高強度レーザー励起 vp×B プラズマ系

Generation of Short Electromagnetic Wave
via Laser Plasma Interaction Experiments
N. Yugami,
Utsunomiya University, Japan
US-Japan Workshop on Heavy Ion Fusion and High Energy Density Physics
Sep. 28-30,2005
Department of Energy and Environmental Science,
Graduate School of Engineering,
Utsunomiya University
7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, JAPAN, TEL: +81-28-689-6083, FAX: +81-28-689-7030
Outline
A proof-of-principle experiment demonstrates the
generation of the radiation from the Cerenkov wake
excited by a ultra short and ultra high power pulse
laser in a perpendicularly magnetized plasma. The
frequency of the radiation is in the millimeter
range(up to 200 GHz). The intensity of the radiation is
proportional to the magnetic field intensity as the
theory expected. Polarization of the emitted radiation
is also detected. The difference in the frequency of
the emitted radiation between the experiments and
previous theory can be explained by the electrons'
oscillation in the electric field of a narrow column of
ions in the focal region.
Oscillating Current in Plasma Wave
Electron Current

 j  0
t
 i  ikj x  0
jx 
Frequency ;   p (plasma
freq.) 16 cm-3
n=1.0x10
f=0.9 THz

k

Skin depth ; d  c/p
n0 = 1016 cm3
lp = 300 mm
d  50 mm
[1] J. Yoshii et al., Phys. Rev. Lett. 79, 4194 (1997).
Experimental Setup for Radiation
Gas: N2
Permanent Magnet Pairs
Window
Radiation
Laser Pulse
1 TW
100 mJ
100 fs
Lens
f = 150 mm
Window
x
y
z
Receiver
fc = 31.4 GHz
Beam Damper
Typical Example of Emitted Radiation
Intensity (normalized units)
1
Expected Pulse Width
0.8
w02
xR 
: Rayleigh length
l0
200 ps
(FWHM)
0.6
0.4
0.2
0
-2
-1
0
Time (ns)
1
Experimental condition
Laser Power: 0.5 TW
B0: 8.5 kG, He 375 mTorr
2
2
2 xR  p


 190 ps
2
vg
c c
Lp
for Lp  2 xR and  p   c
Radiation intensity was proportional to the
strength of B field
Power (arb. units)
40
Laser Power ; 0.5 TW
N2 750 mTorr
(
30
pµ G
Exp.
20
Cal.
10
0
0
2
4
6
8
Magnetic field strength (kG)
10
2
2
2
,B0 , E x
)
Plasma cavity for radiation
Wavelength of the radiation
is satisfied the matching
condtion,
plasma
boundary
boundary
Plasma column works as
“cavity” for radiation
Strong radiation is expected
to observe.
The data suggests the
wavelength(frequency)
depends on the strength of B
field.
Damping at the plasma-vacuum surface
boundary
p02
p2 ( x ) 
A
L
0
Ramp Boundary
Plasma
E vacuum
aL0
G 
 exp(
)
Eplasma
c
Vacuum
B
0

R
Lx 2
p0
L
x
L
Stop Band
h
  p
k
1
Transmission coeffecient G
p2(x)
Ratio
L  5 mm
0.8
0.6
0.4
0.2
0
2
4
6
8
Magnetic field strength (kG)
10
Typical Waveform of Radiation
--- 2 peaks were observed ---
Cut-off freq. of waveguide
fc = 31.4 GHz
Pulse width
200-250 ps
Each peak of radiation has different
polarization

Estatic
Horn Antenna
Two kinds of radiation in GHz region
Freq. Spectrum measured by TOF method
--- Each peak has different freq. ---
Flight length L = 1.2 m
1st peak
2nd peak
~74 GHz
~40 GHz
Freq. of 1st peak does NOT depend on B field.
Freq. of 2nd peak depends on B field.
1st peak of Radiation
•Freq. : 74 GHz．
•Polarization of radiation // B field
•Frequency of radiation dose not depend on the
strength of B field
Electrons' oscillation in the
electric field of a narrow
Electron
column of ions in the focal
region.
B
E
w
+++++++++++++++++
+++++++++++++++++
+++++++++++++++++
ion column
x
z
2nd peak pulse
•Freq. : ~40 GHz．
•Polarization of radiation
B field
•Frequency of radiation depends on the strength of B
field
Bernstein mode
4
3
2
1
0
1
2
3
4
Conclusion
• The radiation from the interaction between laser and
magnetized plasma was observed.
• Higher freq. and lower freq. components were observed.
• Higher Freq. Component
• Freq : 74 GHz
• Polarization …. Parallel to B field
• Frequency does not depend on the strength of B
field.
• Due to the electron motion parallel to the B field.
• Lower Freq. Component
• Polarization …. Perpendicular to B field
• Frequency does depends on the strength of B field.
• Bernstein mode?

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