22P116-P
Conceptual design of dispersion interferometer with 1 micron light source
波長1ミクロン帯光源を用いたディスパーション干渉計の概念設計
Tsuyoshi Akiyama, Ryo Yasuhara, Kazuo Kawahata, Kazuya Nakayama and Shigeki Okajima
秋山毅志1, 安原 亮1，川端一男1，中山和也2，岡島茂樹2
1

National Institute for Fusion Science, 322-6 Oroshi-cho, Toki-shi, 509-5292, Japan
核融合科学研究所 〒509-5292 土岐市下石町322-6
2Chubu University, Matsumoto-cho, Kasugai-shi, 487-8501 Japan
中部大学 〒487-8501 春日井市松本町

A dispersion interferometer is less sensitive to measurement errors due to mechanical vibrations. It alsobecomes free from fringe jump errors in principle by selecting a short enough wavelength. Hence it issuitable for density measurement on future steady-state fusion reactors. A wavelength around 1 micron is
one of candidates from viewpoints of the variety of optical components, the efficiency of the frequency
doubling and a phase shift due to plasmas. In this paper, a conceptual design of the dispersion
interferometer with 1 micron light source is shown.

1. Introduction
Electron density measurement is indispensable
for fueling control and for physical analysis.
Considering an operation of future high-density andsteady-statefusionreactors,
thedensity
measurement should be reliable and accurate.
Interferometers are used for the continuous density
measurement on most of fusion devices at present.
Although it has high time and density resolutions, ameasurement error due to mechanical vibrations has
to be suppressed. It sometimes suffers from “fringejump errors”, which significantly degrade thereliability. Two-color measurement, which uses twodifferent wavelengths, can compensate the phasecomponents due to the vibrations and ashort-wavelength laser can reduce a risk of thefringe jump errors. However, they are notfundamental solutions and introduction of different
schemes in principle is necessary to establish
highly-reliable density measurement for thesteady-state operation.
A dispersion interferometer (DI) is one ofalternatives to the conventional interferometer. It isless sensitive to the mechanical vibrations. Hence itdoes not need vibration compensation systems.
Since the phase error due to the vibrations isnegligible, the phase shift due to a plasma smaller
than one fringe is acceptable from the viewpoint ofsignal to noise ratio. This means that the DI can befree from the fringe jump errors by selecting anappropriate wavelength. From these advantages, theDI is suitable to future fusion reactors.
The DIs with CO2 lasers (a wavelength of 10.6m) are operated or tested at present1-3. The

wavelength, however, is so long that the phase shiftexceeds one fringe in the case of large andhigh-density plasmas such as ones in ITER andSuper Dense Core (SDC) plasmas in LHD. Hence,a development of the dispersion interferometer witha shorter wavelength, around 1 m, is necessary.
2. Principle of Dispersion Interferometer Using a
Ratio of Modulation Amplitudes
Figure 1 shows a schematic view of the DI. A
mixed beam of the fundamental and the second
harmonic components, which is generated with anonlinear component, is used as a probe beam. The
phase of the fundamental component is modulatedwith a photoelastic modulator (PEM). This
modulation and the phase extraction method using aratio of modulation amplitudes4 are to solve thedisadvantage of the DI: measurement errors caused
by variations of the detected intensity. After passingthrough a plasma, another second harmoniccomponent is generated from the fundamental again.

Fig. 1: Schematic view of the dispersion
interferometer using a ratio of modulation amplitude

CO2 laser (10.6 m)
YAG laser (1.064 m)

100

10-1

1.4

1410-2 -2
1010-1
100
101
Line Integrated Density (×1020 m-2)

2nd Harmonics Power (mW)Fringe shift

101

103
102
101
100

PPKTP (d eff of PPKTP: 17 pm/V)
PPMG:SLT (d eff of PPKTP: 9 pm/V)

62.2
13.8
5.6
1.2

10-1 -1
100.3

1

Wavelength: 1.064 nm
crystal length: 10 mmbeam waist at crystal:
25 m

100
Incident Power (W)Fig. 2: Dependence of the fringe shifts of thedispersion interferometer on the laser sources.

Fig. 3: Generated power of the second harmonics
with PPMG:SLT and PPKTP.

The fundamental is cut with a filter and then aninterference signal between two second harmoniccomponents is detected. While the phase shift due
to mechanical vibrations are the same between thesecond harmonic components, those due to theplasma are different because of the dispersion.
Hence the phase shifts due to the vibrations arecancelled optically and that due to plasma onlyremains. This is the reason why the DI is lesssensitive to the mechanical vibrations.

are expected to be installed in a vacuum vessel andthe reflectivity might degrade by two orders ofmagnitude because of impurity deposition orsputtering on the mirror surface for near IR andvisible range5. The transmission losses in several
optical components (window, beam splitter, and soon) are much smaller than the effect of in-vessel
mirrors. Even in such a harsh situation, an output
voltage of about 1 V will be obtained by the bothcrystals for a laser source with a power of 1 W.
Since a cw YAG laser with a power of about 1 W iscommercially available. The PEM is also available
for this wavelength range. An electro-optic
modulator (EOM), whose modulation frequency ishigher (~1 MHz) than that of the PEM (~50 kHz),is also available. Hence it is possible to compose
the phase-modulated YAG laser DI without any
new developments of the optical components.

3. Conceptual design of Optical System
The fringe shift due to the plasma measured with
the DI is 3c p ne L 2  where cp is the constant,

ne is the line averaged density, L is the path length
in a plasma,  is the angular frequency of the laserlight. Figure 2 shows the fringe shift of CO2 andYAG (1.064 m) lasers. If the expected maximum
phase shift is smaller than one fringe, the phase isuniquely determined: no fringe jump in principle.
While the line integrated density ne L whichcorresponds to one fringe is 1.4×1020 m-2 for theCO2 laser, 14×1020 m-2 for the YAG laser.
Assuming ITER ( ne =1×1020 m-3, L=8 m) for
example, the YAG laser is preferable.
One of the key issues of the DI is the efficiencyof the second harmonic generation. Generally
speaking, the power density of a continuous-wave(cw) laser is too small to generate the secondharmonic. Candidates of nonlinear components are
periodically poled crystals such as PPMgSLT andPPKTP, which have a large conversion efficiencyof the second harmonics. Figure 3 shows thecalculated power of the second harmonic withabove crystals (the crystal length is 10 mm and thebeam radius at the crystal is 25 m). A Si
photodetecter with a conversion gain of 104 V/W(OE-200-SI, FEMTO Messtechnik GmbH) will be
used for detection of the second harmonics. Mirrors

4. Summary
The dispersion interferometer (DI) is one ofcandidates of reliable density measurement on
future fusion reactors. The DI with 1 m laser
source is appropriate for the reactor-size devices
and the optical system can be composed ofcommercially-available components.
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