THE DESIGNS OF HIGH EFFICIENCY LAUNCHER OF QUASI-OPTICAL MODE CONVERTER FOR HIGH POWER GYROTRONS
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THE DESIGNS OF HIGH EFFICIENCY LAUNCHER OF QUASI-OPTICAL MODE CONVERTER FOR HIGH POWER GYROTRONS R. Minami, A. Kasugai, K. Takahashi, N. Kobayashi, Y. Mitsunaka1 and K. Sakamoto Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, Ibaraki, Japan 1 Electron Tube Engineering Department, Toshiba Electron Tubes & Devices Co., LTD, Tochigi, Japan Received 5 August 2005 ABSTRACT A high efficiency launcher of quasi-optical (QO) mode converters for high power gyrotrons have been designed and tested. A helical cut launcher radiates the RF power via its straight cut onto the first phase correcting mirror. The launchers have been optimized for the TE31,8 mode at 170 GHz and TE22,6 mode at 110 GHz by numerically optimizing a launcher surface. The helical cut of the launcher has been optimized by taking the taper angle into account. Further more, the amplitude of the surface perturbation have been optimized for improved focusing in order to reduce the diffraction losses at the helical cut. Low power measurement shows a good agreement with the design. High efficiency characteristics of the design have also been calculated on the assumption of frequency downshift due to the thermal expansion of the cavity and stepwise frequency tuning by changing the operating mode. Besides, the possibility of high efficiency launcher for higher mode is discussed, and these results give the prospect to high efficiency long pulse gyrotrons.
Keywords: mode converter, gyrotron, high order mode 13
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1. INTRODUCTION A gyrotron is well known as an excellent high power source of millimeter wave, which is commonly used for plasma heating and current drive of fusion devices. In International Thermonuclear Experimental Reactor (ITER), high RF power source with total output power of 24 MW at 170 GHz has been planned for electron cyclotron heating (ECH), current drive (ECCD), suppression of plasma instabilities and start up of plasma. A 170 GHz gyrotron with an output power of 1 MW, continuous wave (CW) operation and 50% of total power efficiency has been developed to satisfy the requirements of the ITER1-3. For modern gyrotrons, a quasi-optical (QO) mode converter arrangement is adopted to convert initial oscillation mode in a cavity to the Gaussian mode, and the RF beam is extracted through a diamond window. On the other hand, stray radiation in the gyrotron due to diffraction loss of RF beam extracted from the QO mode converter disturbs long pulse operation, because it causes temperature increase of internal components and/or breakdown. Therefore, the diffraction loss from the QO mode converter should be suppressed as much as possible. Up to now, a dimple wall type launcher is widely used4. The high efficiency QO mode converter arrangement was designed and tested at a high power 140 GHz gyrotron5. A numerical algorithm surface integral analysis of the launchers of QO mode converter was developed in consideration of edge diffraction losses with magnetic field integral equation (MFIE) approach (Surf3D code) by Nei
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