Megumu Miki

Initial stage in lightning initiated from tall objects and in rocket‐triggered lightning

We examine the characteristics of the initial stage (IS) in object-initiated lightning derived from current measurements on the Gaisberg tower (100 m, Austria), the Peissenberg tower (160 m, Germany), and the Fukui chimney (200 m, Japan) and their counterparts in rocket-triggered lightning in Florida. All lightning events analyzed here effectively transported negative charge to ground. For rocket-triggered lightning the geometric mean (GM) values of the three overall characteristics of the initial stage, duration, charge transfer, and average current, are similar to their counterparts for the Gaisberg tower flashes and the Peissenberg tower flashes, while the Fukui chimney flashes are characterized by a shorter GM IS duration and a larger average current. The GM IS charge transfer for the Fukui chimney flashes is similar to that in the other three data sets. The GM values of the action integral differ considerably among the four data sets, with the Fukui action integral being the largest. The observed differences in the IS duration between the Fukui data set and all other data considered here are probably related to the differences in the lower current limits, while the differences in the action integral cannot be explained by the instrumental effects only. There appear to be two types of initial stage in upward lightning. The first type exhibits pulsations (ringing) during the initial portion of the IS, and the second type does not. The occurrence of these types of IS appears to depend on geographical location. The characteristics of pulses superimposed on the initial continuous current (ICC pulses) in object-initiated (Gaisberg, Peissenberg, and Fukui) lightning are similar within a factor of 2 but differ more significantly from their counterparts in rocket-triggered lightning. Specifically, the ICC pulses in object-initiated lightning exhibit larger peaks, shorter risetimes, and shorter half-peak widths than do the ICC pulses in rocket-triggered lightning.

Model experiments of laser-triggered lightning

Experiments to guide electric discharges with a chain of apparently discrete air breakdown plasmas (plasma channel) produced by a laser are reported. The electric discharge was guided up to 2 m with a high-power CO/sub 2/ laser focused by a 10 m focal length mirror. Voltage was applied at selected delay times tau following laser radiation. The relationships between 50% flashover voltage of a gap filled with laser-produced plasmas and delay times, and between guided length and the peak of the applied voltage, were obtained experimentally. The effects of the polarity of the applied voltage and the position of the plasma channel on flashover voltage are described. The development of the guided discharge is discussed. >

Development of long gap discharges guided by a pulsed CO2 laser

The mechanism of the guidance effect of laser plasmas on electrical discharges has been studied. A 45 J CO2 laser pulse produces many spherical plasmas over a gap of up to 2 m, and the laser-guided discharge is formed along the plasma by the application of a lightning impulse voltage. Unusual properties of the laser guided discharge are obtained from streak photographs of the discharge development. The laser produced plasmas guide streamers and leaders from a negative electrode more effectively than from a positive electrode. The discharge development from the negative electrode plays an important role in the guidance effect.

Guiding of electrical discharges under atmospheric air by ultraviolet laser‐produced plasma channel

Guiding and triggering of electrical discharges in 0.34 m air gaps by an ultraviolet laser‐produced plasma have been demonstrated. An ultraviolet laser pulse (KrF laser, wavelength=248 nm, energy=750 mJ) produced the plasma channel by a focusing lens with a focal length of 1 m. This plasma channel forms a straight discharge channel along the laser beam (guiding ability) and reduces the flashover voltage (triggering ability). We measured the guiding and triggering ability of the laser‐produced plasma channel and estimated the state of the plasma channel by calculating the rate equations. From these experiments and simulations, we found that the guiding ability of the ion plasma is mainly due to the formation of the leader from the plasma channel by enhancement of the local fields. The guiding ability of the electron‐ion plasma is different from that of the ion plasma.

An Improved Model for Prediction of the Dynamics of Lightning Channel Corona Sheath

We consider dynamics of the lightning-channel corona sheath that is implicitly specified by lumped-current-source lightning return-stroke models. Two slightly different corona models for prediction of charge motion in the corona sheath are proposed. Both models can be viewed as generalizations of the model proposed by Maslowski and Rakov (2006) [2] and are in agreement with measurements of the horizontal (radial) electric field component made in the immediate vicinity of triggered lightning channel.

Mechanisms of guiding ability of laser-produced plasmas on pulsed discharges

Bead-like plasmas produced by a CO 2 laser have guided and triggered electrical discharges in air gaps. The physical parameters (electron density, electron temperature, geometric structure) of the laser-produced plasmas have been measured using spectroscopy and laser interferometry. We compared the results of these measurements with the characteristics of laser-guided discharges, and found that the mechanisms of the guiding ability of the laser can be explained by the behaviour of the plasma as a conductor (metal ball model). We conducted model experiments of the laser-guided discharges using small metallic balls. The features of the discharge guided by metal balls were similar to those guided by laser-produced plasmas. We thus concluded that the behaviour of the plasma plays an important role in triggering and guiding the discharges.

Lidar measurement of constituents of microparticles in air by laser-induced breakdown spectroscopy using femtosecond terawatt laser pulses

We experimentally demonstrated remote sensing of the constituents of microparticles in air by combining laser-induced breakdown spectroscopy (LIBS) and lidar, using femtosecond terawatt laser pulses. Laser pulses of 70 fs duration and 130 mJ energy generated filaments when focused at a focal length of 20 m and the pulses irradiated artificial saltwater aerosols in air at a 10 Hz pulse repetition rate. Na fluorescence was observed remotely at a distance of 16 m using a 318 mm diameter Newtonian telescope, a spectrometer, and an intensified CCD camera. These results show the possibility of remote measurement of the constituents of atmospheric particles, such as aerosols, clouds, and toxic materials, by LIBS-lidar using femtosecond terawatt laser pulses.

Leader effects on femtosecond-laser-filament-triggered discharges

Dynamics of laser filaments in strong nonuniform electric fields is studied with high temporal and spatial resolution. Considerable reduction of the breakdown potential is found and is attributed to a filament-induced leader. Two breakdown modes, fast and slow, are found in 0.4MV positive dc-voltage discharges activated by filaments that are induced by 65fs, 170mJ laser pulses. In the fast mode with duration order of a few microseconds, the filament may acquire the electrode potential and temporarily maintain it, becoming a leader. This gives rise to an average electric field over the attachment instability threshold between a leader head and cathode. Ionization waves precede the breakdown with maximal voltage reduction up to 40% for this mode. The slow mode with its duration order of 1ms appears with a considerably smaller voltage reduction when the leader decays before the secondary streamer; the breakdown delay depends on negative and positive ion mobilities in this case.

Some Inferences From Radial Electric Fields Measured Inside the Lightning-Channel Corona Sheath

Radial electric field waveforms at a horizontal distance of 10 cm from the triggered lightning channel attachment point (inside the corona sheath) have been measured with a Pockels sensor. The electric field change during the leader stage was often overcompensated by the opposite-polarity electric field change during the return-stroke stage. The opposite polarity overshoot in radial electric field waveforms is consistent with the expected existence (during the return-stroke stage) of two concentric zones within the corona sheath: an inner zone containing net positive charge and an outer one with negative charge. The measured radial electric field waveforms exhibited quasi-exponential decay (very similar for all the waveforms), which was used for estimating the apparent electrical conductivity inside the corona sheath.

Submicrosecond laser-filament-assisted corona bursts near a high-voltage electrode

Long, about a half of microsecond, nonuniform corona UV burst is observed after a femtosecond-laser-filament plasma appears nearby an electrode biased (positively or negatively) slightly higher than the corona discharge threshold and well-isolated from the natural streamer discharge. A bright UV emission area moving outwards, over a 20 cm distance, with the velocity of 0.6% of the speed of light and tearing from the filament plasma in the case of the negative voltage is observed. In the case of positive voltage, a bright, bouncing UV cone is formed at around 4 cm far from the filaments exposing the appearance of a leader. Both phenomena could be explained upon supposing the formation of runaway electrons in the vicinity of the filament plasma and electrode.