Relativistic electrons from sparks in the laboratory

Abstract

Discharge experiments were carried out at the Eindhoven University of Technology in 2013.The experimental setup was designed to search for electrons produced in meter-scale sparks using a 1 MV Marx generator. Negative voltage was applied to the high voltage (HV) electrode. Five thin (1 mm) plastic detectors (5 cm 2 each) were distributed in various configurations close to the spark gap. Earlier studies have shown (for HV negative) that X-rays are produced when a cloud of streamers is developed 30-60 cm from the negative electrode. This indicates that the electrons producing the X-rays are also accelerated at this location, that could be in the strong electric field from counterstreamers of opposite polarity. Comparing our measurements with modeling results, we find that ∼300 keV electrons produced about 30-60 cm from the negative electrode are the most likely source of our measurements. A statistical analysis of expected detection of photon bursts by these fiber detectors indicates that only 20%-45% of the detected bursts could be from soft (∼10 keV) photons, which further supports that the majority of detected bursts are produced by relativistic electrons.

X‐ray Emissions in a Multiscale Fluid Model of a Streamer Discharge

“…The most feasible and currently accepted hypothesis of production of these photons is by energetic electrons in the process of bremsstrahlung. The source electrons of 200‐ to 300‐keV energies may have also been observed by Østgaard et al (). The source of energetic electrons themselves, however, remains an unanswered topic.…”

Spontaneous Emergence of Space Stems Ahead of Negative Leaders in Lightning and Long Sparks

“…It is also responsible for counterstreamers seeded by charges in existing stems. Remarkably, all of these mechanisms have been linked to X-ray emissions from long sparks (Babich et al, 2015;Babich & Bochkov, 2017;Ihaddadene & Celestin, 2015;Köhn et al, 2017;Kochkin et al, 2015;Luque, 2017;Østgaard et al, 2016), and these X-rays are in turn linked to leader stepping (Dwyer et al, 2005).…”

Radio Frequency Electromagnetic Radiation From Streamer Collisions

“…Carlson et al () estimate the average energy of runaway electrons in high‐voltage discharges as 200 keV to 400 keV, a result that was later refined by Østgaard et al () with the direct detection of electrons with around 300 keV, so we hypothesize that a significant fraction of this energy is provided by the penetrating fields. Bringing the electron energy to around 100 keV is important because at those energies the stopping power of air is significantly lower than at energies around 1 keV so after reaching 100 keV an electron, even in a relatively weak electric field, can be possibly accelerated to the 300 keV estimated by Østgaard et al ().…”

“…These electrons, upon a collision with a nucleus, emit X‐ray photons as detected around long discharges (Dwyer et al, ; Kochkin et al, , ; Montanyà et al, ; Nguyen et al, , ; Rahman et al, ) and lightning leaders (Dwyer et al, ; Moore et al, ). This hypothesis is supported by laboratory measurements where the time of X‐ray emissions coincides with the time of approach of counter‐propagating streamers (Kochkin et al, , ; Østgaard et al, ). Recent simulations by Ihaddadene and Celestin () and Köhn et al () confirm that the electric field in a streamer collision exceeds the thermal runaway threshold but these authors estimate the time scale of the field peak at around 10 −11 s, which they claim is too fast to produce the detected X‐ray signals because not enough electrons are accelerated to high energies.…”