Comparison of light emissions and Schlieren structures
Both positive and negative high voltage impulses were applied to each electrode geometry. Although the critical reduced field strength of ambient air is typically around 33 kV/cm, it can vary due to polarity, geometry, temperature, humidity, etc., and so it was found experimentally by stepping up the voltage by 0.4 kV and applying an impulse until a breakdown occurred. Once this had happened, the voltage was stepped down by 0.4 kV and a further impulse applied which produced visible streamers, usually without a breakdown occurring. Both cameras were then used to image the light emissions and Schlieren structures simultaneously, and the voltage and current profiles were recorded. In all experiments, the light emissions observed by the first camera and Schlieren structures observed by the second camera were both seen at the same time as the high voltage field was applied. However, the light emissions were only observed during the first frame, i.e., they were present for less than 47.62 µs, whereas the Schlieren structures were observed from the first frame onwards. The first Schlieren frame consisted of both light emissions and Schlieren structures whereas the second Schlieren frame consisted of only the Schlieren structures. As there was no real change in the Schlieren structures in the frames that immediately followed, the first light emission frame, displaying only light emissions, was compared to the second Schlieren frame, displaying only Schlieren structures. Examples of the data obtained are presented in Figure 2.
The high voltage impulse profile, as measured by the voltage divider, was verified to conform to the BS60060-1 electrical standard in all cases. The current profile, as measured by the current transformer, exhibited several small spikes in the Ampere range, consistent with a relatively small flow of charge resulting from ionisation of the air, often classified as partial discharge. It can be seen from Figure 2 that both the light emissions and Schlieren structures coincide along the same streamer filaments as expected. However, there is light emitted from parts of these filaments where no Schlieren effect could be seen, notably in areas further from the high voltage electrodes. This would indicate that, even though filaments have been formed which connect the electrode and ground plane, the density change within these filaments has only started to occur where the field is most intense, i.e., immediately next to the high voltage electrode. There is also a visible distinction between light emissions for positive and negative streamers in Figure 2. The positive streamers consist of uniform channels emanating from the high voltage electrode and, in some cases, from the ground plane. These channels decrease in intensity the further away they are from the electrode and plane, whereas the negative streamers consist of distinct areas of more intense light emissions within the air gap. This is consistent with the current theory as illustrated in Figure 1.
Analysis of Schlieren structures
The settings on the second camera were adjusted to give a magnified view of the Schlieren structures as well as an extended running time up to one second after the high voltage impulse had been applied, with the first camera no longer being used. The above experiments were repeated using the same high voltage 0.4 kV stepping method, although the point electrode was replaced with a second, sharper point electrode to minimise streamer formation from the sides as seen in Figure 2. Examples of the data obtained over time for positive high voltage impulses are presented in Figure 3 and negative high voltage impulses are presented in Figure 4. Note that, in these figures, the first Schlieren images also include the light emissions.
In all cases, the Schlieren structures were found to continue developing into the millisecond timeframe along filaments which connect between the previously energised electrode and the ground plane. Data from the current transducer confirmed that there was no charge flow during this period. There were notable density changes, which appear as brighter regions, not only along the filaments but also at both the high voltage electrode and ground plane. This is reminiscent of the filaments seen in the light emission images presented in the previous section, despite the light only being emitted in the first frame, i.e., within 47.62 µs, and the high voltage impulse having completely diminished by the fourth frame at 190.43 µs. The Schlieren structures then expand in a perpendicular direction to their alignment, i.e., horizontally, whilst moving towards the centre of the air gap in a parallel direction, i.e., vertically, eventually dissipating into the surrounding air over tens of milliseconds before disappearing completely within 100 ms. In this respect, the Schlieren structures can be considered a longer-lived imprint of the short-lived streamers within the air.
The perpendicular expansion of the Schlieren structures can be tracked from one image to the next by measuring the diameter of the more prominent filaments at the same point in each image. Similarly, the movement of brighter regions towards the centre of the air gap can be tracked by measuring how far the centre point of the region has moved away from its starting position in the first image. Plots of both the perpendicular expansion and movement towards the centre are shown in Figure 5.
Schlieren structures during electrical breakdowns
During the experiments, several electrical breakdown events were captured by both cameras. Such events emit a large amount of light within the first frame which saturated both cameras in the first set of experiments. However, during the second set of experiments, the Schlieren camera was able to capture data following the breakdown, as demonstrated in Figures 6 and 7. In most cases, typically when the breakdown occurs to one side of the electrode, the Schlieren structures connecting the electrode to the ground plane are still visible after the high voltage impulse has diminished, as seen in previous experiments when no breakdown occurred. This would indicate that, although many filaments existed, the conditions were right for one to become a leader, heating the air and increasing the conductivity to a point where a current could flow between the electrode and ground plane. The Schlieren imprint then remains in place until either the turbulent air of the breakdown disrupts it, or it dissipates into the surrounding air.