22P143-P
Improved design of 28 GHz Mega-Watt Gyrotron
28GHz-MWジャイロトロンの改良設計
Tsuyoshi Kariya1, Ryutaro Minami1, Tsuyoshi Imai1, Tomoharu Numakura1,Hidetaka Nakabayashi1, Taku Eguchi1, Yoshika Mitsunaka2, Keishi Sakamoto3, Hiroshi Idei4
假家 強1, 南龍太郎1, 今井 剛1, 沼倉友晴1,中林英隆1, 江口 濯1, 満仲義加2, 坂本慶司3, 出射 浩4
1

Plasma Research Center, University of Tsukuba, 1-1-1, Tennoudai, Tsukuba, Ibaraki, 305-8577, Japan
Toshiba Electron Tube & Devices Co.,Ltd., 1385, Shimoishigami, Otawara, Tochigi, 324-8550, Japan3Japan Atomic Energy Agency, Naka Fusion Institute, 801-1, Mukouyama, Naka, Ibaraki, 311-0193, Japan
4
Kyushu University, Research Institute for Applied Mechanics, 6-1,Kasuga-koen, Kasuga, Fukuoka, 816-8580, Japan1
筑波大学プラズマ研究センター 〒305-8577 つくば市天王台 1-1-1
2東芝電子管デバイス株式会社 〒324-8550 大田原市下石上 1385
3
日本原子力研究開発機構 那珂核融合研究所 〒311-0193 那珂市向山 801-1
4
九州大学 応用力学研究所 〒816-8580 春日市春日公園 6-1
2

At Plasma Research Center (PRC) in University of Tsukuba, a 28 GHz 1 MW gyrotron has been developed
to upgrade the electron cyclotron heating (ECH) systems of GAMMA10 tandem mirror. In consideration ofthe first tube test results, the design improvement for the second tube is being performed for the higherefficiency, the higher power and the longer operation. This 28 GHz improved gyrotron design aims tosatisfy the design specification of the gyrotrons for the ECH system of QUEST in Kyushu University or
NSTX in Princeton Plasma Physics Laboratory (PPPL), too.

1. Introduction
A 28 GHz 1 MW 1 sec. gyrotron with TE8,3
cavity has been developed to upgrade the ECHsystems of GAMMA10. The picture and thestructural cross-section of the first tube are shownin Fig.1. In the initial experiment with the shortpulse, the maximum power of 1.05 MW wasobtained, which is in agreement with its design
target value. And the high efficiency of 40 %without collector potential depression (CPD) wasobtained with 0.8 MW [1]. In the long pulse test,
the pulse length extended to 2 sec. with 0.45 MW
which was limited by the power supply and thewater dummy load [2].
In these days, 28 GHz gyrotrons are required insome plasma experimental devices. For the ECH

Fig.1. Picture and structural cross-section of28 GHz 1 MW gyrotron.

system of QUEST in Kyushu University, a 0.4 MW
CW gyrotron is needed. A few sec. with 1～2 MW
gyrotron is useful for Electron Bernstein wave
(EBW) heating system of NSTX in PPPL.
In considerations of the first tube test results andthe requirements for 28 GHz gyrotrons, the design
improvement for the second tube is being
performed for the higher efficiency, the higher
power and the longer operation. The first results of
the improved design of 28 GHz gyrotron arepresented.
2. Improved Design of 28 GHz Gyrotron
Each component design of first tube is examined,as the design targets are 1.5 MW 2 sec. and 0.4MW
CW operations.
2.1 Magnetron Injection Gun (MIG)
The experiment of the first tube were performedby using three SCM for NIFS 77 GHz #3 gyrotron,#1 gyrotron and PRC 28 GHz gyrotron in 2009,2010 and 2011 respectively. The different saturation
tendencies of the output power at higher beam
current were observed in each SCM experiments [2].
It is thought that these differences are caused by thesmall difference of the magnetic field profile at thegun region of the each SCM and adjustment
between gyrotron and SCM. The first tube MIG hassame cathode structure with the NIFS 77 GHz 1.5

MW #3 gyrotron to get compatibility. Therefore thelaminar flows of the electron beam in front of thecathode ware not necessarily good.
The improvement point of MIG design is that thecathode angle has been made deeper for laminarflow of electron beam in front of the cathode. Anelectron beam trajectories calculated by MIG code
are shown in Fig.2. The upper figure is first tube
design and the lower figure is improved one,respectively. As shown in Fig2, the laminar flow ofthe electron beam in front of the cathode isimproved. A good anode voltage dependences ofthe electron beam parameter is obtained by MIG
simulation, that is the pitch factor α are 1.1~1.2
with small α spread (<5%). The higher α operation
with the lower α dispersion leading to the higher
oscillation efficiency at the higher beam current
will be expected by these improvements.

Fig.2. Electron beam trajectories of first tube MIG(upper) and improved MIG design (lower).

2.2 Cavity
In the cavity calculation results of the first tube,
an oscillation power of 1.44 MW with theoscillation efficiency of 45.1% was obtained atbeam voltage Vk=80 kV, beam current Ik=40 A and
α=1.2. In the improved design tube, the cavity
design is the same as the first tube cavity, because
oscillation power is not saturating at Ik=40 A andhigher power will be obtained at the higher Ik by
MIG improvement.
2.3 Radiator and Mirrors
The TE8,3 mode RF wave is converted to a
Gaussian-like beam by a built-in quasi-opticalmode converter, and the RF beam is transmitted byfour pieces of mirror system to the outside of thetube through a output window. The output RF beamis adjusted its profile and phase by Matching Optics
Unit(MOU), and couples to a corrugated waveguideas HE11 mode. The total transmission efficiency
from the mode converter to the output window is94.7% and to the corrugated waveguide is 90.2% in
the first tube. The improved designs that reduce theside loves of RF beam launched from the radiatorand increase the transmission efficiency of themirrors is being performed now with the target of
total transmission efficiency over 95%.
2.4 Output Window
The output window of the first tube is a single
disk sapphire window with the diameter of 136 mm.
The temperature raise of the output window
measured by IR camera was 9 K with 0.45 MW 2
sec.[2]. From this result, the dielectric loss (tanδ) ofsapphire is estimated to be 3.3×10-5 at 28 GHz. The
RF pulse width dependences of the window
temperature calculated with this tanδ considering
the experiment and the temperature dependence oftanδ [3] are shown in Fig.3. As shown in Fig.3(a),the operation of 1.5 MW 2 sec. is possible by single
disk sapphire window, but the operation of 0.4 MW
CW isn’t possible. As shown in Fig3.(b), theoperation of 0.4 MW CW is possible by double disk
sapphire window with the heat transfer rate h>0.03
W/cm2K..

(a)(b)
Fig.3. RF pulse width dependences of the window
temperature. (a) Single disk, (b) Duble disk.

2.5 Other Components
Other important points of the design
improvement for second tube are the adoption ofCPD type collector to reduce heat load of the spentelectron beam and the power supply requirement,
the cooling reinforcement against the heating bystray RF and the ion pump enlargement for exhaustof outgassing.
Acknowledgments
The author thanks the members of the NIFS ECHgroup and GAMMA10 group for their collaborationand valuable discussion during this study. This
work is partially supported by NIFS Collaborative
program (NIFS11KUGM050, NIFS11KUGM053).
References[1] T. Kariya, et al.: J. Infrared Millim. Waves 32 (2011)295.
[2] H. Nakabayashi, et al.: Plasma Conference 2011
(2011) 22P144-P.
[3] A. Kasugai, et al.: Jpn. J. Appl. Phys. 36 (1997)
2883.