23P169-B
Stabilization of high frequency gyrotron output
高周波ジャイロトロンの発振出力の安定化
Isamu Ogawa, Ryosuke Ikeda, Motonao Hosokubo and Toshitaka Idehara
小川 勇, 池田亮介, 細窪素直, 出原敏孝
Research Center for Development of Far-Infrared Region, University of Fukui
9-1, Bunkyo 3-Chome, 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番1号
High frequency gyrotrons are the most promising candidates for high power radiation sources in millimeterto submillimeter wavelength range. In some applications, the output of a radiation source needs to be keptconstant for a long time. Gyrotron output power depends on beam current. As the first step, the beam
current was stabilized by feedback control. The fluctuation level of output power has been reduced from
15% to 2% by the feedback control.

1. Introduction
High frequency gyrotrons are characterized bytheir ability to deliver high powers in millimeterto submillimeter wavelength range. For many
applications where more intense radiation isrequired, the gyrotrons are the most promising
candidates for radiation sources in this range ofthe electromagnetic spectrum. Gyrotron FU CW
Series [1] were developed as high power CW
radiation sources in our research center FIR FU.
In some applications, the output of a radiation
source needs to be kept constant for a long time.
Gyrotron output power depends on beam current
and voltage between cathode and anode. Ripplesof cathode and anode voltage produce thefluctuation of gyrotron output power. The
fluctuation of the output power in a short timeinterval (~several seconds) was decreased from
4 % to 1 % by introducing the smoothing circuit
consisting of a resistor, an induction coil and acapacitor [2]. The change of beam current brings
about slow fluctuation of the output power. It isnecessary to remove this beam current fluctuation
for a long time stabilization of the output power.
The fluctuation of the beam current is effectively
adjusted by changing heater voltage. The
fluctuation level of output power has been
reduced by the feedback control to adjust beam
current.
2. Gyrotron FU CW III
Gyrotron FU CW III [3] is one of gyrotron
device belongs to Gyrotron FU CW Series. Figure 1
shows the cross section and the side view ofGyrotron FU CW III. The gyrotron device consistsof a 20 T superconducting magnet, three gun coils
and gyrotron tube. The gyrotron tube consists of atriode electron gun, a sapphire window and a cavity.
The installed cavity has the same dimensions as thatof the THz pulse gyrotron [4]. The cavity is asimple cylinder and that of the radius and the length
are 1.95 mm and 10 mm (Fig.2), respectively. The
designed cavity mode is TE4,12. The frequency is1.013 THz and operation is at second harmonicoperation.
Fig. 1. Gyrotron FU CW III

Fig. 2. The shape of cavity.

3. Experimental results
The experimental test was carried out under thefollowing conditions: TE03 mode, f=248.72 GHz,
P~20 W, magnetic field intensity in the cavity

region B0 =9.05 T, cathode potential Vk=-17.6 kV,
anode potential Va=-13.95 kV. The gyrotron device
is connected with cathode and anode power
supplies (Fig. 3). The beam current is adjusted bychanging heater voltage (Fig. 4).
The beam current gradually increases anddecreases with the time (the upper trace of Fig. 5).
The beam current fluctuates up to 17% in the timeinterval of 4000 s. This fluctuation level isdecreased to 0.5% by the feedback control to adjust
heater voltage (the lower trace of Fig. 5).

The output power also changes with the time (theupper trace of Fig. 6). The fluctuation level of
output power is about 15%. By the feedback
control of the beam, the fluctuation level of output
power is decreased to 5%. If the gyrotron output
power depends on only the beam current, more
effective stabilization of output power will berealized to the level of 0.5%. This result hassuggested that not only change of beam current but
also other causes bring about change of output
power. This is understood from more effective
stabilization up to 2% being attained by carryingout feedback control to adjust heater voltage of theoutput power (the lower trace of Fig. 6).

Fig. 5. The time evolutions of beam current.
Upper trace corresponds to free running ofgyrotron operation. Lower trace corresponds tofeedback control.

Fig. 3. Connection of power supplies with gyrotron
device.

Fig. 6. The time evolutions of gyrotron output power.

4. Conclusion.
The fluctuation level of output power has been
reduced from 15% to 2% by the feedback control of
the output power to adjust heater voltage.

Fig. 4. Beam current as a function of heater voltage.

References[1] T. Idehara et al., Thin Solid Films 519, 1503-1506(2008).
[2] I.Ogawa et al., Journal of Plasma and Fusion
Research Series, 5, 205-209 (2002)[3] T. Idehara et al., J. Plasma Fusion Res. SERIES, 8,1508-15118 (2009).
[4] La Agusu et al., J. Infrared and Millimeter Waves,28, 315-328 (2007).