Proceedings of ITC/ISHW2007

Observation of Novel Instability by using
Microwave Imaging Reflectometry in LHD
S. Yamaguchi1), Y. Nagayama1), Z.B. Shi2), Y. Kogi3), and A. Mase3)
1)
2)
3)

National Institute for Fusion Science, Toki 509-5292, Japan

Graduate University for Advanced Studies, Miura 240-0193, JapanArt Science and Technology Center for Cooperative Research, Kyushu Univ., Kasuga 816-8580, Japan(Received 15 October 2007 / Accepted day 15 October 2007)

A new magnetohydrodynamic (MHD) mode with equally separated higher harmonics has been observed in the Large
Helical Device (LHD). The MHD mode appears as the density and magnetic fluctuations in the case of perpendicular ion
heating. The poloidal and toroidal mode numbers are m = 3 / n = 4 respectively, and the fundamental frequency is 1 ~ 10kHz, which looks depending on the ion temperature. The density fluctuation is radially localized in the edge region where
the rotational transform is nearly ι = 4/3. The feature of the newly observed mode is different from the interchange mode
which is frequently observed in LHD.
Keywords: MIR, microwave, imaging, reflectometry, LHD1. Introduction
The heliotron configuration is one of the promising
candidates of the thermonuclear fusion reactor since theplasma is confined in the steady-state without hazardous
disruption by using external superconducting coils and afull helical diverter. The Large Helical Device (LHD) is asuperconducting heliotron device with the poloidal period
number of L = 2, the toroidal period number of M = 10, themajor radius of Rax = 3.5 ~ 4.1 m and the averaged minor
radius of 0.6 m [1]. The considerable experiment andtheoretical efforts have been devoted to the study ofinstabilities in helical plasmas, however, the present
understanding are still not satisfactory.
The recent advance in the microwave technology
provides a new generation of the imaging diagnostics inmagnetically confined plasma [2-5]. The microwave
imaging diagnostics has a potential to obtain 2-D/3-D
images of the turbulences and the instabilities with goodtime and spatial resolutions [6-8].
It brings
understanding of the basic physics of the turbulence andthe plasma confinement.
The microwave imaging
reflectometry (MIR) utilizes the radar technique for themeasurement of the electron density profiles and its
fluctuations by probing the density-dependent cutoff layer
in the plasma. The MIR system is under development in
the National Institute for Fusion Science, and it canobserve the electron density fluctuation with good
author’s e-mail: yamaguchi.soichiro@LHD.nifs.ac.jp

sensitivity in LHD [9]. In this paper we will present a
newly observed magnetohydrodynamic (MHD) mode,
which appears during perpendicular ion heating of neutralbeam injection (NBI) or ion cyclotron resonancefrequency (ICRF). This mode is accompanied with theequally separated higher harmonics. The experimental
setup of the MIR system is described in Section 2, andthe newly observed MHD mode is described in Section 3,followed by the conclusion in Section 4.

2. MIR System
Figure 1 shows the schematic view of the MIR system
in LHD. The MIR system illuminates the LHD plasma
by using the probe wave with frequencies of 53, 66 and 69
GHz in X-mode, and the probe beam is parallel with thediameter of 20 cm. The illumination wave is reflected bythe three cutoff layers, which are determined by the local
density and the magnetic field. The density fluctuation
causes the amplitude modulation and the phase modulationin the reflected beam. The reflected wave is diverged in
the case of the density fluctuation with short wavelength.
An optical system focuses the reflected wave on thereceiver in order to reproduce the microwave image of thefluctuation.
The primary imaging mirror with thediameter of 45 cm is installed inside the LHD vacuum
vessel. The distance between the mirror and the plasma isabout 210 cm. The receiver contains three horn antennas

Proceedings of ITC/ISHW2007

Fig.1

Schematic of the Microwave Imaging
Reflectometry (MIR) in LHD.

to detect the microwave image. A pair of receiving
antennas is separated by 8.4 cm in the poloidal direction,
and another pair by 10.7 cm in the toroidal direction on thecutoff layer. The fluctuation signal is measured by theheterodyne receiver with the sampling time of 1 μsec.
The fluctuation signal of the reflected wave represents thedensity fluctuation on the cutoff layer in the plasma.

3. Observation
A typical example of this MHD mode is shown infigure 2. The LHD configuration is as follows: Rax =3.575 m, a = 0.6 m, Bt = 2.870 T, γ = 1.254 and Bq = 100 %.
The plasma is basically heated by the co-injected NBI withthe power of 8.6 MW and the counter-injected NBI withthe power of 3.8 MW. Figure 2 shows time evolutions of
the heating power, the beta (β) measured by thediamagnetic diagnostics, the line averaged electron density
ne, the central electron temperature Te, the central ion
temperature Ti0 and the MIR signal. Figure 2(e) shows
the MIR and the magnetic signals showing the mode,
which look semi-sinusoidal oscillation.
Figure 3 shows the time evolution of the FFT power
spectrum of the MIR signal and the magnetic probe signal.
When the plasma is heated by NBI in the co, counter andperpendicular directions, the target mode appears at t =1.45 sec with the sharp spectrum in the FFT power
spectrum of the MIR signals. This mode is accompaniedwith the equally separated several higher harmonics. The
same mode appears in the FFT power spectrum of themagnetic fluctuations, however, another mode also appears
with the broad spectrum at t = 1.3 sec. Right after turning
off the perpendicular injected NBI with the power of 3.2
MW at t = 1.7 sec, the frequency of the target mode starts
dropping, and it looks depending on the ion temperature.
Right after turning on the ICRF heating with the power of
1.7 MW, the mode with equally separated several
harmonics appears again. So, the perpendicular ion
heating looks playing an important role to destabilize thismode. However, the present mode is sometimes observed
during only the tangential NBI heating in the case of lowdensity and high temperature plasma.

Fig.2 Typical discharge with a newly observed MHD
mode. (a) The heating power; (b) the
diamagnetic beta (dotted line) and the line
averaged electron density (solid line); (c) the
central electron temperature (dotted line) andthe central ion temperature (solid line); (d) theMIR signal; (e) the MIR signal and themagnetic probe signal in a fast time scale.

Fig.3

FFT Power spectrum of (a) the MIR and (b)the magnetic probe signal.

The mode numbers is estimated from MIR signals
since the magnetic probe signal is not strong enough toidentify the mode numbers in the case of low beta. MIR
ch.2 and MIR ch.8, which detect the frequency of 63 GHz,
are poloidally separated with the distance of about 84 mm
at R = 4.55 m. It corresponds to the poloidal angle of θ= 4.9 degrees. The phase difference in the fundamental
mode can be obtained from the cross power spectrum ofthese channels. At t = 1.55 sec, the phase difference δθ isabout 15 degrees. So the poloidal mode number is m = 3,
and the error is about 1. The mode number of higher

Proceedings of ITC/ISHW2007

Fig.4

Radial profiles of (a) the electron density (ne)and temperature (Te) at t = 1.6 sec, (b) therotational transform, and (c) the amplitude of
the fundamental mode of the density
fluctuation.

harmonics is proportional to the frequency.
Figure 4(a) shows the radial profile of the electron
temperature and the electron density at t = 1.6 sec. These
are measured with the YAG laser Thomson scattering with
200 spatial channels [10, 11]. By fitting to the 8th orderpolynomials, smooth Te and ne profiles are obtained. The
absolute value of the electron density is calibrated by using
the 2 mm wave 2-color interferometer [12], of which
viewing line has the same configuration of the laser beamof the Thomson scattering in the different port. The radial
density profile has the flat top shape and the steep gradientin the peripheral region of the plasma in this case. The
cut-off layer for the probe wave in X-mode is obtained
from this density profile and the calculated magnetic fieldin vacuum. Figure 4(b) shows the radial profile of therotational transform ι. Figure 4(c) shows the amplitude of
the fundamental mode between t = 1.5 sec and 1.7 sec.
These are obtained from 9 discharges of similar operational
conditions with slightly different densities. The mode
amplitude is localized near r ~ 0.9 m where the rotationaltransform ι is about 4/3, as shown in Figure 4(c). Here, r
is the minor radius in the horizontal port section, defined asr = R - R0 and R0 = 3.575 m in this case. The profile widthof the mode amplitude is about 3 cm. At t = 1.6 sec themode amplitude is large on the intermediate cutoff layer of66 GHz (r = 0.88 m). However, it is quite weak on theslightly outer layer of 69 GHz (r = 0.85 m) and it
disappears on the inner layer of 53 GHz (r = 0.98 m).
Therefore, the mode is well localized in the radial
direction.
The mode presented in this paper is different fromknown MHD instabilities in LHD. Usual MHD modes
observed in LHD are the ideal and the resistive interchange
modes [13, 14]. The edge region of LHD plasma isFig.5 (a) Comparison of the fundamental frequency
fMIR vs. the ion diamagnetic frequency fid; (b)fMIR vs. the toroidal precession frequency fpr intokamak.
theoretically stable for the ideal interchange mode due tothe high shear [15]. As shown in Figure 3, the resistive
interchange mode appears in the magnetic fluctuations
after the pellet injection at t = 1.06 sec. The frequency of
the resistive interchange mode increases from t = 1.1 sec
and decreases from t = 1.5 sec. The frequency range ofthe resistive interchange mode is similar to the target mode.
However, features (spectrum and localization) are different.
The present mode has sharp spectrum and is localized innarrow space, while the observed resistive interchange
mode has a broader spectrum and is not very localized in
the radial direction. Actually it spreads more than 12 cm
in the MIR measurement. Usual beam driven MHD
instabilities are Alfven eigen modes, and some of them
have been observed in LHD [16]. The frequency of TAE
mode is more than 100 kHz, and that of GAE mode is theorder of 50 kHz. Those frequencies are much higher thanthat of the present mode.
The frequency characteristic of the target mode issimilar to the fishbone oscillations in tokamaks [17]. In
the case of tokamak there are two models for explaining
the mechanism of the fishbone mode with high frequency
branch and low frequency branch. In the high frequency
branch the trapped energetic ion destabilizes the mode withthe toroidal precession frequency fpr. In the low frequency
branch, the energetic ion destabilizes the mode withfrequency close to the thermal ion diamagnetic frequency
fid. Those frequencies are calculated using the formula fortokamaks [17], as follows:

where fpr is the toroidal precession frequency of theresonant trapped fast ions, and fid is the finite diamagnetic
frequency evaluated at the safety factor q = 1 surface.

Proceedings of ITC/ISHW2007
n = 4 mode that is localized in the narrow layer with thewidth of 3 cm near the ι = 4/3 surface in the edge region.
The mode frequency is much less than that of known
Alfven eigen modes, and is similar to the ion diamagnetic
frequency. However, we have not obtained a clear
evidence of the fishbone instability yet.

Acknowledgement
Fig.6

The parameter regions of (a) ne0 vs. Te0, (b) ne0
vs. Te0, where the mode appears (O) or
disappears (X).

Here, E denotes the energy of the ions, ω0 = eB/M the gyro
frequency and M the mass of the particles, ∇pi denotes theion pressure gradient of the plasma bulk and n the plasma
density.
In Figure 6, these frequencies are compared with thefundamental frequency of the mode. Here, the fid iscalculated by using the density profile and the ion
temperature profile, which is measured by the chargeexchange spectroscopy. The fundamental frequency isabout fMIR = 1 ~ 7 kHz, which is several times higher than
the ion diamagnetic frequency of fid = 0.5 ~ 1.6 kHz. The
perpendicular NBI injects the beams with energy of 40 keV,
therefore, the toroidal precession frequency around the ι =1 surface is about fpr = 1 kHz. The toroidal precession
frequency does not depend on the bulk ion temperature,
while the mode frequency increases as Ti0 increases in theexperiment. The fundamental frequency is slightly higher
than fid and fpr, however, the clear correlation between thetarget mode and the low frequency branch of the fishbone
mode has not been found yet.
Figure 6 shows the parameter regions of ne0 vs. Te0 and
ne0 vs. Tei, where the target mode appears in LHD. In
tokamaks, the plasma parameter of the fishbone activity isin the region of high ne and high Te [17]. The target mode
appears in the cases of high temperature or low density,and is also different from fishbone instability in tokamak.
The mode sometimes appears or not with similar plasma
parameters. The neutral particle analyzer (NPA) [18, 19]
and other diagnostics of LHD suggest that the appearanceof the target mode might depend on both the verticalcomponents of the fast ion and the pressure gradient at theresonance surface. The details will be presented soon.

4. Summary
In conclusion, we have observed the new MHD modewith the sharp spectrum and the equally separated higher
harmonics by using the MIR in LHD. This is the m = 3 /Technical assistance from S. Sugito and Y. Ito in
Department of Engineering and Technical Services ofNIFS is gratefully acknowledged.
This work is
supported by the budgets of the National Institute forFusion Science (ULPP525), and by the National Instituteof Natural Sciences (KEIN0021).

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