Laboratory Measurements of X‐Ray Emissions From Centimeter‐Long Streamer Corona Discharges

Abstract

We provide extensive evidence that runaway electron acceleration and subsequent bremsstrahlung X-ray emission are a common feature in negative electrical discharges with voltages as low as 100 kV, indicating that all negative lightning could potentially produce runaway electrons. Centimeter long streamer corona discharges produce bursts of X-ray radiation, emitted by a source highly compact in space and time, leading to photon pileup. Median photon burst energies vary between 33 and 96 keV in 100 kV discharges. Statistical analysis of 5,000+ discharges shows that X-rays are observed in as many as 60% of the triggers, depending on the configuration. X-ray detection is more frequent when streamers are not followed by a spark, the detector is oriented perpendicular to the gap, and a thicker anode is used. In an 8-cm-long gap, X-rays are produced when runaway electrons hit the anode, and the electron acceleration is not necessarily correlated with streamer collisions. Plain Language Summary Small electrical sparks prolifically emit X-rays. These discharges are produced in the lab by electrical energy sources 100 to 1,000 times weaker than what is available in thunderstorms and lightning. Therefore, this is strong evidence that all lightning should produce X-rays and energetic electrons.

Modeling of a New Electron Acceleration Mechanism Ahead of Streamers

“…However, from the present results we find two issues that prevent the electrons from running away when short and large dielectric ellipsoids representative of streamers are used. However, there is no strong experimental evidence for streamer collisions to occur and to produce X-rays (e.g., da Silva et al, 2017). Based on Table 1, we would require a single head-on collision to produce electrons run > 60 keV at the collision point to allow them to runaway in E c = 10 kV/cm positive streamer channel, if one assumes an average discharge ambient field of ≃20 kV/cm.…”

“…Streamer head-on collisions are believed to happen in the middle of the discharge gap during the encounter of long negative and positive streamers (e.g., Kochkin et al, 2012). However, there is no strong experimental evidence for streamer collisions to occur and to produce X-rays (e.g., da Silva et al, 2017). The detector response time is much longer than the picosecond interaction times of a single-streamer head-on collision, so the detector cannot pinpoint the source.…”

“…However, such a high field exists over a very short time scale <10 ps. The X-ray bursts observed in long gap experiment are more intense than those observed in a short gap experiment presumably because the electrons experience a larger potential difference when they cross the gap (e.g., Kochkin et al, 2012Kochkin et al, , 2015da Silva et al, 2017). The 2,000 runaway electrons were estimated in two separate steps.…”

“…The streamers they observed were 1-cm long and the X-rays were not associated with any streamer head-on collision (see also Kochkin et al, 2015, section 3.5.2). The X-ray bursts observed in long gap experiment are more intense than those observed in a short gap experiment presumably because the electrons experience a larger potential difference when they cross the gap (e.g., Kochkin et al, 2012Kochkin et al, , 2015da Silva et al, 2017). Köhn et al (2017) used full self-consistent particle in cell (PIC) simulations to investigate the acceleration of electrons during a single streamer head-on collision and they found no more than one electron with energy >500 eV was produced.…”

Influence of Elastic Scattering on Electron Swarm Distribution in Electrified Gases

“…The discovery that in a thunderstorm environment electrons are sometimes accelerated to relativistic energies (Dwyer et al, ) has prompted a renewed interest in the propagation of energetic particles through air. There is consensus that electron thermal runaway is the key process behind the X‐ray emissions from lightning (Dwyer et al, ; Moore et al, ) and in long sparks in the laboratory (Dwyer et al, ; Kochkin et al, , , ; Montanyà et al, ; Rahman et al, ; da Silva et al, ) and possibly it also plays a role in the production of terrestrial gamma‐ray flashes (Fishman et al, ). Thermal runaway is possible because below a few megaelectronvolts the rate of energy loss by an electron in air peaks around 300 keV/cm for an electron energy close to 200 eV (Dwyer et al, ; Gurevich et al, ).…”