J. Plasma Fusior Res. SERIES, Vol.5 (2002) 333-336

70 GHz Electron Cyclotron ResonanceHeating System for Heliotron J
SHIDARA Hiroyuki. NAGASAKI Kazunobur, SAKAMOTO Kinzor.
YUKIMOTO Hidetoshi, NAKASUGA Masahiko, SANO Fumimichi', KONDO Katsumi,
MIZUUCHI Tohrur, OKADA Hiroyukir, BESSHOU Sakae, MANABE Yoshito,
ANG Wan Leng, KAWAZONIE Hayato, MAENO Shogo, TAKEDA Masafumi, TAKAMIYA Tasho,
TOMIYAMA Keishi, TSURU Hiroki, OHNO Yoshinori, KUBO Hiroyasu,NISHIOKA YusuKe. IRICUCHI Masao. ORLOV Victor2, PAVELYEV Alexander2,TOLKACHEV Alexander2, TRIBALDOS Victor3 and OBIKI Tokuhirol
Graduate School of Energy Science, Kyoto University, Uji 611-0011, Japan
1
Insritute of Advanced Energy, Kyoto (Jniversity, Uji 611-0011, Japan
2

3

GYCOM, Ulyanov Street, 603600 Nizhny Novgorod, Russia
Asociacion Euratom-Ciemat para Fusion, Madrid 28040, Spain(Received: ll December 2001 / Accepted: 4 June 2002)

Abstract
A 70 GHz electron cyclotron resonance heating (ECRH) system has been constructed in Heliotron J,
a helical-axis heliotron device, in order to rcalize localized heating and current drive experiments. ThisECRH system consists of a gyrotron, a matching optics unit (MOU), a waveguide transmission line with
the HE,1 mode, polarizers, a barrier window and a launching system. The low power transmission testshows lhat the beam radius of the focused Gaussian beam is 22 mm at the magnetic axis, which is smallenough compared to the averaged minor plasma radius (170 mm), and the launching system covers awide steering range which is necessary for controlling the power localization in the three-dimensionalhelical-axis configuration. In the high power transmission test, the transmission efficiency of thecomrgated waveguide of 20 m in length is 92 Vo, and the available output power to the vacuum vessel isup to 0.4 MW.

Keywords:
ECH, Fteliotron J, localized heating, current drive, Gaussian beam, transmission efficiency

1. Introduction

system has the injected power of TE62 axisymmetricmode, the power absorption was not localized, and theECRH has widely been used in many magnetic
confinement di:vices. Especially in helical coil devices,
the current-fr€,e plasma is routinely produced by thislow single pass absorption caused a low heating
efficiency. We introduced a new 10 GHz ECRH system
for the purpose of localized heating, which makes itheating method. The helical-heliotron plasma device,Heliotron J, h:rs begun its experiment since December
1999, and a 53.2 GHz ECRH system has been used forplasma production and heating []. Since the 53.2 GHz

possible to perform various experiments such as plasma

profile control and electron cyclotron current drive. The@2OO2 by The Japan Society of Plasma

Corresponding author's e-mail: shidara@ center.iae.kyoto-u.ac.jp-)-l -lScience and Nuclear Fusion ResearchShidara H. et al., 70 GHz Electron Cyclotron Resonance Heating System for Heliotron J

Gaussian shape with the linear polarization, is put into
the MOU for correcting the beam shape to a circularone. The MOU has two phase correcting mirrors andHeliotron J device has three-dimensional magnetic fieldstructure and spatial magnetic axis. Due to its complex
shape, a wide range of injection angle and polarization
with precise control is required to achieve the efficient
single pass absorption. The 70 GHz ECRH system isdesigned and constructed to satisfy these requirements.
In this paper, we report the overview of the 70 GHz
ECRH system and the results of the low and high power

water-flowing pipe for absorbing spurious modes anddiffracted waves. The corrected Gaussian beam iscoupled to the HE11 waveguides mode. The transmission
line is composed of comrgated waveguides of totally 20m long and 63.5 mm diameter, and three miter bends.
Two grooved plane mirrors are assembled in the bends
for adjusting the beam polarization. One is designed as alinear polarizer which can rotate the polarization plane,
and the other is designed as a circular one which cancontrol the ellipticity. The transmission line can beevacuated to avoid arc breakdown. The transmitted
beam is injected into the Heliotron J plasma by thelaunching system as shown in Fig. 2. The launcher has
two mirrors, a focusing ellipsoidal mirror l2l and asteering plane mirror. The focusing mirror focuses thebeam of which the waist is located at the magnetic axis
in the perpendicular injection case. The steering mirror
can control the beam direction both toroidally andpoloidally.

transmission tests.

2. 70 GHz ECRH System
We have designed the ECRH system for the second
harmonic of the extraordinary (X-) mode at the resonant
magnetic field of 1.25 '1. The system consists of thefollowing components: a gyrotron, a MOU, atransmission line, a Boron Nitride barrier window and alaunching system as shown in Fig. l. The gyrotron
manufactured by GYCOM has the operational frequencyof 70.0 GHz, the maximum power of 600 kW at theoutput window, the pulse duration up to 500 ms and theGaussian beam output mode. The output beam irradiatedfrom the gyrotron window, which has an elliptic

3. Performance Results
3.1 Low power test
The performance of the polarizers and thelaunching system has been examined in a low power
condition. The TE16 rectangular output mode from a 70(+0.2) GHz,20 mW output Gunn oscillator is converted
into the HE11 mode using a waveguide mode converter.
The purity of the HE11 mode is 97 Vo for this low power
beam. We set a polarizer in a miter bend by replacing

the reflecting plane mirror. We can change thepolarization by rotating the polarizer. The miter bend isconnected to 1-m waveguides for both sides. The beam

Fig. 1 Schematic view of the 70 GHz ECRH system inHeliotron J.

shape and the polarization mode are measured with adiode detector in free space to suppress the effect ofstanding waves.

Figure 3 shows the results for the linear andcircular polarizers. The definition of the rotation angle,
cr, and the ellipticity, p, is the same as in ref. [3]. The
theoretical curves are also plotted, which are given byusing integral method developed in the vector theoriesof diffraction gratings [3]. According to the calculation,
the sinusoidally grooved mirror of the deep depth,dl),=O.50, can rotate the polarization arbitrary withsmall ellipticity in the range of the polarizer rotation
angle,60" < Q < 115", and the mirror of the shallow
depth, dlL=0.30, makes almost circular polarization at
0=55" and 125". Here d and h are the grooved depth

Fig. 2 Poloidal cross section of launching system.

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.90Fig. 4 Beam radius, as a function of the distance from,t5 60 75 90 105 t20 135 150 165

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Distance from contor [mm]Fig. 3 Results of polarization measurement; (a) linear
polarizer and (b) circular polarizer.

Fig. 5 Beam distribution in vertical direction at 390 mm

after the focal point in the low power measurement.

waveguide exit [4,5]. The spatial resolution isdetermined by the waveguide size of the detector. Aand the wavelength, respectively. Since the experimental

results are in a good agreement with the theoreticalvalues for each polarizer, we conclude that the grooved
mirrors are manufactured as designed. It is possible tocontrol the rotation and the ellipticity independently, bycombining these two polarizers.
The launching system is also tested in the same low
power measurement system. We use a three-axis
measuring system, which can measure the beam
distribution and polarization plane for estimating the1/e2 power radius from the radiation pattern. The size ofthe focusing mirror is 215 x 140 mm2, and the steering
plane mirror is 157 x 86 mm2, which are 1.5 times
larger than the beam diameter. The mirrors of this size
can cover over 98 7o of the power. Figure 4 shows thedependence ol the beam radius as a function of thedistance from the waveguide exit. The solid line gives
the value derived from a quasi-optical theory, where thespot size of the beam is assumed 18.8 mm at therectangular waveguide of W band (2.54 x 1.27 mm21 isused to detect the beam radiation. The measurement
results shows that the Gaussian beam is focused and thebeam radius is 22 mm at the major axis for
perpendicular injection, although the beam radius is alittle larger than the theoretical value. The beam shape iskept circular Gaussian after the focal point. The
expected power density, which is defined by the power

divided by the lle2 folding area at the focal point, is ashigh as 26kWlcm2. Figure 5 shows an example of beam
shape after the focal point. After passing the focal point,
the beam diverges as expected and the shape is kept
Gaussian.

The direction range of the launching beam has beenmeasured with a laser pointer (diameter 3 mm) at first
set on the beam axis. Then we have confirmed the beam

direction at several points using the low power
335

Shidara H. et al., 70 GHz Electron Cyclotron Resonance Heating System for Heliotron J

millimeter wave oscillator. The steering range is +l5o
and, *23" in toroidal and poloidal directions, respectively. These ranges are limited by the mechanicalrestriction of the mirror system in the launcher. The4. Conclusion

toroidal range coresponds to the angle from 60o to l20o
between incident beam wave number and the magnetic
field direction, and the poloidal range corresponds to 0 <rla 3 0.7, where r is the averaged plasma minor radius
and a is the averaged radius of the last closed flux
surface. This poloidal range is sufficient for controlling
the power absorption distribution from on-axis to off-

experimental research on the heat and particle transport.
The localization of the power absorption and the control
capability of the injection angle and polarization enables
us to perform a wide variety of plasma experiment.
We have tested the performance of the transmission
components such as the waveguide in the low and highpower measurement. The each component has a good
performance as designed. The grooved mirror works aslinear or circular polanzer depending on the groove

We have designed and constructed the 70 GHz
ECRH system for Heliotron J. The main objectives areto construct the system which can carry out the flexible

axis.

3.2 High power measurement

depth. The launching system can focus the Gaussian

A high power transmission measurement has been
performed using the gyrotron in order to confirm thereliable operation and to measure the transmission
efficiency. The beam power is measured with abeam well, and control the beam direction both
toroidally and poloidally, which is a necessary conditionin three dimensional confinement system as Heliotron J.
This shows a wide control capability of the power
absorption from on-axis to off-axis. We have started theplasma experiment using this 70 GHz ECRH system.
The calculation of the power absorption profile under
this ECRH launching condition is also underway by

calorimetric load at the gyrotron window, MOU output,

and the waveguide mouth after transmission. Themaximum power is 600 kW at the gyrotron output
window under the condition of the beam voltage of 75
kV and the beam current of24 A. The power loss rate at
the MOU is about 16 Vo, and the transmission loss
between the MOU exit and the waveguide mouth aftertransmission is 8 7o. The transmission efficiency ofusing a ray tracing code 'TRECE' t6l. The plasma
experimental results will be shown in the forthcoming
paper.

Acknowledgements

corrugated waveguide, 92 7o is estimated by the surveyof over 20 shots at the conditions of the output powerfrom 150 kW to 500 kW. The transmission loss may becaused by the misalignment of the waveguide at theMOU output including the offset and tilting, the ohmic
consumption and the mode conversion. The loss at thebarrier window is estimated less than I 7o. These results

The authors would like to thank the Heliotron J
group for operating the Heliotron J device.

References

lll T. Obiki er al., Nucl. Fusion, 41,833 (2001).
t2l J. A. Murphy, Int. J. Infrared Millimeter Waves 8,

indicate that the system is performed reasonably as

l 165 0987).

t3l K. Nagasaki et al., Int. J. Infrared Millimeter

designed. The power injected to the Heliotron J vacuum

vessel is up to 400 kW at the nominal gyrotron
operation,530 kW (beam voltage,Vs=75 kV, and beam
current, Ip,=21 A). Following the successful operation ofWaves 20, 823 (1999).

t4l A. Yariv, Quantum Electronics, Wiley, New York,1967.

the 70 GHz ECRH system, we have started the plasma
experiment in Heliotron J. The current-free plasma can[5] L. Rebuffi et al.,lnt. J. Infrared Millimeter Waves

be produced at the second harmonic resonancet6l V. Tribaldos et al.,Plasma Phys. Control. Fusion

r0,29r (1989).

condition, that is. the resonant masnetic field around40,2rr3 (1998).

1.25 T.

336