24aD29P
高周波ジャイロトロンにおけるエミッタの熱絶縁と電子ビーム特性への影響
Influence of thermal-insulation structures for thermionic cathode onelectron-beam characteristics in a sub-THz gyrotron
山口 裕資，立松 芳典，辻 幸介，平野 泰成，小寺 政輝，笠純，斉藤 輝雄
Y. Yamaguchi, Y. Tatematsu, K. Tsuji, Y. Hirano, M. Kotera, J. Kasa and T. Saito
福井大学 遠赤外領域開発研究センター
Research Center for Development of Far-Infrared Region, University of Fukui
are extracted with a variety of initial velocities. As
the result, the laminarity decreases and Δα isestimated as several tens of percent.
Oscillation tests of FU CW GIA were carriedout with the two MIGs under the same operating
conditions. The output power with MIG-1 wasapproximately 10% higher than that with MIG-2.
Moreover MIG-1 could be stably operated in wider
regions of parameters as compared with those for
MIG-2. At least in a short term operations, MIG-1
showed better characteristics than those of MIG-2.
The potential distribution near the emitter in MIG-2
should be improved by optimizing the electrodeshape in order to decrease the velocity spreads.
25AnodeR [mm]-11.9 kVα ~ 1.3, Δα ~ 3%

Potential
Contour
350 mA

Emitter
Cathode0-100-17.3 kV

10Z [mm]ElectronBeamEmitter
2030Fig. 1 (MIG-1) The thermal insulation gaps are placed
a little bit apart from the emitter.
25

α ~ 1.3, Δα ~ 50%

Anode-11.9 kV

R [mm]

Thermionic cathode is a critical component ofmagnetron injection guns (MIGs) which are widelyused in electron tube devices including highfrequency gyrotrons [1, 2]. The emitter is made ofporous tungsten impregnated with barium
compounds. There are small gaps in order tothermally isolate the emitter from the otherelements of the cathode [3]. In this study, two kindsof MIG cathodes with different gap position
arrangements were fabricated to investigate theeffect of those gaps on electron beam characteristics.
Each cathode was installed in a 0.2 THz gyrotron,
FU CW GIA [4].
Figure 1 and 2 show the two types of MIG
electrodes with different thermal insulation
structures, and show electron-beam trajectories
calculated with a computer code EGUN. In case ofFig. 1 (MIG-1), the thermal insulation gaps areplaced a little bit apart from the emitter which is incontact with the non-emissive elements in bothsides. The surface between two gaps is flat suchthat the electric potential is uniform in the vicinityof the emitter. The EGUN calculations predict
generation of high-quality laminar electron beams
in a wide range of expected operation voltage VK,current IB and velocity pitch-factor α up to -20 kV,
0.5 A and 1.3, respectively. The spreads in thevelocity pitch-factor Δα is no more than 5%.
However, this structure has also disadvantage thatundesired emissions might occur from thehigh-temperature areas other than the emitter owing
to dispersion and adhesion of impregnated bariumfrom the emitter. In such a situation the beam
quality is deteriorated, and electrical breakdown canbe induced between the cathode and anode.
On the other hand, only the emitter is heated tohigh-temperature in case of Fig. 2 (MIG-2). This
structure is more preferable from the view point of
avoiding the undesired emissions and discharges forlong term operations. In addition, the loads to theheater can be significantly reduced. However,
distributions of the electric potential are deformednear both sides of the emitter, from which electrons

350 mA
CathodeGaps for thermal
Insulation-17.3 kV0200-1010Z [mm]Emitter
30Fig. 2 (MIG-2) Only the emitter is isolated from theother parts of the cathode.

References[1] Y. Yamaguchi et al., Phys. Plasmas 19 113113(2012)[2] Y. Yamaguchi et al., Nucl. Fusion 55 013002 (2015)[3] K. T. Nguyen et al., IEEE Trans. on Plasma Science26 799 (1998)[4] Y. Tatematsu et al., J. Infrared Millim. TerahertzWaves 33 292 (2012)