ABSTRACT
We compare the water condensation and snow formation induced by a femtosecond laser filament with that when the filament is assisted by an aluminum target located at different positions along the filament. We reveal that the laser-filament-induced water condensation and snow formation assisted by the aluminum target are more efficient compared with those obtained without the assistance of the aluminum target. We find that the mass of the snow induced by the laser filament is the largest when the aluminum target is located at the end of the filament, smaller when it is at the middle of the filament, and the smallest at the beginning of the filament. These findings indicate that a higher plasma density and the generation of vortex pairs below the filament are important for enhancing the efficiency and yield of the laser-induced water condensation and precipitation. The higher plasma density provides more cloud condensation nuclei and facilitates the water condensation; vortex pairs below the filament are favourable to the growth of particles up to larger sizes.
ABSTRACT
AgI-type pyrotechnics are widely used in the field of weather modification, as a kind of artificial ice nuclei. However, their precipitation yield remains an intensively studied area. In this paper, we present a study of AgI-type pyrotechnic nucleant-induced water condensation promoted by femtosecond laser filaments in a cloud chamber. It is found that when 50-ml sample was irradiated by the laser filaments, the particles condensed on the glass slide are more soluble and slightly larger (5-15 µm). The irradiation of the laser filament on the nucleant rarely induces the generation of particles of sizes larger than 1 µm; however, it increases the decay time of particles from 13 to 18 min by the creation of numerous small particles. The amount of snow on the cold bottom plate increases by 4.2-13.1% in 2 h, compared to that without the irradiation of the laser filament. These results are associated with the production of high-concentration HNO3 by the laser filament. The concentration of HNO3 in the melt water increases by more than ten times when the sample was irradiated by the laser filaments.
ABSTRACT
We report on a method to experimentally generate ionic wind by coupling an external large electric field with an intense femtosecond laser induced air plasma channel. The measured ionic wind velocity could be as strong as >4 m/s. It could be optimized by increasing the strength of the applied electric field and the volume of the laser induced plasma channel. The experimental observation was qualitatively confirmed by a numerical simulation of spatial distribution of the electric field. The ionic wind can be generated outside a high-voltage geometry, even at remote distances.
ABSTRACT
Water condensation and precipitation induced by 22-TW 800-nm laser pulses at 1 Hz in an open cloud chamber were investigated in a time-resolved manner. Two parts of precipitation in two independent periods of time were observed directly following each laser shot. One part started around the filament zone at t < 500 µs and ended at t â 1.5 ms after the arrival of the femtosecond laser pulse. The other following the laser-induced energetic air motion (turbulence), started at t â 20 ms and ended at t â 120 ms. Meanwhile, the phase transitions of large-size condensation droplets with diameters of 400-500 µm from liquid to solid (ice) in a cold area (T < -30 °C) were captured at t â 20 ms.
ABSTRACT
Artificial rainmaking is in strong demand especially in arid regions. Traditional methods of seeding various Cloud Condensation Nuclei (CCN) into the clouds are costly and not environment friendly. Possible solutions based on ionization were proposed more than 100 years ago but there is still a lack of convincing verification or evidence. In this report, we demonstrated for the first time the condensation and precipitation (or snowfall) induced by a corona discharge inside a cloud chamber. Ionic wind was found to have played a more significant role than ions as extra CCN. In comparison with another newly emerging femtosecond laser filamentation ionization method, the snow precipitation induced by the corona discharge has about 4 orders of magnitude higher wall-plug efficiency under similar conditions.
ABSTRACT
We present a novel method based on plasma-guided corona discharges to probe the plasma density longitudinal distribution, which is particularly good for the weakly ionized plasmas (~1014 cm-3). With this method, plasma density longitudinal distribution inside both a weakly ionized plasma and a filament were characterized. When a high voltage electric field was applied onto a plasma channel, the original ionization created by a laser pulse would be enhanced and streamer coronas formed along the channel. By measuring the fluorescence of enhanced ionization, in particular, on both ends of a filament, the weak otherwise invisible plasma regions created by the laser pulse were identified. The observed plasma guided coronas were qualitatively understood by solving a 3D Maxwell equation through finite element analysis. The technique paves a new way to probe low density plasma and to precisely measure the effective length of plasma inside a filament.
ABSTRACT
We investigated water condensation in a laboratory cloud chamber induced by picosecond (ps) laser pulses at ~350 ps (800 nm/1-1000 Hz) with a maximum peak power of ~25 MW. The peak power was much lower than the critical power for self-focusing in air (~3-10 GW depending on the pulse duration). Sparks, airflow and snow formation were observed under different laser energies or repetition rates. It was found that weaker ps laser pulses can also induce water condensation by exploding and breaking down ice crystals and/or water droplets into tiny particles although there was no formation of laser filament. These tiny particles would grow until precipitation in a super-saturation zone due to laser-induced airflow in a cold region with a large temperature gradient.
ABSTRACT
We investigated femtosecond laser-filamentation-induced airflow, water condensation and snow formation in a cloud chamber filled respectively with air, argon and helium. The mass of snow induced by laser filaments was found being the maximum when the chamber was filled with argon, followed by air and being the minimum with helium. We also discussed the mechanisms of water condensation in different gases. The results show that filaments with higher laser absorption efficiency, which result in higher plasma density, are beneficial for triggering intense airflow and thus more water condensation and precipitation.
ABSTRACT
A unified picture of femtosecond laser induced precipitation in a cloud chamber is proposed. Among the three principal consequences of filamentation from the point of view of thermodynamics, namely, generation of chemicals, shock waves and thermal air flow motion (due to convection), the last one turns out to be the principal cause. Much of the filament induced chemicals would stick onto the existing background CCN's (Cloud Condensation Nuclei) through collision making the latter more active. Strong mixing of air having a large temperature gradient would result in supersaturation in which the background CCN's would grow efficiently into water/ice/snow. This conclusion was supported by two independent experiments using pure heating or a fan to imitate the laser-induced thermal effect or the strong air flow motion, respectively. Without the assistance of any shock wave and chemical CCN's arising from laser filament, condensation and precipitation occurred. Meanwhile we believe that latent heat release during condensation /precipitation would enhance the air flow for mixing.
ABSTRACT
Laser based lightning control holds a promising way to solve the problem of the long standing disaster of lightning strikes. But it is a challenging project due to insufficient understanding of the interaction between laser plasma channel and high voltage electric filed. In this work, a direct observation of laser guided corona discharge is reported. Laser filament guided streamer and leader types of corona discharges were observed. An enhanced ionization took place in the leader (filament) through the interaction with the high voltage discharging field. The fluorescence lifetime of laser filament guided corona discharge was measured to be several microseconds, which is 3 orders of magnitude longer than the fluorescence lifetime of laser filaments. This work could be advantageous towards a better understanding of laser assisted leader development in the atmosphere.
ABSTRACT
We numerically simulated the air-flow motion in a diffusion cloud chamber induced by femtosecond laser filaments for different chopping rates. A two dimensional model was employed, where the laser filaments were treated as a heat flux source. The simulated patterns of flow fields and maximum velocity of updraft compare well with the experimental results for the chopping rates of 1, 5, 15 and 150 Hz. A quantitative inconsistency appears between simulated and experimental maximum velocity of updraft for 1 kHz repetition rate although a similar pattern of flow field is obtained, and the possible reasons were analyzed. Based on the present simulated results, the experimental observation of more water condensation/snow at higher chopping rate can be explained. These results indicate that the specific way of laser filament heating plays a significant role in the laser-induced motion of air flow, and at the same time, our previous conclusion of air flow having an important effect on water condensation/snow is confirmed.
Subject(s)
Gases/chemistry , Gases/radiation effects , Lasers , Models, Chemical , Rheology/methods , Computer Simulation , Heating/methods , Scattering, RadiationABSTRACT
1 kHz, 2 mJ, 45 fs, 800 nm laser pulses were fired into a laboratory diffusion cloud chamber through a subsaturated zone (relative humidity â¼73%, T â¼ 4.3 °C). After 60 min of laser irradiation, an oval-shaped snow pile was observed right below the filament center and weighed â¼12.0 mg. The air current velocity at the edge of the vortices was estimated to be â¼16.5 cm/s. Scattering scenes recorded from the side show that filament-induced turbulence were formed inside the cloud chamber with two vortices below the filament. Two-dimensional simulations of the air flow motion in two cross sections of the cloud chamber confirm that the turbulent vortices exist below the filament. Based upon this simulation, we deduce that the vortices indeed have a three-dimensional elliptical shape. Hence, we propose that inside vortices where the humidity was supersaturated or saturated the condensation nuclei, namely, HNO(3), N(2)(+), O(2)(+) and other aerosols and impurities, were activated and grew in size. Large-sized particles would eventually be spun out along the fast moving direction towards the cold plate and formed an oval-shaped snow pile at the end.
ABSTRACT
Using 1 kHz, 9 mJ femtosecond laser pulses, we demonstrate laser-filamentation-induced spectacular snow formation in a cloud chamber. An intense updraft of warm moist air is generated owing to the continuous heating by the high-repetition filamentation. As it encounters the cold air above, water condensation and large-sized particles spread unevenly across the whole cloud chamber via convection and cyclone like action on a macroscopic scale. This indicates that high-repetition filamentation plays a significant role in macroscopic laser-induced water condensation and snow formation.
