22D03
Development of the high power 170GHz gyrotron for ITER
ITER用170GHz大電力ジャイロトロンの開発
Ken Kajiwara, Yasuhisa Oda, Atsushi Kasugai, Koji Takahashi and Keishi Sakamoto
梶原健, 小田靖久, 春日井敦, 高橋幸司, 坂本慶司
Japan Atomic Energy Agency
801-1, Mukoyama, Naka-shi, Ibaraki 311-0193, Japan
日本原子力研究開発機構 〒311̶0193 茨城県那珂市向山801-1
A high power 170GHz gyrotron has been developed in JAEA for ITER. Two recent topics are introduced.
The first is an anode voltage modulation for on/off output RF power with a high-speed switch. Full beam
current suppression by shorting the cathode and the anode electrodes by the switch is tested. 5kHzmodulation is successfully achieved with the power of 1.1MW and 60 sec with full beam current
suppression (up to now). The other is the design and the tests of 170/137GHz dual frequency gyrotron witha triode MIG. In short pulse experiments, the maximum power of more than 1.3MW are achieved withhigh efficiency for both frequencies. For the long pulse experiments, 1MW/5sec and 760kW/60sec for
170GHz are achieved.

1. Introduction
A gyrotron is a high power millimeter wavesource that is applied for electron cyclotron
heating and current drive (EC H&CD) and plasma
breakdown for the fusion devices including ITER.
In ITER, a 5kHz modulation of the gyrotron
system for ECCD is important in order tosuppress a Neo-classical Tearing Mode (NTM). It
is also important frequency tunability of thegyrotorn for the fusion devices, which is effective
to widen a range of plasma heating position
and/or to correspond to the various range ofplasma confinement magnetic field. The progressof these two topics on gyrotron development isintroduced.
Table I. List of the modulation methods.
Mod. type1) Turn on/off
main switch
2) Acceleration
voltagemodulation
3) Anode voltage
modulation (onlyfor the triodeMIG)

AdvantageLow heat load on thegyrotron collector
High-speed
modulation and low
cost of the power
supplyHigh-speed
modulation and low
cost of the power
supplyDisadvantage
Slow modulation
speed (<1kHz)High heat load on
the gyrotron
collector

electron beam is not reduced dramatically when theRF is off. In that period, the electron beam istraveled through the cavity to the collector without
dumping the energy, which increases the heat loadon the collector. By combining the two methods,
ITER plans the type 1) for 1kHz / 0-100% power
modulation and the type 2) or 3) for 5~kHz /50-100% power modulation to protect thecollector[1].
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2. A 5kHz modulation
The gyrotron has a couple of modulation
strategies listed in Table I. As shown in the table,
the type 1) is not suitable to adopt the high speed
modulation, because the current of the main circuit
is several tenth of A, then, heat removal of the main
switch could be issue if the turn on/off frequency is
too high. Contrarily, 2) and 3) need to handle lessthan several tenth of mA. It allows high-speed
modulation of more than 5kHz. However, the"#$%&'(,"'(-!"#$%56.-!"#$+,%')*)*#$!&+#,-./!&$+0$!1,-. ,%-&#$'

72/.'*'(#..%/*#0
89:

High heat load on
the gyrotron
collector

1234)

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89:

Fig. 1 Modulation circuit with anode switch.

An alternative modulation type for the gyrotron
with a triode magnetron injection gun (MIG) is
considered for the high-speed modulation with lowheat load on the collector by using a short-circuitedswitch. The electric field around the emission belton the cathode can be reduced by shorting thecircuit between the anode and the cathodeelectrodes. As a result, the beam current isdecreased as close as zero, which decrease the heatload on the collector. The current between theanode and the cathode for CW operation is less thantenth of mA, therefore, the short-circuited switch
between the anode and the cathode (=anode switch)turn off
phase

0(a)
Vc
VaVb

Ib[A]

403020(b)10402000RF[a.u.]

50

500-40600.3(c)0.2
0.10Freq. [GHz]60170GHz
137GHz1000*note: duty 45% case

20-20

1500

Power[kW]Voltage [kV]turn onphase

suitable pitch factor, which is the advantage for thedual frequency gyrotron. Another frequency of137GHz is selected in order to use 1.853mm
thickness single disk output window. Cavity modes
are TE31,11 and TE25,9 for 170GHz and 137GHz
operation, respectively, which have high mode
conversion efficiency to the RF beam mode withsimilar radiation angles.

Efficiency[%]

can be small compared to the main switch and there
is no problem about the cooling even for the 5kHz
modulation. The power supply circuit including thenewly installed anode switch is shown in Fig. 1.
There is another advantage to suppress theelectron beam during turn off phase. Because theheat load on the collector and the cavity are about
half in this operation, there is a good possibility toincrease the beam current to generate more than
1MW RF power with 1MW CW designed gyrotron.
It is very attractive option for ITER.

170(d)

169
168
167

500

500.1

500.2

500.3

500.4

500.5

Time[ms]

Fig. 2 Typical waveform of 5kHz modulation withanode switch.

Typical waveform of 5kHz modulation
experiments with the anode switch is shown in Fig.
2. The anode voltage (Va) is shaped as rectangular
5kHz waveform with the anode switch turning on
and off (see Fig.2(a)). The switch successfully
shorts between the anode and the cathode within~45µs. The perturbation of the cathode voltage Vc
and the body voltage Vb due to the rectangularmodulation of the Va is acceptable. The ramp up
speed of the Va is about ~20µs. The beam current Ib
follows the Va as shown in Fig.2(b). The Ib goes tozero when the Va equals to the Vc as expected.
Figure 2(c) shows the RF waveform measured by adiode detector and Figure 2(d) shows a time domain
frequency measurement. After the careful
adjustment of the parameters for high power
operation, the output power of 1.1MW/60s wasachieved with total electrical efficiency of 48%.
There was no restriction about the pulse length, soit could be extended if required.
3. A dual frequency gyrotron
A high power dual frequency gyrotron isdesigned and tested. The design is based on
170GHz single frequency gyrotron with a triode
MIG[2]. The triode MIG enables to choose variety
of oscillation modes for different frequencies with

102030

40

50

60

7080

0Ic[A]

Fig. 3 Power and efficiency without a depressedcollector as a function of the beam current.

The power and the oscillation efficiency
measured with repetitive 0.5ms short pulse shots
are shown in Fig.3. More than 30% efficiency
without depressed collector for the beam current of
40-50A is achieved. The maximum power of
1.39MW for 170GHz operation and 1.33MW for
137GHz operation are obtained with beam currentof 72A and 70A, respectively. The measured
frequencies at 30ms are 170.2GHz for 170GHz
operation and 137.1GHz for 137GHz operation. Up
to now, in the long pulse experiments, 1MW/5sec
and 760kW/60sec for 170GHz operation areachieved. The pulse length will be extended and137GHz long pulse experiments will be conductedin near future.
4. Summary
The recent progress of the gyrotron development
for ITER is introduced. A 5kHz modulation byusing the anode switch is demonstrated and the1.1MW/60s is achieved (up to now). This is the first
5kHz modulation experiment with full current
suppression for high power with long pulse. A dual
frequency gyrotron (170/137GHz) with the triode
MIG is successfully designed and tested. The highefficiency operations for both frequencies aredemonstrated.
References[1] J. How and R. Reichle, Eds., ITER 2009 Baseline:
Plant Description (PD).
[2] K. Sakamoto, A. Kasugai, K. Takahashi, R. Minami,
N. Kobayashi, and K. Kajiwara, Nat. Phys., 3 (2007)
411.