Geoscience Reference
In-Depth Information
the beam propagated at distance
L
(=
L
1
− L
2
) is obtained as follows:
L
2
π
λ
δ
=
(
n
+
− n
−
)
dl
=2
.
62
×
10
−
13
λ
2
L
2
L
1
L
1
n
e
Bdl,
(2)
where
λ
is the wavelength of the propagating beam. Since
δ
is proportional
to
λ
2
, the rotation angle for visible light is small. Therefore, the polarization
angle rotation must be measured with high accuracy in order to detect
lightning discharge.
When the Faraday effect is applied to lightning measurement, the
atmosphere needs to be partially ionized, and the magnetic flux due to
the lightning discharge must exist. Cloud-to-cloud discharge, which causes
20-30 times continuous discharge, satisfies those conditions.
3. High-voltage Discharge Experiment
3.1
. Experimental model
An experimental apparatus incorporating multiple reflection optics was con-
structed to verify whether the polarization plane rotation of a propagating
beam due to an electrical discharge can be detected. The experimental setup
is shown in Fig. 2, and the specifications are shown in Table 1. The propagating
beam runs repeatedly around the discharge path so that the beam can interact
with the high-voltage discharge multiple times, providing enough rotation in
the polarization plane due to the Faraday effect. The optics consists of input-
output optics and a square mirror. The latter is installed inside a discharge
chamber.
The discharge gap consisting of two needle electrodes is at the center of
the square mirror. The polarization and divergence of the beam are adjusted
on entering the square mirror. The total length of the optical path can be
changed by controlling the tilts of the four sides of the square mirror.
The outgoing beam from the square mirror is divided by a polarized
beam splitter and detected separately at mutually orthogonal polarizations.
The polarization of the incident beam is adjusted to balance the detected
intensities. An impulse voltage is applied to the needle electrode, causing a
spark discharge inside the chamber. This causes a rotation in the polarization
plane of the propagating beam, and the difference between the intensities
of the two mutually orthogonal polarizations is detected by a differential
amplifier.
