Geoscience Reference
In-Depth Information
Table 2.
Estimation of rotation angle of beam polarization under various conditions.
Mirror
Number of
Necessary
Total optical Discharge
Rotation
Polarization
length
reflections/ reflectance
path length
gap
angle (max)
ratio
(mm)
step (mm)
(%)
(m)
(cm)
(deg.)
280
12/21.4
91
19
10
0.21
1:1.007 (21 dB)
280
28/10.0
96
44
10
0.51
1:1.018 (18 dB)
280
56/05.0
98
89
10
0.95
1:1.034 (15 dB)
500
12/41.7
91
34
10
0.074
1:1.003 (26 dB)
500
50/10.0
98.3
141
10
0.51
1:1.018 (18 dB)
500
12/41.7
91
34
20
0.074
1:1.003 (26 dB)
500
50/10.0
98.3
141
20
0.52
1:1.018 (17 dB)
The magnetic flux densities along the beam propagation projected on a
side of the square mirror are shown in Fig. 3(b). Multiplying the magnetic
flux density
B
by the electron density
n
e
along the propagation distance,
the rotation angle of the beam polarization was estimated by Eq. (2).
Figure 3(c) shows the change of the magnetic flux density with respect
to the distance from the discharge path at 13
ยต
s after the discharge. The
flux density changes its value along the elapsed time. The magnetic flux
density
B
is inversely proportional to the distance from the discharge path.
The electron density
n
e
is assumed to be high only within 2 cm of the
discharge path. Based on the results of Fig. 3(a)-(c), the rotation angle of
the polarization was estimated, and is shown in Fig. 3(d). The change of
the rotation angle had the same time response as the discharge current.
The result shows that the frequency response of a few MHz was sucient
for the detector and amplifiers. Other calculations with different discharge
gaps and the beam propagation conditions are summarized in Table 2.
Selecting those conditions, we can distinguish the rotation angle of the
beam polarization by differential detection with a dynamic range of 30 dB.
4. Experiment
4.1
. Experimental setup
The experimental setup of Fig. 2 was constructed and installed. The
specification of experimental apparatus is the same as Table 1. Figure 4
shows photographs of the optics (left) and the square mirror in the discharge
chamber (right). The sides of the square mirror were 28 cm in length, and
its reflectance was about 95%. The number of the reflections on a single
mirror was fixed at 12-14. The total round-trip optical path length was
