22P141-P
Design of MIG for 300 GHz High-Power Pulsed Gyrotron
300 GHz 高出力パルスジャイロトロンの実現に向けた電子銃設計
Yuusuke YAMAGUCHI, Yoshinori TATEMATSU, Teruo SAITO, Naoki YAMADA,Ryosuke IKEDA, Jagadish C. MUDIGANTI, Isamu OGAWA and Toshitaka IDEHARA
山口裕資，立松芳典，斉藤輝雄，山田尚輝，池田亮介，Jagadish C. Mudiganti，小川勇，出原敏孝
Research Center for Development of Far-Infrared Region, University of Fukui, Fukui 910-8507, Japan
福井大学 遠赤外領域開発研究センター 〒910-8507 福井市文京3丁目9-1
A 300 GHz pulsed gyrotron with a power exceeding 100 kW has been under development. This paper
describes the design optimization of a magnetron injection gun for this gyrotron. An optimum geometry of
the electrodes is investigated to provide a laminar electron flow with a small velocity spread.

2. Design of new MIG
The oscillation frequency in the cavity is set at
295 GHz with the magnetic field strength of 11.4 T.
In order to achieve the output power of over 100
kW, beam current of more than 10 A is requiredwith the beam voltage of 65 kV. Furthermore, toensure the high quality beam, the velocity
pitch-factor α (the ratio of perpendicular to parallelvelocities to the magnetic field line) of 1.2 and theα spread (defined by the formula below) less than
10 % are set as the target design values.
α spread ≡ (α max - α min ) α average40
1st Anode

2 nd Anode
Magnetic fieldstrength

R [mm]

GNDCavity
Emitter

1. Introduction
Gyrotrons are capable of providing high-powers
at millimeter and sub-millimeter wave lengths, andwidely used in the various fields of physics study
and technological use. Recently, development of ahigh power source in the sub-Terahertz region isexpected for the measurement of collective
Thomson scattering in the fusion experiments. Toachieve simultaneously a higher signal-to-noise
ratio and scattering angles sufficient for good
spatial resolution, an oscillation power of more than
100 kW is required at the frequency range of 300 ~400 GHz [1, 2]. In FIR, Univ. of Fukui, design ofover 100 kW tube is currently in progress. The
oscillation mode of this gyrotron is the TE14,2 at thefundamental harmonic.
In the gyrotrons, formation of a laminar electron
beam with a small velocity spread is essential forrealization of the high power oscillations. In thisstudy, a design of magnetron-injection gun (MIG)
is performed by use of an electron trajectory code
EGUN [3].

040Electron trajectories

Cathode- 65 kV60Fig. 1.

Equi-potential lines

80Z [mm]100v

vv

120140A schematic diagram of MIG.

A triode-type structure is adopted to control thebeam properties without changing the total beamenergy. The MIG parameters were initiallyestimated on the basis of the adiabatic theory. Thedimensions of the cavity are 3.4 mm in radius, and9.0 mm in length. The radius of the electron beam
Rb should be 2.42 mm in the cavity in order tocouple to the TE14,2 mode. The radius of theemitting ring is set to 17.6 mm, from which theguiding center of the electron arrives at the Rb =2.42 mm in the cavity. In this geometrical design,
the magnetic compression ratio between the cavity
and the emitting ring becomes 53.2. Figure 1
represents the schematic drawings of the electrodes
which are used for the numerical computation of
the beam trajectory. The distance between thecathode and first anode of 6.5 mm is derived under
the condition, where the applied voltage between
the cathode and first anode (VKA) is 25 kV andexpected α at the cavity is 1.2. Here, the maximum
value of VKA is restricted up to 30 kV to provide anenough electrical insulation, which depends on thestructures of the electric terminals located on theoutside of the vacuum region.
The design optimization has been conducted bycalculating the beam trajectories, the averaged
value of α (α average) and α spread at the cavity

80

120

F_laminar

0.5
α= 1.2Vaverage

F40laminar[kV], αspread [%]60KAα_spread
V-KA [kV]

016202428
θ [deg.]32036EFig. 2.
Variation of the VKA, the α spread at the cavity
and the degree of the lamination is given as functions ofthe slant angle θ. In the calculations, the averaged valueof α at the cavity is fixed by changing the VKA.

220MIG for 100 kW, 300 GHz Pulsed Gyrotron
α average (65 kV, 10 A)
α average (65 kV, 15 A)
α average (65 kV, 20 A)

151

α10spread[%]1.5
αaveragefor the various shapes of the electrodes. An
example of the beam trajectories calculated with
EGUN is shown in Fig. 1. For the higher beam
current, the space charge has stronger effect on theα spread. To reduce the influence of the spacecharge, formation of a laminar flow becomes
important. The degree of the lamination is strongly
affected by the slant angle θ of the emission surface
to the magnetic field line. The value of θ should belarger than 20 degrees for realizing a well laminatedflow [4]. In Fig. 2, the applied voltage VKA, the αspread at the cavity and the degree of the lamination[5] are plotted as functions of the slant angle θ. In
the calculations of Fig. 2, the α average at thecavity is kept to 1.2 by adjusting the applied voltage
VKA. In Fig. 2, the left side vertical axis corresponds
to the required VKA to obtain the α average of 1.2. It
is clearly shown that θ has an optimum value ataround 28 deg. to suppress the α spread. It has been
also confirmed that the α spread decreases withincreasing the degree of lamination. In addition, theoptimum value of θ depends on the detailed shapesof the first anode and the cathode. Therefore, it issuggested that the formation of a well laminatedflow depends not only on the slant angle θ, but also
on the spatial distribution of the electric field in
between the cathode and the first anode associatedwith the other shapes of the electrodes. The value ofθ is finally determined at 28 deg., and the othershapes of the electrodes have been carefullyoptimized. The sensitivity of the beam quality tosmall deviation of the shapes of the electrodes fromthe designed one is checked.

α spread (65 kV, 10 A)
α spread (65 kV, 15 A)
α spread (65 kV, 20 A)

0.502352425VKA26[kV]27028Fig. 3. Calculated pitch-factor α and α spread for 65 kV,
10-20 A electron gun.

As a result of the design optimization, a good
configuration of the electrodes is obtained with awide operation window. In Fig. 3, α average and α
spread at the cavity are plotted as functions of VKA.
In the parameter range shown in Fig. 3, no mirror
reflection of the electrons occurs. The desired valueof the α average of 1.2 can be realized even at thebeam current larger than 10 A. The α average
decreases with increasing the beam current. The
reason for this is thought to be the voltage
depression in between the cathode and first anode,
because of the space charge. Similarly the α spread
tends to increase with the beam current, and evenwith VKA. In any case, very small α spread less than
5 % has been achieved in the expected operating
region of α average = 1.0 ~ 1.3. Finally, it isconfirmed that the ratio of the beam current density
Jb to the Langmuir current density JL is kept low(Jb/JL < 20 %) [4]. Besides that, it is checked thatthe electric field in the vicinity of the emitting
surface becomes higher than 3 ~ 4 kV/mm to avoid
too strong influence of the space charge forces on
the beam properties [6]. An electron gun which issuitable for the new gyrotron has been successfullydesigned.
References[1] T. Notake et al., Phys. Rev. Lett. 103 (2009) 225002.
[2] T. Saito et al., IRMMW and THz 2010, Th-E1.5[3] W. B. Herrmannsfeldt, Stanford Linear AcceleratorCenter Report, SLAC-331-UC-28 (1998)[4] P. V. Krivosheev et al., Int. J. Infrared and Millimeter
Waves, 22 No. 8 (2001) 1119
[5] Y. Yamaguchi et al., IRMMW and THz 2011, Tu-5.1[6] V. N. Manuilov et al., Int. J. Infrared and Millimeter
Waves, 29 (2008) 1103