Production of runaway electrons and x-rays during streamer inception phase

Abstract

Streamers play a key role in the formation and propagation of lightning channels. In nature streamers rarely appear alone. Their ensemble behavior is very complex and challenging to describe. For instance, the intricate dynamics within the streamer zone of negative lightning leaders give rise to space stems, which help advance the stepped-leader. Another example is how the increasing morphological complexity of sprites can lead to higher sprite current, and greater energy deposition in the mesosphere. Insights into the complex dynamics of a streamer corona can be obtained from laboratory experiments that allow control of the conditions of streamer formation. Based on simultaneous nanosecond-temporal-resolution photography, and measurements of voltage, current, and x-ray emissions, we report the characteristics of negative laboratory streamers in 88 kPa of atmosphere. The streamers are produced at peak voltages of 62.2\,$\pm$\,3.8 kV in a point-to-plane discharge gap of 6 cm. While all discharges were driven to the same peak voltage, the discharges occurred at different stages of the relatively slow voltage rise (177 ns), allowing us to study discharge properties as a function of onset voltage. The onset voltage ranged between 24 and 67 kV, but x-ray emissions were observed to only occur above 53 kV, with x-ray burst energies scaling quadratically with voltage. The average delay between the current pulse and x-ray emission was found to be 3.5\,$\pm$\,0.5 ns, indicating that runaway electrons are produced during the streamer inception phase or no later than the transition stage, when the inception cloud is breaking into streamer filaments. During this short time span, runaway electrons can traverse the gap, hit the ground plate and produce bremsstrahlung x-ray photons. However, streamers themselves cannot traverse more than 3.5 mm across the gap, which supports the idea that runaway electron production is not associated to streamer connection to the ground electrode.

Spectral Hardness of X‐ and Gamma‐Ray Emissions From Lightning Stepped and Dart Leaders

“…Panels (a) and (b) show the linear response of the Labr and NaI detectors, respectively. The green line corresponds to the response of the detectors using our DAS, while the blue line corresponds to the response of the detectors using a LeCroy HDO6014a oscilloscope (same as in Contreras‐Vidal et al., 2022). The red symbols correspond to data collection with the DAS at the field site.…”

“…Figures 2a and 2b show that both detectors have a linear response in the range of interest. In a separate study, the linearity of the LaBr detector was verified down to 20 keV (Contreras-Vidal et al, 2022).…”

“…Photon pile-up is a major issue in the interpretation of laboratory measurements due to the compactness of the spatial scales involved, of the order of 10s of centimeters (Contreras-Vidal et al, 2022;da Silva et al, 2017; Journal of Geophysical Research: Atmospheres 10.1029/2023JD040397 Pantuso et al, 2022). This issue is also present in the measurements of rocket-triggered lightning X-rays, where detectors are 10s of meters away from the source (Saleh et al, 2009;Schaal et al, 2012).…”

Optimal Conditions for the Generation of Runaway Electrons in High-Pressure Gases

“…In works [99,100], attention was focused on the study of RAEs in TOKAMAK-type installations and devices for their diagnostics in these conditions. Works [101][102][103][104][105] studied RAEs registered in atmospheric discharges, and work [106] investigated the application of RAE discharges for materials processing.…”

Optimal Conditions for Generation of Runaway Electrons in High Pressure Gases

“…In works [99,100], attention was focused on the study of the RAEs in TOKAMAK-type installations and devices for their diagnostics in these conditions. Works [101][102][103][104][105] studied the RAEs registered in atmospheric discharges, and work [106] investigated the application of the RAE discharges for materials processing.…”