22P146-P
Development of a Millimeter-Wave Beam Profile Monitor
in Mega-Watt CW ECH Transmission Line
メガワット定常ミリ波伝送系におけるミリ波強度分布モニターの開発
Takashi Shimozuma, Sakuji Kobayashi, Satoshi Ito, Yasuhiko Ito, Shin Kubo, Yasuo Yoshimura,
Hiroe Igami, Hiromi Takahashi, Masaki Nishiura, Yoshinori Mizuno, Kohta Okada,Yasuyuki Takita and Takashi Mutoh
下妻 隆，小林 策治，伊藤 哲，伊藤 康彦，久保 伸，吉村 泰夫，伊神 弘恵，
高橋 裕己，西浦 正樹，水野 嘉識，岡田 宏太，多喜田 泰幸，武藤 敬
National Institute for Fusion Science322-6 Oroshi-Cho, Toki-City, Gifu 509-5292, Japan核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6

In an ECRH system for magnetic confinement devices, over 1 MW/ CW millimeter-wave istransmitted through an over-sized corrugated waveguide in vacuum. Precise alignment between amillimeter-wave beam and waveguide axis is required to improve transmission efficiency. For that
purpose, the profile of the propagating millimeter-wave in the evacuated corrugated wave-guide
should be measured even during an actual high power operation. We have been developing amillimeter-wave beam position and profile monitor (BPM) that consists of a Peltier-device arrayinstalled in atmospheric-side of a miter-bend reflector and a heat sink. A proto-type BPM wasdesigned, assembled and high-power tested. The voltage profile of the devices well coincided with thetemperature profile of the reflector measured by an infrared camera. A new BPM for higher spatial
resolution was constructed and is under test.

1. Introduction
The Large Helical Device (LHD), which is thelargest Heliotron-type plasma confinement device
in the world (Major radius 3.9 m and average minor
radius 0.6 m), confines high-temperature andhigh-density plasmas by the external magnetic field.
Electron Cyclotron Resonance Heating (ECRH)
using high power millimeter-waves (mmw) is oneof the main plasma production and heating methodsin LHD.
We have been developing and installing threehigh-power 77 GHz gyrotrons under collaboration
with University of Tsukuba. The output power of
each gyrotron has been improved from 1 MW to 1.5
MW step by step, and the gyrotrons also achieved
0.3MW CW operation [1].
It is important to precisely align a propagating
mmw-beam to a transmission line to avoid mode
conversion to the other modes. We have beendeveloping a real-time beam-position and profilemonitor (BPM) to measure the intensity profile of ahigh power (Megawatt level) mmw propagatingeven in the evacuated corrugated waveguidewithout any disturbances. It was improved to obtainhigher spatial resolution and is under test.
2. Structure of a Millimeter-Wave Beam
Position and Profile Monitor (BPM)

In the long-distance transmission lines, thereliable gyrotron operation can be much improvedby the evacuation, sufficient cooling and precisealignment of the system. A real-time beam-positionand -profile monitor is required to evaluate theposition and profile of a high power (Megawattlevel) mmw propagating even in the evacuatedcorrugated waveguide. Two-dimensional array of
Peltier devices is installed and aligned on theatmospheric side of a thin miter-bend reflector, as
schematically shown in Fig. 1. A mmw-beampropagating in the corrugated waveguide isreflected on the mirror surface of the miter-bendand partly absorbed in the reflector plate. TheFig. 1 Schematic diagram of a mmw beam position
monitor.

generated heat by the Ohmic loss diffuses to theoutside of the reflector and removed by the Peltierdevices. A voltage of each Peltier device isapproximately expressed as a following equation.

V = IR + S(TH − Tc ) ,€(1)

where I, R and S are a current, resistance andSeebeck coefficient, respectively. TH and TC
represent hot- and cold-side temperature of thePeltier device. When these devices are connected
serially and driven by the constant current control(I=constant), the voltage change of each device isalmost linearly proportional to the temperaturechange of the cooled side of the device, if thetemperature at the hot-side of the Peltier device iskept constant. The information of thetwo-dimensional temperature profile of themiterbend reflector can give the real-time
information of the position and profile of the mmw
beam.
3. High Power Test results of a Proto-Type BPM
and Its Upgrade
A prototype beam profile monitor, which
consisted of 20 Peltier devices with a heat sink, wasassembled and tested [2]. The equipment wasinstalled in one of the miter-bends in the LHD
transmission line, which is connected to an 82.7
GHz gyrotron. Another side of the miter-bend wasconnected to a dummy load. High power test wasperformed using 82.7 GHz power with about 200
kW and 100 ms pulse every 30 s. The temperature
increase of the reflector measured by an RTD wasabout 0.5 degree around the center. Figure 2 a)shows the voltage change normalized by the initial
voltage of each Peltier device. Each quadrate in thefigure corresponds to the device position. The
ellipse indicated by a white dotted line shows thecross section of the corrugated waveguide at themiter-bend. In this case, the peak of the voltagechange is slightly off-center. After the test, we tried
to obtain a thermal image using an absorber-coatedreflector and IR camera at the same position. Figure2 b) shows the obtained thermal image. The peaktemperature position and profile well coincide withthe position with the highest voltage change shownin Fig.2 a).
For higher spatial resolution, 52 Peltier devices
with 10 mm square are now aligned on a circuitboard and the voltage of each Peltier device can bemeasured at the same moment (Fig. 3). A heat sinkis cooled by water to remove the heat generated bymega-watt, CW millimeter-waves. This device will
be installed in the high power transmission line ofECRH system and will be tested.

Fig. 3 Hot side of Peltier-device (52 devices) array on abackside of a reflector

This work was supported by National Institutefor Fusion Science under grants NIFS10ULRR002
UJRR001, NIFS11ULRR701 and is also supported
by a grant for scientific research from the Ministryof Education, Science and Culture of Japan(21560058).

Fig. 2 Comparison between the output of the beam
position monitor in a) and measured IR imageof the reflector in b) .

References[1] T. Shimozuma, S. Kubo, et al., Journal ofMicrowave Power & Electromagnetic Energy,
43(2009) pp.60 - 70.
[2] T. Shimozuma, S. Kubo, Y. Yoshimura, et al., Proc.
on IRMMW & THz conference 2010, Tu-P.04.