We observed that introducing filaments in a cloud chamber saturated with water vapor results in the spectacular condensation of a cloud, which is very well visible with the bare eye.
position of the filaments.
The Teramobile container
includes a comprehensive
optics laboratory that has air
conditioning, power supplies
and a lidar (light detection
and ranging) remote sensing
system—in addition to the
laser itself and thermal and
mechanical protections from
to the formation of lar- ger droplets by promot- ing the coalescence of the smaller particles bearing opposite charges. These mechanisms, which
Source: Eur. Phys. J. Appl. Phys 20, 183 (2002)
mobile femtosecond terawatt laser ]
2. 6 m
Air cond. D S Laser exit L5 L6 L7 L3 L1 L2 L4 Switch board and air cond.
trigger laser-induced nucle-
ation in sub-saturated atmo-
spheres, are not restricted to
laboratory experiments. We
have also observed con-
densation in the real atmo-
sphere. For that purpose,
we launched the Teramobile
beam vertically into the
atmosphere over the city of
Berlin, Germany. We used a
second, low-power laser beam
as a probe overlapping with
the filaments and detected its
backscattering in a lidar configuration. Although the atmo-
sphere was sub-saturated (90-93 percent relative humidity),
switching the Teramobile laser on increased the backscatter-
ing from the filaments by a factor of 20. This effect can be
unambiguously attributed to the formation of new particles
and demonstrates the effect of the laser.
2. 4 m
L1-L7 are laser elements; S = sending telescope; C = power supplies and temperature control; D = Lidar detection system.
A century ago, Lord C. T.R.
Wilson observed the effect
of charges on the condensation of vapors in saturated
conditions. Wilson noted the condensation of droplets along
the trajectory of cosmic rays in a so-called “cloud chamber.”
In this experiment, the charges initiate condensation nuclei,
which will further grow in the saturated atmosphere of the
condensation chamber. Laser filaments can be expected to have
an even stronger effect, since they produce at least 106 times
more charges than cosmic rays do. They are therefore excellent
candidates to assist in the condensation of water droplets in
Indeed, we observed that introducing filaments in a cloud
chamber saturated with water vapor results in the spectacular
condensation of a cloud, which is very well visible with the
bare eye. Surprisingly, the same effect can also be seen in sub-saturated conditions. Such unexpected droplet stability and
growth, in spite of relative humidity that is insufficient to balance the surface tension, is the sign of a specific mechanism
at play in the context of laser-induced water condensation.
We expect that this mechanism implies both photochemistry
initiated by the high intensity of the incident laser pulses and
oxidative chemistry due to the concentration of electrons up
to 1015 cm- 3.
These processes promote the formation of O3 as well as
•OH radicals, which will in turn generate hygroscopic H2SO4
and HNO3 condensation nuclei from the SO2 and nitrogen of
the atmosphere, respectively. These hygroscopic particles then
grow, forming cloud condensation nuclei, which are chemically
stabilized, even in sub-saturated atmospheres. They can therefore further expand into the droplets that we have observed
Filaments are not the only source of electrification at play in
the clouds. Due to internal air mass movements (especially
convection), particles such as water droplets or ice crystals
undergo collisions at speeds up to 20 m/s, in which they get
charged. The segregation of different particle types within
separate clouds regions results in overall electric fields across
the cloud. The influence of these charges also initiates fields
up to 10 or 15 kV/m at ground level and 50 kV/m several
hundreds of meters above. Such huge fields are at the root of
lightning—one of the most spectacular and potentially destructive atmospheric phenomena known to humans.
A lightning discharge begins with the elongation of cloud
water droplets into ellipsoids, which enhances the electric field at
their ends, initiating corona discharges. These discharges connect
and form an ionized channel, or leader, which proceeds by steps
of several tens of meters, separated by idle times of 50 to 100 µs.
During its propagation, the leader can split into several
branches. When one reaches close to the ground, an ascending leader is generated from a tree, building, mountain or other
elevated point and progresses towards the descending leader.