Measurement of RF Transmission Mode in ITER Relevant EC H&CD Transmission Line
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Measurement of RF Transmission Mode in ITER Relevant EC H&CD Transmission Line Yasuhisa Oda & Ken Kajiwara & Koji Takahashi & Atsushi Kasugai & Michael A. Shapiro & Richard J. Temkin & Keishi Sakamoto
Received: 20 November 2009 / Accepted: 4 June 2010 / Published online: 15 June 2010 # Springer Science+Business Media, LLC 2010
Abstract The preliminary results of mode measurement in the ITER relevant 40 m long transmission test line composed from 63.5 mm diameter corrugated waveguides and miter bends are presented. The field patterns were measured by taking temperature profiles on a paper screen placed in front of the waveguide end using an infrared imaging camera. The complex electric field at the waveguide end was retrieved from the measured temperature profiles. As a result, the transmission power includes 87% of HE11 mode and 6% of LP11 odd (HE21+TE01) mode and small ratios of other modes. The mode content had small dependence on length of the transmission line. This result indicated that the higher order mode content generated at the input of the transmission line is conserved and propagated through the transmission line. This suggests that the initial RF coupling to the waveguide is critical since it affects the launcher efficiency. Keywords ITER . Corrugated waveguide . Gyrotron . Mode content
1 Introduction The 170 GHz transmission line (TL) for the electron cyclotron heating and current drive (EC H&CD) system in the International Thermonuclear Experimental Reactor (ITER) requires more than 90% power transmission efficiency from the gyrotrons to the plasma using about 160 m length corrugated waveguides. The dominant loss in TL is caused at miter bends as the ohmic loss and the mode conversion loss. According to J. Doane’s
Y. Oda (*) : K. Kajiwara : K. Takahashi : A. Kasugai : K. Sakamoto Plasma Heating Research Group, Japan Atomic Energy Agency, Naka Fusion Research Institute, 801-1 Mukoyama, Naka, Japan e-mail: [email protected] M. A. Shapiro : R. J. Temkin Plasma Science and Fusion Center, Massachusetts Institute of Technology, 167 Albany St., Cambridge, MA, USA
950
J Infrared Milli Terahz Waves (2010) 31:949–957
theory, a single miter bend generates about 0.4% transmission loss which was caused by mode conversion loss and ohmic loss on the mirror [1]. However many EC H&CD transmission lines in large devices report about 80% of transmission efficiency which is a far smaller efficiency than the theoretical prediction [2, 3]. This is because the RF transmission mode in corrugated waveguides contains higher order modes (HOMs). For high transmission efficiency, the good purity of HE11 mode is necessary because the mode conversion loss increases with increasing content of the higher order modes (HOMs) [4]. The requirement of the HE11 mode purity in the ITER TL is more than 95%. There were several works on measurement of millimeter wave transmission mode in a corrugated waveguide. The transmission mode was estimated from the amplitude and phase measured in low power testing facilities [5]. The measur
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