and hypotheses have been advanced for the transition mechanism [2,4], no direct measurement of the speed at which it advances through the powder have been reported.…”

Multiple-pulse, twin-output, high-voltage generator

“…Because of the high-impedance loads on the transformers, the voltage pulse on the load closely follows the rate of change with time of the primary current shown in figure 3(b). The almost equal magnitudes of the negative and positive pulses means that the familiar way of removing the unwanted long negative pulse by means of a simple spark gap [4] is difficult to implement, and a novel form of high-voltage diode was therefore introduced.…”