skip to main content

Explore Research Products in the NSF-PAR

Title: 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 us to control 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 ± 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 ± 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.  more » « less
Award ID(s):
1917069
NSF-PAR ID:
10439535
Author(s) / Creator(s):
; ;
Date Published:
Journal Name:
Journal of Physics D: Applied Physics
Volume:
56
Issue:
5
ISSN:
0022-3727
Page Range / eLocation ID:
055201
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; ( , Geophysical Research Letters)
    Abstract

    Terrestrial gamma‐ray flashes are linked to growth of long bidirectional lightning leader system consisting of positive and stepping negative leaders. The spatial extent of streamer zones of a typical lightning leader with tip potential exceeding several tens of megavolts is on the order of 10–100 m. The photoelectric absorption of bremsstrahlung radiation generated by avalanching relativistic runaway electrons occurs efficiently on the same spatial scales. The intense multiplication of these electrons is triggered when the size of the negative leader streamer zone crosses a threshold of approximately 100 m (for sea‐level air pressure conditions) allowing self‐replication of these avalanches due to the upstream relativistic electron seeds generated by the photoelectric absorption. The model results also highlight importance of electrode effects in interpretation of X‐ray emissions from centimeter to meter long laboratory discharges, in particular, a similar feedback effect produced by generation of runaway electrons from the cathode material.

     
    more » « less
  2. ; ; ; ( , Physics of Plasmas)
    Runaway electron acceleration is the keystone process responsible for the production of energetic radiation by lightning and thunderstorms. In the laboratory, it remains undetermined if runaway electrons are merely a consequence of high electric fields produced at the ionization fronts of electrical discharges, or if they impact the discharge formation and propagation. In this work, we simulate photon pileup in a detector next to a spark gap. We compare laboratory measurements to ensembles of monoenergetic electron beam simulations performed with Geant4 (using the Monte Carlo method). First, we describe the x-ray emission properties of monoenergetic beams with initial energies in the 20 to 75 keV range. Second, we introduce a series of techniques to combine monoenergetic beams to produce general-shape electron energy spectra. Third, we proceed to attempt to fit the experimental data collected in the laboratory, and to discuss the ambiguities created by photon pileup and how it constrains the amount of information that can be inferred from the measurements. We show that pileup ambiguities arise from the fact that every single monoenergetic electron beam produces photon deposited energy spectra of similar qualitative shape and that increasing the electron count in any beam has the same qualitative effect of shifting the peak of the deposited energy spectrum toward higher energies. The best agreement between simulations and measurements yields a mean average error of 8.6% and a R-squared value of 0.74. 
    more » « less
  3. ; ; ; ; ; ; ; ( , Journal of Geophysical Research: Atmospheres)
    Abstract

    This paper reports a study to understand the radio spectrum of thunderstorm narrow bipolar events (NBEs) or compact intracloud discharges, which are powerful sources of high‐frequency (HF) and very high frequency (VHF) electromagnetic radiation. The radio spectra from 10 kHz to about 100 MHz are obtained for three NBEs, including one caused by fast positive breakdown and two by fast negative breakdown. The results indicate that the two polarities of fast breakdown have similar spectra, with a relatively flat spectrum in the HF and VHF band. The ratio of energy spectral densities in the very low frequency and HF bands is (0.9–5) × 105. We develop a statistical modeling approach to investigate if a system of streamers can explain the main features of fast breakdown. Assuming that the current moment peak and charge moment change of individual streamers vary in the ranges of 5–10 A‐m and 5–20 μC‐m, respectively, the modeling results indicate that a system of 107–108streamers can reproduce the current moment, charge transfer, and radio spectrum of fast breakdown. The rapid current variation on a time scale of nanoseconds required for fast breakdown to produce strong HF/VHF emissions is provided by exponentially accelerating and expanding streamers. Our study therefore supports the hypothesis that fast breakdown is a system of streamers. Finally, suggestions are given regarding future streamer simulations and NBE measurements in order to further develop our understanding of NBEs and lightning initiation.

     
    more » « less
  4. ; ; ; ; ; ; ; ; ; ; et al ( , Journal of Geophysical Research: Atmospheres)
    Abstract

    In this paper we report the first close, high‐resolution observations of downward‐directed terrestrial gamma‐ray flashes (TGFs) detected by the large‐area Telescope Array cosmic ray observatory, obtained in conjunction with broadband VHF interferometer and fast electric field change measurements of the parent discharge. The results show that the TGFs occur during strong initial breakdown pulses (IBPs) in the first few milliseconds of negative cloud‐to‐ground and low‐altitude intracloud flashes and that the IBPs are produced by a newly identified streamer‐based discharge process called fast negative breakdown. The observations indicate the relativistic runaway electron avalanches (RREAs) responsible for producing the TGFs are initiated by embedded spark‐like transient conducting events (TCEs) within the fast streamer system and potentially also by individual fast streamers themselves. The TCEs are inferred to be the cause of impulsive sub‐pulses that are characteristic features of classic IBP sferics. Additional development of the avalanches would be facilitated by the enhanced electric field ahead of the advancing front of the fast negative breakdown. In addition to showing the nature of IBPs and their enigmatic sub‐pulses, the observations also provide a possible explanation for the unsolved question of how the streamer to leader transition occurs during the initial negative breakdown, namely, as a result of strong currents flowing in the final stage of successive IBPs, extending backward through both the IBP itself and the negative streamer breakdown preceding the IBP.

     
    more » « less
  5. ; ; ( , Journal of Physics D: Applied Physics)
    Abstract

    This study delves into the dynamics of cold atmospheric plasma and their interaction within conductive solutions under the unique conditions of nanosecond pulsed discharges (22 kV peak voltage, 10 ns FWHM, 4.5 kV ns−1rate-of-rise). The research focuses on the electrical response, breakdown, and discharge propagation in an argon bubble, submerged in a NaCl solution of varying conductivity. Full or partial discharges were observed at conductivities of 1.5µS cm−1(deionized water) to 1.6 mS cm−1, but no breakdown was observed at 11.0 mS cm−1when reducing the electrode gap. It is demonstrated that at higher conductivity electric breakdown is observed only when the gas bubble comes into direct contact with the electrode and multiple emission nodes were observed at different timescales. These nodes expanded in the central region of the bubble over timescales longer than the initial high-voltage pulse. This work offers a temporal resolution of 2 ns exposure times over the first 30 ns of the initial voltage pulse, and insight into plasma formation over decaying reflected voltage oscillations over 200 ns.

     
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email