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Dispersion interferometer using
modulation amplitudes on LHD
中科院等离子体物理研究所
王兴立，曹骑佛，孙兆轩，周凡，魏然，江堤
T.Akiyama,R. Yasuhara,K.Kawahata,S.Okajima,and K.Nakayama,Rev. Sci. Instrum.85, 11D301 (2014)
Introduction---Background



Electron density measurement with an interferometer
is indispensable on present fusion devices.
However,the mechanical vibrations cause measurement
errors for interferometer and a fringe jump error
in the high density range leads to
uncontrollability of the density.
Hence,great efforts have been paid for the
development of the electron density measurement
which overcomes the above limitation of the
interferometer.
Introduction---Polarimeter

O n e
o f
t h e
candidate methods
is a polarimeter
which measures the
electro-magnetic
interactions
between a plasma
and waves such as
the Faraday effect
a n d
t h e
CottonMouton effect.
Introduction---Polarimeter



These interactions depend upon not only the
electron density but also the magnetic field
strength.
The dependence on the magnetic field is an
advantage because information such as the magnetic
fluctuations can be obtained.
the dependence on the magnetic field is also a
disadvantage because the dependence makes the
density evaluation complicated and the precise
magnetic field strength for a high density
resolution is necessary.
Introduction---DI


A dispersion interferometer ( D I ) is also insensitive to the
mechanical vibrations. Since the measurement errors caused by
the vibrations are small, the phase shift due to a plasma
less than one fringe would be acceptable. Then the DI is
principally free from the fringe jump error by selecting the
short wavelength of the laser because the electron density
can be determined from the phase shift without uncertainty of
2π.
the DI is also attractive for the electron density
measurement of an atmosphericpressure plasma because the DI
can significantly reduce the phase shift caused by a change
in the gas-pressure, which is cancelled as the same as the
mechanical vibrations, during the discharge.
Principle of phase extraction method
T. Akiyama, K. Kawahata, S. Okajima, and K. Nakayama,Plasma Fusion Res.5, S1041 (2010).
Principle of phase extraction method
Principle of phase extraction method
Principle of phase extraction method
By using the ratio of the amplitudes, the variations are canceled in the electron density evaluation
T. Akiyama, K. Kawahata, S. Okajima, and K. Nakayama,Plasma Fusion Res.5, S1041 (2010).
T.Akiyama,R. Yasuhara,K.Kawahata,S.Okajima,and K.Nakayama,Rev. Sci. Instrum.85, 11D301 (2014)
Optical setup of bench testing
Optical setup of bench testing
Optical setup of bench testing
Experiment results
This small variation of the baseline indicates that the phase shift due to the
mechanical vibrations is well cancelled.
Experiment results

The remaining variations might come from the following
reasons.
One is the uncanceled vibration components.
The walk-off observed
As for long time measurement,
in the
slow variations are speculated
to be
related
Similar
to that, the distortion of the
If there is large to
displacement
nonlinear
changes in the room temperature.
The
A wedge
plate
wavefronts,
which
is which
due to multiof the optical axis
between
crystalmight
and change
temperature variations
thecan change
the direction
reflectionin
the window
material
the fundamentaloptical
and constant of some
The wavefront
should
be
window
optical
components.
of the also
beamcauses
axis may
or the PEM,
the
the second harmonic beams,
completely.
materialoverlapped
reduce the
baseline
imperfections
of vibration
the cancellation might be
causes the
variations.
cancellation.
imperfect.
beam
displacement
of around
One of the solutions is
0.1 mm.
an adaptive optics
Experiment results

Measurement results of low and high density plasma with the DI and
the existing far infrared laser interferometer are shown in thes e
two figures.
Experiment results


One of the possible reasons of
the offset in the modulation
amplitudes
is
the
multireflection inside the optical
element in the PEM.
In
order
to
reduce
the
offsets, optimization of the
incident angle of the laser
beam to the PEM to minimize
the multi-reflections may be
effective.
Experiment results

While a fringe jump
error occurred in the
far infrared laser
interferometer due to
decrease
in
the
detected intensity by
beam refraction, the
DI
can
continue
measurement.
The
decrease
in
the
detected
intensity
was not significant
due
to
shorter
wavelength.
Discussion

co2 laser
Nd:YAG laser
Although there were no fringe
jump errors in the pellet
injected discharge by the
sufficient sampling frequency
and the time constant, there
still remains the risk of
fringe jump errors for the CO2
laser DI principally because
the phase shift is larger than
one fringe. A Nd:YAG
laser,
m
whose wavelength is 1.064
can reduce the phase shift
down to 1/10 compared with
that of the CO2 laser.
Discussion
T. Akiyama, K. Kawahata, S. Okajima, and K. Nakayama,
Plasma Fusion Res.5, S1041 (2010).

As for the phase modulator, not
only
the
PEM
but
also
an
electrooptic modulator (EOM) is
available in this wavelength range.
P. A. Bagryansky, A. D. Khilchenko, A. N. Kvashninet al.,
Rev. Sci. Instrum.77, 053501 (2006).