Proceedings of ITC/ISHW2007

Observation of Internal structure of energetic particle driven MHD
modes in the Large Helical Device
T. Tokuzawa, A. Ejiria, K. Kawahata, and LHD Experimental Group
National Institute for Fusion Science, Toki 509-5292, Japana

Grad. School of Frontier Sciences, Tokyo Univ., Kashiwa 277-8561, Japan(Received day month year / Accepted day month year should be centered 10-point type)In the burning plasma the energetic alpha particles enhance MHD instabilities such as toroidal Alfvén
eigenmodes and these modes affect the particle transport and the plasma confinement. The study of the energetic
particle driven MHD mode is important as the preparation study of the future alpha particle transport. In the Large
Helical Device three microwave reflectometer systems have been installed to measure the density fluctuation in thewide radial area with a high spatial resolution. These systems are used for knowing the internal structures of each
MHD mode and these temporal behaviors. One of the results is the energetic particle mode driven by the injecting
neutral beam is strongly localized at the plasma core region and it is agreement with the theoretical expectation.
Keywords: Alfvén eigenmode, MHD, Density Fluctuation, Microwave Reflectometer, Large Helical Device

1. Introduction
In the burning plasma energetic alpha particles
enhance magneto hydrodynamics (MHD) modes such as
toroidal Alfvén eigenmodes (TAEs). Also the study ofthe energetic particle transport is one of the important
issues and it is found that the correlation between thetemporal behavior of the instability and the lost particle isquite high. That is an MHD mode affects the alphaparticle transport and changes plasma confinement.
Therefore energetic particle driven MHD instability has
been studying in several magnetic confinement devices[1-4]. Usually MHD phenomena are observed bymagnetic probes and the excellent analytical technique isdeveloped to know toroidal and poloidal mode numberand travelling direction. Also theoretical analysis using
three dimensional code has been developing [5]. For thecomparison between the simulation code result and theexperimental result, it is important to measure directly theinternal radial distribution of these modes.
In Large Helical Device (LHD [6]) recently we havebeen applying three types of microwave reflectometersystem for measuring the radial distribution of thefluctuation, because the microwave reflectometer has apotential of the localized measurement by using thecut-off effect in the plasma core region. The density
perturbations δn associated with the displacement ξ of ashear Alfvén mode is described by [2],author’s e-mail: tokuzawa@nifs.ac.jp

δnn= −∇ ⋅ ξ − ξ ⋅∇n  − 2 Rˆ nˆ ≅+⋅ ξ . (1)n  R 2
Ln 

Here n is the plasma density, n̂ is the density unit vector
normal to the magnetic surface, R is the major radius, R̂
is the unit vector along a major radius direction, and Ln isthe density scale length. Therefore it was found that thereflectometer can measure the internal structure of MHD
phenomena such as energetic particle mode and Alfvén
eigenmodes.
At first we have applied a fixed frequency
reflectometer system to measure the low frequency
fluctuation such as an interchange mode, etc. One of thereasons is the limitation of the memory size of the dataacquisition. Recently the real-time fast data acquisition
system has been developed in LHD [7] and the sampling
rate of up to 10 MSample/sec is available to use. It
makes the high frequency fluctuation measurable during
the whole plasma discharge.
Also we make thefrequency variable reflectometer systems, so called
Hopping reflectometer. To measure the fluctuation
profile in the wide range, a reflectometry needs a lot offrequency sources. If the plasma condition seems to be
steady during the frequency changing period, the radial
profile can be measured each sweep in one plasma
discharge. In this paper we present these reflectometersystems and some experimental results.

Proceedings of ITC/ISHW2007

2. Frequency Fixed E-band Reflectometer System
E-band fixed frequency heterodyne reflectometersystem is utilized for the fluctuation measurement [8].
Currently the system has three channels of fixed
frequencies of 78, 72, and 65 GHz. Power combined
three microwaves travel to/from the LHD through thecorrugated waveguide for avoiding the transmission loss.
The extraordinary polarization wave is used. The
simplified super heterodyne detection technique is used
Reflectometer 78GHz

ρ~0.35Magnetic Probe

PSD (<500kHz)

Frequency [kHz]PSD (<250kHz)

Fig.1

Time evolution of reflectometer signal andfrequency spectrum (Left) and these of magnetic
probe signal (right)

for the receiver system. In LHD the real-time dataacquisition system has been able to be utilized and thesampling rate is up to 10 MSample/sec by using acompact PCI based digitizer. The system is very
convenient to observe MHD phenomena such asenergetic particle driven Alfvén eigenmodes.
The
example of the temporal behavior of the reflectometersignal of 78 GHz and magnetic probe signal and thesefrequency power spectra are shown in Fig. 1. In thisplasma discharge some bursts are repeatedly and in thisfigure one of the bursts is presented. Coherent spectraof around 8 and 16 kHz are caused by low-n modeoscillation. In the range of 100~150 kHz there are a lotof coherent mode. These modes are identified the n=1(n: toroidal mode number) TAE mode by the magnetic
probe analysis. Also on the reflectometer signal it isobserved higher mode around 230 kHz. Just after
t=1.82s MHD-burst is occurred and at the same time TAEmode frequency components are rapidly disappearedshown in Fig. 2. Then passing 0.02s this mode is started
to revive. This sudden disappearance may be caused
that the distribution of high energy particle is changed bya MHD-burst. In this experiment the birth source ofenergetic particle is generated by the neutral beam.
During this phenomenon is occurred, the injection powerof neutral beam is kept constant. Therefore the TAE
mode is re-exited quickly and then it keeps to a nextburst.

RMS

0.4

2. Frequency Hopping Ka-band Reflectometer
System>200kHz

To know the radial distribution of fluctuation thereare two methods in a reflectometry. One is themulti-channel system, and another is the wide band
frequency source system. For the latter system, source
frequency sweeps step by step in the whole frequency

0.2

00.8

50< f <200kHz
0.4

VCO
11-18.5GHz

00.8

22-37GHzx2<50kHz
0.4

42GHz

01.5

Fig.2

1.6

1.7

time [s]1.8

1.92Time evolution of fluctuation power ofreflectometer signal. Including each frequency
component is higher than 200 kHz (top),
higher than 50 kHz and less than 200 kHz(middle), and less than 50 kHz (bottom),
respectively.

Arb.
Wave Gen.

E/O

O/E

60Hz-15MHz

Fig.3

DataAcquisition
SystemSchematic view of Frequency Hopping
Ka-band Reflectometer system

Proceedings of ITC/ISHW2007
n=1500(a)PSD (<500kHz)
frequency [kHz]

400

300

200

1000Frequency spectrum of interferometer mode
CW Reflectometer

range. The step width is limited by the characteristic
time of the measuring fluctuation frequency. Of course,
during the frequency change, the plasma condition andthe fluctuation level are assumed to be constant. The
schematic of frequency hopping Ka-band reflectometersystem is shown in Fig. 3. The system uses voltage
controlled oscillator (VCO) as a source. The output
frequency of this source is easily changed by the external
controlled signal. The output wave is amplified and also
this frequency is multiplied by two. The reflected wave
is mixed with a local wave for the heterodyne detection
and intermediate frequency (IF) signal is amplified anddetected. Data acquisition system is the same as theprevious E-band reflectometer system.
The experiment is carried out that the axial magnetic
field strength is 1.0 T, the averaged electron density isunder 0.5x1019m-3, and neutral beam is injected withconstant. The source frequency is swept full range every
200 ms and the number of the frequency step is 20.
Each time of the launching frequency is 10 ms and data
sampling rate is 1 µs, then the data point is 10,000 andthe frequency resolution is 100 Hz. It is enough toobserve the MHD phenomena such as TAE. Figure 4
shows the frequency spectrum of the previous frequency
fixed 78 GHz reflectometer signal. In this plasma
condition there is no corresponding cut-off layer of 78GHz wave and then this system is operated as aninterferometer mode. We can see several continuous
coherent frequency components. Figure 5 shows theradial profile of the fluctuation strength of the frequency
hopping reflectometer signal during t=4.0-4.8s (4 sweep
periods).
It can be obtained that the frequency
component around 200 kHz is large near at ρ=0.8 and theother component around 150 kHz is localized in theplasma centre. Here the meaning of the data points
which are located under ρ=0 is that these frequency
waves are not reflected from the plasma and they are

fluctuation level [arb.units]

Fig.4

fluctuation level [arb.units]01

0.40.6

ρ

non Reflect. (b)Reflect.

0.8

1175-220kHz
0.10.01(c)120-170kHz

1010Fig.5

0.2#57835 t= 4.0-4.8 s

0.2

0.4

0.6

ρ

0.8

1(a) Shear Alfvén spectra for n=1 and Radial
profile of the fluctuation component of Ka-band
Hopping reflectometer in the integrated frequency
range that (b) 175-220 kHz and (c) 120-170 kHz

come back from the opposite wall. The calculated shear
Alfvén spectra is shown in Fig.5(a). The frequency gap ofaround 200 kHz is located near at ρ=0.7. It is wellagreement with the measured profile data. On the otherhand, the coherent frequency mode of around 150 kHz islower than the gap frequency and the temporal behavior
is different with the 200 kHz frequency mode. It looks
like the energetic particle mode (EPM). However,strictly speaking, this signal is not only the phasefluctuation of the reflected wave and it is not directly
related to the density fluctuation. Therefore the directProceedings of ITC/ISHW2007
phase measurement of reflected wave is necessary for thestrict analysis of fluctuation.

LHD
プラズマ

Synthesizer
12.5-18GHz f050-72GHz

SSB

For more accurate fluctuation measurement we havedeveloped a new system shown in Fig.6.
Somecomponents are added to the previous Ka-band system.
Especially the single side band (SSB) frequency
modulation is utilized for the direct phase measurement.
Also the synthesizer is used as a source as a low phasenoise source. The signal to noise ratio is up to 50 dB in
the test of the system noise.
An example of this hopping system’s measurement
is shown in Fig.7. In this time window the launchingfrequency is changed from 52 GHz to 63 GHz and thestep size is 1GHz with 50 ms duration. Some Alfvén
eigenmodes can be observed in the whole launchingfrequency range. Disappointingly in this discharge thefluctuation is not kept constant and it can not be got thestructure of the fluctuation mode. We plan to add more
improvement in this system and will get the internal
information of Alfvén eigenmodes in near future.

SummaryTo study the internal structure of Alfvén eigenmodes,
some reflectometer systems are installed on LHD. BothTAE and EPM modes can be observed with highresolution. Using frequency hopping technique theinternal structure can be obtained. For the strictly
analysis the direct phase measurement system has been
applied. We will add more improvement in the systemand study the Alfvén eigenmodes physics.

Acknowledgements
This work was partially supported by aGrand-in-Aid of the Japan Society for the Promotion ofScience to one of the authors (TT) and also by
NIFS07ULHH507 from the budget grant-in-aid of theNational Institute for Fusion Science.

References[1][2][3][4]
[5]
[6]
[7]
[8]

S. E. Sharapov et al., Phys. Rev. Lett. 93, 165001 (2004)
R. Najikian et al., Phys. Rev. Lett. 91, 125003 (2003)
H. Kimura et al., Nucl. Fusion 38, 1303 (1998)
S. Yamamoto et al., Nucl. Fusion 45, 326 (2005)
C. Nührenberg, Phys. Plasmas 6 137 (1999)
O. Motojima et al., Nucl. Fusion 43, 1674 (2003)
H. Nakanishi et al., Fusinon Eng. Design 66-68827(2003).
T. Tokuzawa et al., 31st EPS Conf. P5-114 (2004)f

20ms/stepfm
4 (f0 - fm )+φMixertx4
4 f0x4BRF
77,82-84GHz

IF AMP
fm =220MHz

BPF
880MHz

4 fmI.Q.

4 fm +φ

Acosφ AsinφFig.6

Schematic view of Frequency HoppingV-band Reflectometer system
Hopping Reflectometer

Launching Frequency [GHz]frequency [kHz]2. Frequency Hopping V-band Reflectometer
System4 (f0 - fm )x4Fig.7

Temporal behavior of the Frequency
Hopping V-band Reflectometer signal andits frequency spectra