W-band millimeter-wave back-scattering system
for high wave number turbulence measurements
in LHD
メタデータ

言語: English
出版者:
公開日: 2021-10-08
キーワード (Ja):
キーワード (En):作成者: TOKUZAWA, Tokihiko, TANAKA, Kenji,Tsujimura, Ii Toru, KUBO, Shin, EMOTO, Masahiko,INAGAKI, Shigeru, IDA, Katsumi, YOSHINUMA, Mikiro,
WATANABE, Kiyomasa, TSUCHIYA, Hayato, EJIRI,Akira, SAITO, Teruo, YAMAMOTO, Koji, LHD,Experiment Group
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所属:URLhttp://hdl.handle.net/10655/00012653
This work is licensed under a Creative Commons
Attribution 3.0 International License.

W-band millimeter-wave back-scattering system for high wavenumber
turbulence measurements in LHD
T. Tokuzawa,1, 2, a) K. Tanaka1, 3, T. Tsujimura1, S. Kubo1, 4, M. Emoto1, S. Inagaki3, K. Ida1,2,M. Yoshinuma1, K. Y. Watanabe1, 4, H. Tsuchiya1, A. Ejiri5, T. Saito6, K. Yamamoto6,
and LHD Experiment Group1, b)
1National Institutes of Natural Sciences, National Institute for Fusion Science, Toki 509-5292, Japan

2SOKENDAI (The Graduate University for Advanced Study), 322-6 Oroshi, Toki-city, 509-5292, Japan3Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan

4Department of Energy Engineering and Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan
5Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa 277-8561, Japan

6Research Center for Development of Far-Infrared Region, University of Fukui, Bunkyo 3-9-1, Fukui 910-8507, Japan(Presented XXXXX; received XXXXX; accepted XXXXX; published online XXXXX)(Dates appearing here are provided by the Editorial Office)90 GHz W-band millimeter-wave back-scattering system is designed and installed for measuring the electron
scale turbulence (k⊥ρs ~ 40). A metal lens relay antenna is used for the in-vessel beam focusing and the beam
diameter of less than 40 mm is achieved in the plasma core region. This antenna can be steered at an angle of
159° ± 6°, which almost covers the plasma radius. The estimated size of the scattering volume isapproximately 105 mm at the edge and 135 mm at the core, respectively. A 60 m corrugated waveguide isused to achieve a low transmission loss of approximately 8 dB. A heterodyne detection system for millimeterwave circuits with probing power modulation can distinguish the scattered signal from background noise.
I. INTRODUCTION

Multi-scale turbulence interaction, especially, mixed
micro-scale turbulence phenomena which includes boththe ion and the electron scale turbulences, is of interest1 in
the high temperature plasma confinement study at present
and there are many theoretical studies2-8. Simultaneous
measurements of each electron scale turbulence and ion
scale turbulence are required. However, the measurementof turbulence, especially on the electron scale, is state-ofthe-art, because this observation requires very high spatial
and temporal resolutions. There are a few previous studiesof this diagnostic in tokamaks such as NSTX9, 10, DIII-D11,KSTAR12, MAST13 and NSTX-U14. We have developedand installed this electron scale turbulence diagnostic toFIG. 1. Bragg relationship between the scattering angle and theturbulence wavenumber as a function of launching frequency. Here, anangle greater than 90 degrees implies back-scattering.
a)Author to whom correspondence: tokuzawa@nifs.ac.jp
Author list of Y. Takeiri et al., Nucl. Fusion 57, 102923 (2017).

b)

the Large Helical Device (LHD15) plasma and this is thefirst report for the application to the helical device to theauthor's knowledge. In addition, LHD has several ion-scale
turbulence diagnostics, such as CO2 laser used phase
constant
imaging(PCI)16,
microwave
Doppler17reflectometer , and beam emission spectroscopy (BES)18.
The ion scale turbulence observations have been comparedwith the gyrokinetic simulation (GKV19, 20) results21-23. The
introduction of this electron scale turbulence measurement
will allow for a more detailed comparative study.
In order to measure an electron-scale turbulence suchas electron temperature gradient (ETG) driven mode which
has a high wavenumber (k⊥ρs ~ 40 and k⊥~ 40 cm-1), a
millimeter-wave back-scattering system is one of thesuitable candidates, where k⊥ is the ﬂuctuation wave

FIG. 2. Salpeter parameter α as a function of temperature and electrondensity. Here, the wavenumber k is calculated as k = 40 cm-1, and the α>1 means that the scattering condition is collective.

number perpendicular to the magnetic ﬁeld and ρs is theion gyroradius at the electron temperature. When theelectromagnetic wave is launched to the turbulent plasma,
the scattered wave which satisfies with the Bragg
relationship 𝑘𝑘 = 2𝑘𝑘𝑖𝑖 sin( 𝜃𝜃𝑠𝑠 ⁄2) is generated24. Here, ki is the
local wavenumber of the probing beam and θs is thescattering angle. To measure higher wavenumbers of k⊥~
40 cm-1, one can choose either forward-scattering ofterahertz waves or back-scattering of W-band millimeterwaves, as shown in Fig. 1. In general, there are two typesof scattering conditions. One is collective, and the other is
non-collective scattering. Considering the current situation,
the scattering parameter (so-called Salpeter parameter)
α = (k⊥λDe)−1 is calculated in several plasma conditions oftemperature and density as shown in Fig. 2. Here, λDe isthe electron Debye length. The electron density is higher
than 1x1019m-3 in the usual LHD plasma experiments, thus
the diagnostic region is almost in α >1. This means that thescattering signal is collective, and the scattering signal
originates from correlated density fluctuation 𝑛𝑛�𝑒𝑒 behavioras in the following equation12, 24,
1
𝑃𝑃𝑠𝑠 = 𝑟𝑟02 |𝑛𝑛�𝑒𝑒 |2 𝜆𝜆2𝑖𝑖 𝐿𝐿2 𝑃𝑃𝑖𝑖 ,4
where Ps is the scattered power, r0 is the classical electron
radius, λi is the probing wave’s wavelength, L is thescattering length, and Pi is the probing wave power.
Considering the installation of a millimeter-wave
scattering system on the LHD, W-band 90 GHz is suitable
as a probing frequency. There is no electron cyclotron
resonance layer in the plasma confinement region in theline of sight as shown in Fig. 3. When we use the ordinarymode (O-mode) propagation in the plasma, we can accessthe plasma core region. However, the plasma center islocated approximately 3 meters away from the vacuum
window because of the complicated LHD vacuum chamber.
We need the focusing optics to be precise for small scale
turbulence observation. For this aim, we developed the invessel focusing antenna. In addition, the complex magnetic
structure generates stray light, especially electron
cyclotron emission (ECE), which plays an important rolein the background noise.
In this paper, we describe the in-vessel focusingantenna and the millimeter wave circuits of back-scatteringsystem and its characteristics as follows.

FIG. 3. Radial profiles of characteristic frequencies (upper) and magnetic
flux surfaces in the horizontal plasma cross section. (bottom). Here, fce, 2fce, 3 fce, fp, fr, fl, and fuh mean fundamental, second harmonic, third
harmonic electron cyclotron frequency, electron plasma frequency, righthand cutoff frequency, left-hand cutoff frequency, and upper hybrid
frequency, respectively.

FIG. 4. Three-dimensional LHD CAD design of in-vessel antenna pair forthe back-scattering system. Here, the plasma shape is illustrated by themagnetic surface layers (pink).

FIG. 5. Drawing of metal lens combination antenna (left) and photograph of the installation in-vessel of LHD (right).

II. METAL LENS RELAY ANTENNA

Since the turbulence structure is small and theobservation position in the plasma core region is locatedapproximately 3 meters away from the vacuum window,
the probing wave beam must be focused. In addition, thelimited space around the viewing window precludes theuse of large focusing optics. Furthermore, beam steering
capability is also required because of the requirement tomeasure the radial structure of the turbulence. Figure 4
shows the cross section of the observation line of sight ofthe back-scattering system. A possible method is in-vessel
focusing optics. For these aims, the metal lens antenna isapplied as an in-vessel focusing antenna. This antenna iscomposed of a conical horn and two lenses arranged in a
co-linear configuration as shown in Fig. 5. One lens is a biconcave shaped lens with a curvature of 380 mm and adiameter of 200 mm with 2343 2 mm holes. Another lens
is a plano-convex shaped lens with a curvature of 73 mm
and a diameter of 72 mm, and 253 2 mm holes. This relay
lens is capable of focusing a beam width of approximately
38 mm. The beam width of the probing beam is measured
on a test bench. Figure 6 shows the beam profile obtainedwith a sub-THz imaging camera (TeraSense T30, 64x64pixels), where the beam width is less than 40 mm at the 3
meter distance. In addition, remote beam steering ispossible, and the scattering angle of 159º ± 6º can berealized, covering almost the entire plasma radius.
III. MILLIMETER WAVE CIRCUIT

The millimeter wave circuit for the back-scattering
measurements is shown in Fig. 7. Since precise density
fluctuation measurements require the low-noise source forprobing beam, dielectric resonator oscillator withsynchronized quartz oscillator is used as a source. Probing
90 GHz millimeter wave is generated by x6 multipliers. A
part of probe 15.0 GHz wave is led to the mixer (“Mixer1”) and mixed with the local 15.15 GHz wave to generate
the intermediate frequency (IF) component for heterodyne
phase measurement. In W-band mixer (“Mixer 2”), thereceived 90 GHz wave and the local 90.9 GHz wave are
mixed to generate another IF signal component. Each IF
signal is used for quadrature signal detection and theoutput of in/quadrature (IQ) detection is led to the analog
to digital convertors (National Instruments PXI-6115) withthe sampling rate of 2.5 MHz. The LABCOM dataacquisition system collects the acquired signal in real-timeand also controls the acquisition setting via their web site25.
In addition, the above millimeter wave circuits are locatedin the diagnostic room, which is separated from the LHD
experiment room. Because of the need to connect the 60m
distance with low transmission loss, 1.5" and 3.0"
corrugated waveguides are used to connect to the LHD
viewing port. The measured transmission loss is shown inFig. 8. The total transmission loss is approximately 8 dB at
90 GHz. This value includes the losses due to two quartz
windows and 12 miter bends.

FIG. 6. Beam profile pattern at the focusing point of 3 m distant fromthe antenna (upper) and the profile sliced on the center line of left
figure (bottom).

In the current configuration, the main noise isconsidered to be ECE radiation. In order to estimate thenoise level during the plasma discharge, the system ismodified to modulate the power of a probe wave with asingle pole single throw (SPST) switch. When the SPST
switch is off, the probe beam does not launch to the plasma

FIG. 7. Schematic of the W-band 90 GHz millimeter wave circuit forthe back-scattering measurements. Two synchronized oscillators areused as a source. Active x6 multipliers provide from the oscillator
frequencies to the W-band millimeter waves. Power of the probe wave
is modulated by a Single Pole Single Throw (SPST) switch for theestimation of background noise. A local oscillator is used forheterodyne detection. Two intermediate frequency (IF) components,
which are indicated as “Reference” and “Signal,” are generated in eachMixer. Each IF signal is used for quadrature signal detection. The
output of IQ detection is fed to the 2.5 MHz data acquisition system onreal-time (LABCOM data acquisition system). In addition, thismillimeter wave system is located at the diagnostic room and the 60 m
corrugated waveguides (not shown here) connect to the viewing port.

FIG. 8. In-situ transmission loss measurement of 1.5-inch corrugatedwaveguide. Note that this result includes two quartz windows and 12bends.

and only ECE radiation noise can be detected. Figure 9shows the observed frequency spectrum. ECE radiationnoise, which has broad frequency components, is detected
but the scattered signal is found to be large enough toallow the fluctuation measurements. In addition, the caseof the spectrum of “w/o plasma” shows the small intensity.
This means the reflected wave components from theopposite wall, that is, multi-reflection components aresmall. Currently, during normal plasma measurements,
SPST operates at a ratio of 98 (on) : 2 (off) to estimate thebackground noise. Therefore, the high wavenumber
turbulence is routinely observed by this back-scatteringsystem.
In the current antenna setup (Figure 4), the radial
component of the perpendicular wavenumber is dominant.
Theoretically, the large poloidal wavenumber is of moreinterest for ETG study. However, we think both radial andpoloidal components are not independent and radial
wavenumber measurement is meaningful. Simulations
involving both components allow for comparison study tobe made. Since it is important to know this wavevector
information, scattering position, and beam bending effects,
the LHDGauss ray tracing code26, 27 on the AutoAna
system28 is used to analyse the experimental conditions
immediately after each LHD plasma discharge.
ACKNOWLEDGMENTS

The authors would like to thank the technical staff ofLHD for their support of these experiments. The authors
also would like to thank Mrs. K. Takehara and T. Ito for
their support of metallic lens production. This work waspartially supported in part by KAKENHI (Nos. 19H01880,17K18773, 17H01368, 15H02335, and 15H02336), by abudgetary Grant-in-Aid from the NIFS LHD project under
the auspices of the NIFS Collaboration Research Program(ULPP027 and KLPH024), by the NINS program forcross-disciplinary study (Nos. 01311802 and 01311903),
by the collaboration programs of the RIAM of Kyushu
University and FIR-UF of Fukui University. Additional

FIG. 9. Complex frequency power spectrum of the scattering signal fromthe plasma (red). The “w/o RF” line (grey) indicates the system noiselevel and the “w/o RF” line (orange) shows the background plasma noise
which is considered to be mainly radiated ECE.

support was provided by Japan/U.S. Cooperation in Fusion
Research and Development.
DATA AVAILABILITY

The data that support the findings of this study areavailable from the corresponding author upon reasonable
request.

REFERENCES
T. Tokuzawa, Nuclear Fusion 57, 025001(2017).
S. I. Itoh and K. Itoh, Plasma Phys. Controlled Fusion 43, 1055 (2001).
3.
J. Li and Y. Kishimoto, Phys. Rev. Lett. 89, 115002 (2002).
4.
C. Holland and P. H. Diamond, Phys. Plasmas 11, 1043 (2004).
5.
J. Candy et al., Plasma Phys. Controlled Fusion 49, 1209 (2007).
6.
T. Görler and F. Jenko, Phys. Rev. Lett. 100, 185002 (2008).
7.
N. Bonanomi, P. Mantica et al., Nucl. Fusion 58, 124003 (2018).
8.
S. Maeyama, Y. Idomura et al., Phys. Rev. Lett. 114, 255002 (2015).
9.
Y. Ren, W. X. Wang et al., Phys. Plasmas 22, 110701 (2015).
10.
E. Mazzucato, D. R. Smith et al., Phys. Rev. Lett. 101, 075001 (2008).
11.
T. L. Rhodes et al., Rev. Sci. Instrum. 77, 10E922 (2006).
12.
W. Lee , H. K. Park et al., Rev. Sci. Instrum. 87, 043501 (2016).
13.
J. C. Hillesheim, N. A. Crocker et al., Nucl. Fusion 55073024 (2015).
14.
R. Barchfeld, C. W. Domier et al., Rev. Sci. Instrum. 89, 10C114 (2018).
15.
Y. Takeiri, T. Morisaki et al., Nucl. Fusion 57, 102023 (2017).
16.
K. Tanaka, C. A. Michael et al., Rev. Sci. Instrum. 79, 10E702 (2008).
17.
T. Tokuzawa, H. Tsuchiya et al., Rev. Sci. Instrum. 89, 10H118 (2018).
18.
T. Kobayashi et al., Plasma Phys. Control. Fusion 62, 125011 (2020).
19.
M. Nunami, T. H. Watanabe et al., Plasma Fusion Res. 5, 016 (2010).
20.
M. Nunami et al., Plasma Phys. Control. Fusion 59, 044013 (2017).
21.
K. Tanaka, K. Nagaoka et al., Nucl. Fusion 57, 116005 (2017).
22.
K. Tanaka et al., Plasma Phys. Control. Fusion 62, 024006 (2020).
23.
M. Nakata et al., Plasma Phys. Control. Fusion 61 014016 (2019).
24.
D. H. Froula, S. H. Glenzer, N. C. Luhmann, Jr., J. Sheffield, Plasma
Scattering of Electromagnetic Radiation (Academic press. 2011)25.
H. Nakanishi, M. Ohsuna et al., IEEE Trans. Nucl. Sci. 63, 222 (2016).
26.
S. Kubo et al., "ECH Power Deposition Study in the Collisonless
Plasma of LHD" in Proceedings of 11th Int. Congress on Plasma
Physics (July 2002, Sydney, Australia) p.133 (2002).
27.
T. Ii Tsujimura et al., Nucl. Fusion 55, 123019 (2015).
28.
M. Emoto, K. Ida, C. Suzuki et al., Fusion Eng. Des. 89 758 (2014).
1.
2.