Laser-guided lightning

The study addresses whether ultrashort, high-intensity laser pulses that undergo filamentation in air can guide or trigger lightning discharges, offering a controllable alternative or complement to traditional protection methods such as the Franklin rod. Lightning occurs globally at rates of 40–120 flashes per second and causes thousands of deaths and large economic losses. Traditional protection uses grounded conducting masts to attract and safely conduct strikes. Rocket-and-wire techniques can trigger lightning but involve expendable hardware and safety concerns. Laser filamentation—where self-focusing and plasma defocusing maintain long, ionized, low-density air channels with enhanced conductivity—has guided meter-scale discharges in laboratory settings and can extend to hundreds of meters with terawatt-class, picosecond lasers. The hypothesis is that such filamentary channels, formed near a tall instrumented tower under thunderclouds, can provide preferential paths that guide natural lightning leaders and potentially enable initiation under suitable conditions.

Prior work proposed laser-triggered lightning decades ago and executed early field attempts (for example, Uchida et al., 1999) using high-energy sparks, which did not yield convincing guidance. Extensive laboratory studies established that femtosecond/picosecond laser filamentation produces long-lived underdense, conductive channels and can trigger and guide long air-gap discharges, sometimes competing with traditional rods. Field campaigns in New Mexico (2004) and Singapore (2011) using terawatt-class lasers reported no clear guidance or initiation. The physics of filamentation, air heating, and the resultant plasma/underdense channels are well documented, and prior work also developed VHF interferometry to track leader development. This paper situates itself at the intersection of these literatures by leveraging a kilohertz-repetition terawatt laser to maximize interaction with slowly varying pre-flash atmospheric fields and potential precursors, thereby overcoming limitations of earlier, lower-repetition campaigns.

Site and instrumentation: Experiments were conducted atop Säntis mountain (2,502 m), Switzerland, near a 124-m telecommunication tower struck ~100 times/year and instrumented with current sensors (Rogowski coils, B-dot), fast and slow electric-field sensors (20 m and 15 km from the tower), X-ray detectors (20 keV–1 MeV), VHF interferometer (1–160 MHz band) for lightning source localization, and multiple cameras including two high-speed systems. Airspace closures and ADS-B monitoring ensured operational safety. Laser system and beam delivery: A Yb:YAG CPA laser (1030 nm) delivered up to 720 mJ, 920 fs pulses at 1 kHz (TRUMPF Scientific Lasers). For field robustness and optics protection, pulses were chirped to 7 ps and output energy set to ~500 mJ. A 7.14× beam-expanding sending telescope (primary off-axis aspheric mirror 430 mm diameter; secondary 100 mm spherical mirror) produced a 250 mm output beam launched at a 7° elevation toward the tower tip. The telescope focal length (150 m) placed the onset of filamentation 30–50 m above the tower tip to intersect the region where upward lightning typically initiates. A lithium triborate crystal generated visible second harmonic (515 nm) for path visualization. Operations: Between 21 July and 30 September 2021, the laser operated for 6.3 h during thunderstorms within 3 km. Sixteen flashes struck the tower; four events (L1–L4) occurred with the laser on. Instrument limitations and harsh conditions meant not all diagnostics were available for each event. Optical and RF measurements: High-speed cameras at Kronberg (24,000 fps) and Schwägalp (10,000 fps) recorded event L2 on 24 July 2021, 16:24 UTC, under relatively clear sky. Camera calibration used daylight images, nighttime DSLR images during laser operation, and GPS-based reconstruction to overlay the laser path. For other events and perspectives, projected 2D distance histograms to the laser path were computed from individual frames. The VHF interferometer used generalized cross-correlation to locate radiation sources (azimuth, elevation) with meter-scale spatial and microsecond temporal resolution, enabling mapping of leader development relative to the laser path. X-ray bursts were integrated at 50 ns sampling; current and electric field waveforms were recorded synchronously. Leader kinematics: From interferometer data, leader velocities of 1×10^5–6×10^5 m s^-1 were estimated. High-speed imaging of L2 showed an initial apparent speed ~4×10^7 m s^-1 decreasing to ~9×10^6 m s^-1 at the first branching ~120 m above the tip, with an average of ~2×10^6 m s^-1 over that interval. Modeling and simulation: A charge-simulation method evaluated electric fields in the gap between the lower tip of a polarized laser filament and the tower tip for given background fields at 0.75 atm (site altitude). Streamer inception criteria used Townsend ionization coefficients; avalanche-to-streamer transition assumed at ~10^6 positive ions within a ~50 µm radius. Streamer propagation thresholds scaled for altitude (positive ~375 kV m^-1; negative ~0.75–1.5 MV m^-1). If streamers span the gap (final jump), breakdown is unavoidable; otherwise, leader inception and bridging conditions were assessed using established criteria (for example, leader-sustaining fields ~150–200 kV m^-1). Parametric results versus gap length and filament length (10–50 m) compared thresholds for laser-assisted positive and negative flashes and for tower-initiated negative flashes without laser assistance.

- First field observation of laser-guided lightning: On 24 July 2021 (L2), two high-speed cameras recorded an upward negative leader following the laser path over the initial ~50 m above the tower tip. The lowest vertical section exhibited no branching during the laser-guided stage. - Replication across events via VHF interferometry: For events with the laser (for example, L1), VHF sources accumulated along the laser path over ~60 m. Compared to laser-off events, the standard deviation of the distance from VHF sources to the laser beam was reduced by 45% in the 0–60 m range (ON: 25.5 m; OFF: 42.7 m), with weaker effects above 60 m, consistent with a filament length of ~50–60 m. - Enhanced X-ray activity with laser guidance: Across events L1 and L3 (laser on) versus a comparable positive event without laser, the mean number of X-ray bursts per event increased from ~1 (laser off) to ~4.3 (laser on). Most X-ray bursts with the laser occurred within the first 500 µs, coincident with the laser-guided leader propagation. - Event statistics during campaign: Of 16 flashes during laser operations window, four occurred with the laser on (L1–L4); all four were positive flashes (upward negative leaders), contrasting with typical Säntis statistics (approximately 84% negative, 11% positive, 5% bipolar in prior years). A unilateral χ² test indicates the excess of positive flashes with the laser on is highly significant (P = 6.7×10^-7). - Leader velocities: Interferometer-derived velocities were 1×10^5–6×10^5 m s^-1. High-speed imaging for L2 indicated initial apparent speeds decreasing from ~4×10^7 to ~9×10^6 m s^-1 with an average ~2×10^6 m s^-1 over the first 120 m.

The results confirm that filamentary channels produced by a kilohertz-repetition terawatt laser can guide upward negative leaders over tens of meters, providing a preferential conducting path near the tower. This directly addresses the hypothesis that atmospheric laser filamentation can influence natural lightning paths. The success at Säntis, compared to earlier unsuccessful field attempts, is attributed to the two-orders-of-magnitude higher repetition rate (1 kHz), which continuously samples the slowly varying pre-flash electric field and intercepts developing precursors. High repetition also fosters a persistent memory of the laser path via long-lived charged oxygen species and residual free electrons, creating a polarizable medium where charge migration can locally enhance the electric field and promote discharge segment formation. Modeling indicates that laser filaments preferentially assist positive flashes (upward negative leaders) at lower background fields than required for laser-assisted negative flashes. At field levels sufficient for laser-assisted negative flashes, the tower alone would typically initiate negative flashes without laser assistance. Thus, the laser is expected to increase the likelihood of positive upward flashes while not significantly altering negative flash incidence. The observed excess of positive flashes during laser operation is consistent with this prediction. The guidance is further supported by the tighter spatial clustering of VHF sources along the laser path and the increased X-ray burst rate during the guided phase, in line with laboratory observations of X-ray emission from straight, guided discharge segments. The findings suggest that optimizing filament visibility (for example, via second harmonic generation) and adjusting the onset of filamentation toward cloud charge centers could enhance guiding efficiency and potentially enable laser initiation of lightning under favorable conditions, warranting further coordinated campaigns and theoretical work.

This work provides the first field demonstration that ultrashort laser-induced filaments can guide natural lightning leaders, with direct optical evidence over ~50 m and corroboration by VHF interferometry and X-ray diagnostics. The approach opens avenues for active lightning protection strategies for critical infrastructures and advances the understanding of leader dynamics in enhanced-conductivity channels. Future research should explore optimization of laser parameters (wavelength, repetition rate, energy), controlled placement of filament onset closer to cloud charge centers, and comprehensive modeling and multi-season field campaigns to assess initiation capabilities and operational robustness for real-world protection systems.

- Limited number of guided events: Only four events occurred with the laser on during 6.3 h of thunderstorms, and clear optical documentation was available for one event (L2). - Instrumental and environmental constraints: Harsh weather, visibility, buffer sizes, and electromagnetic interference limited simultaneous data across all diagnostics for each event. - Geometry and generalizability: The campaign focused on upward flashes from a tall tower in mountainous terrain; results may differ for other environments or downward flashes. The study demonstrates guidance but does not yet demonstrate routine laser initiation of lightning. - Range of influence: Guidance effects were strongest within the ~50–60 m filament region; influence decreases above this height, consistent with filament length and modeling constraints on gap bridging.