J. Plasma Fusion Res. SERIES, Vol. 3 (2000) 4ll4l4Application of Pulsed Radar Reflectometry/Delayometry
to the Large Helical Device Plasmas
PAVLICHENKO Rostyslav*, TOKUZAWA Tokihiko and KAWAHATA Kazuo
National Institute for Fusion Science, Toki 509-5292, JA2AN(Received: 18 January 2000 / Accepted: 16 March 2000)

Abstract
In the large fusion devices (tokamaks and recently helicalsystems) much more attention will be paid
to enhance thediagnostic tools for reliable measurements during steady stateplasma operation. In thisarticle we describe the bistatic,heterodyne, dual-mode pulsed radar reflectometer, which wasinstalled at
the Large Helical Device (LHD) and present theinitial results obtained by it. This system was designed to
testthe possibility of using radar techniques to monitor edge plasmadensity. If we take into account theoperational frequency andcertain wave polarization this diagnostic tool can be also usedas line density

monitor (like ordinary interferometry). Suchregime of operation is known as a delayometry.

Keywords:
fusion, plasma diagnostics, LHD, long pulse operation,reflectometry, delayometry, pulsed radar

1. Introduction

l. Taking into account the magnitude of the magnetic
field (2.75 T) the operational frequency of 5l GHz and
60 GHz was chosen to meet reflection conditions for theIn the past several years, reflectometry became awell-established and common diagnostic technique formeasurements of the plasma electron density and its
fluctuations [,2]. It has been shown that in tokamaks[3-5] and helical devices [6,7] plasma experiments thepulsed radar works reliably giving density values which

extraordinary polarized (X-mode) radiation at theare consistent to those from the conventional diagnostics(i.e. Thomson scattering and interferometry).
The principle of the pulsed radar is to measure thetime delay between a launched pulse and its reflection
by a plasma cutoff layer. The time delay is directly
defined by the coordinates of cutoff layer and the line
integral density up to that point.

2. Instrumentation and Techniques
For third phase of the LHD plasma experiments
one channel, bistatic, heterodyne, dual-mode pulsed
radar, was installed at 9-O port. Probing of the plasma
was conducted from the low-field (LFS) side. The main
components of the experimental setup is shown in Fig.
*Fig. 1 The block diagram of the LHD U-band pulsed
radar reflectometer.
@2000 by The Japan Society of Plasma

C o rre sp ondin g autho r' s e - mail : s lav ik @ nifs. ac.j p

Science and Nuclear Fusion Research4Ll

pavlichenko R. et al., Application of Pulsed RadarReflectometry/Delayometry to the Large Helical Device Plasmas

plasma edge (for 51 GHz) and at the density profile

500

_-ECH

gradient region (for 60 GHz). Almost all microwave
components (oscillators, switches, mixer, etc.) wereplaced close to the device on D-stage in LHD room.
Short (about 1 ns) microwave pulses are launched into
the plasma. This pulse is produced by a CW oscillation'E4400 j

r33oo

modulated by a fast microwave switch.

r* 5i'-NBl

Ot
coEol

=ota
E
([.-ru,i5II=

To deliver maximum microwave power a^Iftll:\llrntransmitter and receiver are placed close to the0

0.5

1.5

1

2.5

23

3.5

4

time (s)diagnostic port and a special teflon lenses were attached

to launching and receiving corrugated conical horns.

Fig. 2 Time evolution of the electron line density (solidSuch arrangement allows to deliver radiation with bothpolarizations into the plasma. To optimize reflection
conditions during plasma experiments we change
direction of the antennas in the horizontal plane. Thus,
the position of the beams cross-point could be readjustfrom outer edge ofthe plasma to inner port surface. The
output of the receiver (IF frequency amplification unit)
was connected to time measurement system atline) and the plasma diamagnetism for the LHD
combined heating plasmas; shot #12746 (dottedline).

diagnostic room via -100 m 0.01-1.5 MHz electrical-tooptical link. The time measurement system utilizes
classical setup for pulsed radar [3,5,71. This system isbased on a constant fraction discriminator technique,where timing is measured between the leading edges (atthe half of the maximum amplitude) of the transmitted

tFrft
tlt"*'N;.).

ino+**le,r,,*r f

and received pulse, disregarding pulse deformation
effects.

-20

3. Experimental Results

or,,-3,(")34Fig. 3 Measured time of flight (pulse delay) for the shot
#12746.

For the standard (8, = 2.75 T, R^ - 3.60 m) LHD
operation scenario, pulse delay measurements by plasma
were conducted for 51 GHz and 60 GHz using X-mode
polarized radiation, The time evolution of the main
plasma parameters as well as pulsed radar signal for theLHD shot #12146 is shown in the Fig. 2. At thebeginning of the discharge after the small increasing of
the plasma density the pulses start to reflect from cutoff
layer which is located at the edge of the plasma (t = 0.25(from interferometeryEz
2time,(3)s). When NBI heating was applied (t = 0.75-2.0 s) theconsiderable growing of the plasma density cause thefurther increasing of the pulses delay in plasma. Becauseof X-mode operation even after the consecutive plasma
density increasing there are no indications of thechanging in the cutoff position (Fig. 3).

Microwove pulse deloy for O-,X-modes
20
100a.

l.t--10-20

During LHD experiments at low magnetic field (BtT, Ro,= 3.60 m) the situation becomes quite
1.52=different. In this case the initial position of the cutofflayer appear more close to the plasma center.
Measurements show a significant movement of thesutoff position to outer plasma boundary region (r =time,(s)Fig. 4 Low magnetic field operation. upper plot is time

evolution of the electron average density, lower
one is pulse delay measured at 51 GHz andcalculated delays of the O-D(-modes; shot #13228.

412

Pavlichenko R. et al., Application of Pulsed RadarReflectometry/Delayometry to the Large Helical Device plasmas

interferometer
1.5"CEo.'lEn.a6E

iadar

0.53o).go-0.5 t
00.51

1.5

2

2.5

3time,[s]2time,(s)Fig. 6 Delayometry type of operation. A comparison ofline integral density obtained by means ofinterferometry and a pulse radar system.

Fig. 5 Dual-polarized experiments, because of poor
reflection with respect of pure O- or X-mode casetiming system start to be triggered by noise (withReflectometry raw data was smoothed within 15points; shot #12694.

timing about 26 ns), or not reflected backward(-23.5 ns); shot #13771.

flected by the inner wall of the vacuum vessel and passthrough the plasma again where they will be detected.

1.2-1.5s). There is a quite obvious disagreement

As differ from the interferometry the above

between simulation results and measured data at thelower density. This could be explained by the fastchanging of the plasma density profile gradient.
Up to present time, all radar experiments have been
carried out for tokamaks (without separatrix) and mirrorsystems. At those devices magnetic shear isconsiderably low. At helical systems the presence of theshear can deteriorate microwave measurements due tothe mode scrambling. Because of the fact that theorientation of the magnetic field can change within onetwo wavelength the power conversion from the O- to theX-mode and vice versa may occur. That is why,
simultaneously O-/X-mode operation could be of greatinterest. Both antennas were rotated by 45' with respect
to the direction of the magnetic field at the edge. Theenergy of the incident wave suppose to be equallydivided between two modes. As a result the amplitudeof the reflected radiation decreases significantly. One
can see (in the Fig. 5) an appearance of the second
reflected signal. That part of the wave that has O-mode
polarization at the edge can penetrate into the more
dense center region, where it meets the reflection

outlined approach is characterized by a significantly
lower frequency of operation. It makes delayometry a
possible contender in monitoring plasma turbulenceeven for low density case. Also due to the minimal port
requirements for pulsed radar it is possible to use pulsed

radar delayometry for the measurement of diverter
plasma line density. However, it is beyond the scope of

this paper to discuss this problem.

4. Summary
During the third cycle of the LHD experiments thebistatic, heterodyne, dual-mode pulsed radar reflectometer, was installed. The initial measurements weredone at 5l and 60 GHz. It was shown that pulsed radar
could be a promising tool for edge density fluctuation
measurements as well as a line monitoring diagnostic
for the special plasma conditions.
The major advantage of the pulsed radars comparedwith the conventional reflectometers and interferometersis that these systems are insensitive to measurement fails
even after temporal loss of the signal. In the steady state
operation this feature is of major importance.

condition.
In low-density plasmas experiments at LHD, wherefor O-mode polarized radiation, maximum density is below the critical one (for 5l GHz: 3.22 x lOLe m-3 for 60
GHz: 4.46 x 10le m-3), pulsed radar reflectometers operate as interferometers. Microwave pulses launched into
the plasma will not be reflected, but pass through theplasma. At the opposite side these pulses will be re-We have to mention also some distinctive
difficulties that arose from the implementation of thetechnique under consideration to the particular case of
LHD device. Due to the complex plasma geometry
microwave sources with much power are required forsuccessful measurements. The alignment of the horns in
the vertical cross-section seems to be more favorable.
413

Pavlichenko R. et al., Application of Pulsed RadarReflectometry/Delayometry to the Large Helical Device Plasmas

Probing along the smaller half-axis of the plasma (which14)

C.A.J. Hugenholtz, A.J.H. Donn6, B.S.Q.

requires a special antenna systems) could improve

Elzendoorn, J.C.van Gorkom, W. Koijman, H.A.

measurements and data treatment dramatically.

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