Théberge et al. (Phys. Rev. Research 2, 013266, 2020) produced luminous, confined plasma balls (PB) in a tabletop experiment by driving high-voltage discharges across a 50 cm air gap: the hot, low-pressure channel left by the discharge traps the plasma ejected from the ground strike point and transports it along the channel, reproducing many reported parameters of natural ball lightning — except lifetime, which was 9–20 ms in the laboratory versus 1–10 s reported for natural ball lightning. The source work explicitly leaves open: (i) which physical mechanism makes the ambient electric field increase PB lifetime and luminosity (direct electroluminescence, more ejected material at the strike point, or a lower-density channel reducing collisional quenching); (ii) how lightning-like current densities and humid air (hydrated plasma) would quantitatively extend emission lifetimes; and (iii) the optimal combination of field amplitude, channel geometry, and ground material for producing long-lived, stable PB.

Background

Ball lightning (BL) is a luminous, roughly spherical object, typically 10–50 cm in size, reported during thunderstorms, with reported lifetimes of 1–10 s, speeds of a few meters per second, and luminosities comparable to a 100 W incandescent bulb. Its physical origin remains unexplained; nearly everything known comes from eyewitness reports, because instrumental records of natural BL are extremely rare. The best instrumented observation to date (Cen, Yuan, and Xue, Phys. Rev. Lett. 112, 035001 (2014)) captured the optical spectrum of a BL event in Qinghai, China, and found emission lines of neutral silicon, iron, and calcium — soil elements — suggesting the ball formed when a cloud-to-ground lightning strike vaporized the ground. Proposed explanations fall into two classes. Internal-energy models hold that the ball stores its own energy, for example the Abrahamson–Dinniss model (Nature 403, 519 (2000)), in which lightning striking soil creates a glowing aerosol of burning silicon nanoparticle networks, or the hydrated-plasma model (Egorov, Stepanov, and Shabanov, Phys. Usp. 47, 99 (2004)), in which ions produced by a discharge are trapped by water molecules in humid air, delaying recombination and extending fluorescence lifetimes to 0.1–1 s. External-energy models, following Kapitza, propose that an external radio-frequency electromagnetic field continuously powers the ball; this motivated many microwave fireball experiments. Attempts to create BL directly with rocket-triggered natural lightning (Hill et al., J. Atmos. Sol. Terr. Phys. 72, 913 (2010)) produced only short-lived (~300 ms) glows. A recent review of this literature is Shmatov and Stephan, J. Atmos. Sol. Terr. Phys. 195, 105115 (2019).

Théberge, Nadeau, Issa, Vidal, and Kieffer (Phys. Rev. Research 2, 013266 (2020)) built a laboratory analogy of a thunderstorm: a hybrid high-voltage source (a solid-state Tesla coil — a resonant transformer — in series with a high-load 30 kV capacitor) drives discharges across a 50 cm air gap, producing ambient electric fields of 15–60 kV/m at the ground, comparable to thunderstorm conditions. The discharge heats an air channel to about 4800 K; after roughly 0.5 ms the pressure inside the channel falls below ambient, so surrounding air flows into the channel ends and pushes the plasma cloud ejected from the ground strike point into the channel. The radial pressure and density profile of the channel confines this plasma ball (PB) to a quasi-constant diameter while it propagates at 3–6 m/s. Diagnostics included synchronized high-speed color cameras, 2D schlieren imaging (an optical technique that visualizes density gradients), a slitless spectrograph, and photomultiplier-tube light detection. The measured PB properties — speed, erratic path, brightness (3–15 W in the visible), stable diameter, vortical structures, and color/spectrum set by the ground material (iron gives bluish Fe emission turning orange from FeO oxidation; aluminum gives bluish Al I/AlO; copper gives green Cu I/CuO; dry sand reproduces the natural BL spectrum) — quantitatively match eyewitness accounts, and PB formation failed only for ground materials (wet sand, wood) whose vaporization blast destroyed the low-pressure channel. Two laboratory limitations stood out relative to nature: the peak current density (0.2–0.4 kA/cm²) was two to eight times lower than in natural lightning, and the experiment ran in dry air (30–45% relative humidity) to avoid condensation on the high-voltage hardware, which suppresses the hydrated-plasma channel for ion storage. The authors also detected no radio-frequency electromagnetic field above 100 MHz during the PB lifetime and concluded that microwave or radio frequencies do not contribute to PB formation or confinement — evidence against Kapitza-type external powering in this system. The unresolved issues, quoted from the paper's conclusions, are: 'we cannot confirm whether the increase in the PB lifetime and luminosity with the ambient electric field was caused directly by an electroluminescence effect [light emission driven directly by the ambient field, cf. Liu and Zhang, Phys. Rev. Lett. 103, 235002 (2009)], indirectly caused by an increase of the ejected material at the discharge strike point and/or in the generation of a lower-density discharge channel which increased the PB fluorescence lifetime by reducing the collision frequency'; 'further exploration is required, particularly into the optimal conditions for producing them'; and 'the higher current density in lightning and the contribution of high air humidity producing hydrated plasma should be studied to quantify their impacts on the longer emission lifetime of BL observed in nature.'

Problem Statement

For plasma balls generated and confined along laboratory discharge channels in the configuration of Théberge et al. (2020) — a hybrid Tesla-coil/high-load-capacitance source driving discharges over a ~50 cm air gap, with ambient electric fields of 15–60 kV/m and ground materials whose vaporized ejecta form the luminous ball — determine, experimentally and quantitatively: (i) the physical mechanism by which increasing the ambient electric field lengthens the PB lifetime (measured, e.g., as the e−1e^{-1} decay time of visible luminosity, from 9 ms at 15 kV/m to 20 ms at 60 kV/m) and increases its luminosity, discriminating among direct electroluminescence driven by the ambient field, an increase of ejected material at the discharge strike point, and the generation of a lower-density discharge channel that reduces collisional quenching (collisional deactivation of the excited emitting species); (ii) the quantitative impact on PB emission lifetime of higher discharge current density approaching natural lightning (0.7–3 kA/cm² versus the laboratory 0.2–0.4 kA/cm²) and of elevated air humidity producing hydrated plasma (ion trapping by water molecules), i.e., whether these effects can extend laboratory lifetimes from tens of milliseconds toward the 1–10 s reported for natural ball lightning; and (iii) the optimal combination of ambient-field amplitude, discharge-channel geometry (diameter and propagation length), and ground-material properties for producing the longest-lived, most stable PB — the 'optimal conditions for producing them' that the source paper states are still required. The common objective of all three components is to establish the discharge-channel route as a controlled, physically understood laboratory route to ball lightning: to identify the governing mechanisms and to characterize quantitatively how lifetime and luminosity depend on the controlling parameters (Théberge et al., DOI: 10.1103/PhysRevResearch.2.013266).

References

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