J. Plasma Fusion Res. SERIES, Vol.3 (2000) 407410

Development of Frequency Tunable Gyrotrons in Millimeterto submillimeter wave Range for plasma Diagnostics
IDEHARA Toshitaka*, MITSUDO Seitaro, UI Masahiro, OGAWA Isamur),
SATO Motoyasu2) and KAWAHATA Kazuo2)
Research Center for Development of Far-lnfrared Region, Fukui University

Fukui 910-8507, Japanof Engineering, Fukui University
Fukui 910-8507, JapanI)Cryogenic Laboratory, Faculty2)National Instituiefor Fusion Science322-6, Oroshi-cho, Toki 509-5292, Japan(Received: 19 January 2000 / Accepted: 19 May 2000)

Abstract
High frequency, frequency tunable, medium power gyrotrons (Gyrotron FU series) are being
developed in Fukui University as millimeter to submillimeter wave sources. The gyrotron series hasachieved frequency tunability in a wide range from 38 to 889 GHz and medium output power from 0.1

kW to several kW. For the application to plasma diagnostics, modulations and stabilizations of both
amplitude and frequency of their outputs have been achieved. The present status of the Gyrotron FU
series is described. In addition, the development of a frequency tunable, high power, quasi-opticalgyrotron for plasma diagnostics is also presented briefly.

Keywords:
gyrotron, millimeter wave, submillimeter wave, frequency-tunable, plasma diagnostics

1. lntroduction

technologies, for example, ceramic sintering. The

developed. Similar submillimeter wave gyrotron
developments are advancing in Institute of Applied
Physics in Russia [] and in Massachusetts Institute ofTechnology in USA [2]. 'Gyrotron FU series' consistingof 8 gyrotrons has many advantages which have notbeen achieved by other gyrotrons, that is, frequency
tunability from 38 GHz to 889 GHz, high harmonic

second direction is the development of high frequency,
medium power gyrotrons as millimeter to submillimeter

operations up to the fourth harmonics, studies of mode
competition and mode cooperation, high purity mode

wave sources for plasma scattering measurements,
electron spin resonance (ESR) experiments and so on.
Gyrotrons developed in Fukui University belong tofrequency step switching, complete cw operation for a

The development of gyrotrons is proceeding in twodirections. One is the development of high power,
millimeter wave gyrotrons as the power sources forelectron cyclotron heating of plasmas and electron
cyclotron current drive of tokamaks and for industrial

operation, frequency and amplitude modulations,the second group. High frequency, medium power

long time (- 15 hours), high stabilization of theamplitude by feed back control of anode voltage of

gyrotrons covering a broad frequency band in millimeterelectron gun and so on.

and submillimeter wavelength regions have been*

Corresponding author's e-mail: idehara@ maxwell.apphy.fukui-u. ac.jp407@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchIdehara T. et al., Development of Frequency Tunable Gyrotrons in Millimeter to Submillimeter Wave Range for Plasma

Diagnostics

Gyrotron FU IVA has achieved the highest frequency of889 GHz for single mode operation on the TEs,6,1 cavity

A quasi optical gyrotron for plasma diagnostics isalso being developed. The frequency tunabilities in twobands near 92 GHz (fundamental operation) and 184

mode at the second harmonic (f = 2f,). TheGHz (second harmonic operations) will be achieved.
These works are being advanced under cooperations

corresponding wavelength is 337 pm.

with University of Sydney, Stuttgart University,(b) Amplitude modulation 16,71
Gyrotrons FU III and IV have achieved amplitude

Karlsruhe Research Center and National Institute formodulation of their outputs. A modulation of the anode

Fusion Science in Japan.
This paper summarizes achievements of 'Gyrotron

voltage % will modulate the velocity distributionfunction of beam electrons, which, in its turn, will
modulate the gyrotron output. In the experiment, thegyrotron (Gyrotron FU III) operates on the second

FU series' and present status of the quasi-opticalgyrotron development.

2. Gyrotron FU Series
Gyrotrons included in Gyrotron FU series areharmonics of the cyclotron frequency. The cavity mode

frequency step-tunable sources covering a wide
wavelength range from millimeter to submillimeter

power is about 300 W. The modulating frequency is 5
kHz and modulation mode is sinusoidal wave. The

wave region. The output powers are not so high, that is,from several hundreds watt to several tens kilowatt for
fundamental operation and from several tens watt to
several kilowatt for second harmonic operation. The

modulation rate APou,lPo,t of gyrotron output increases
with the modulation rcte AVolVo of the anode voltage.
The 100 percent modulation of the output (APo,,lPour=1.0) is attained, when AV"lVo is only several percent(AV"/V" - 0.055). The modulation rate APou,lPo,, ofoutput power is almost linearly proportional to AVolVo.
This means the sinusoidal modulation of output power ispossible by the sinusoidal modulation of anode voltage.

is TE1.6.', the frequency is 444 GHz, and the output

main results are summarized as following.

(a) Frequency tunabitity in broad band from38 GHz to 889 GHz 13-51

Sinusoidal modulation of Pou' at the modulation
frequency up to 600 kHz has been achieved with theAll frequencies achieved up to now by 5 gyrotrons
included in Gyrotron FU series are summarized asfunctions of field intensity Bs in Fig. l. Solid lines
represent the fundamental Qf = f,), the second and thelow AVolVo value of l.l x l0-3.
The amplitude modulation efficiency (APodlPou)l(AV./V,) is estimated as a function of c and Po6,/Po,6
where Po1, is the ohmic power I oss in a cavity and athe pitch angle of beam electrons. The theoreticalprediction for the efficiency is derived from the energy
transfer function between electrons and electromagnetic
wave, as follows,

third harmonic U = 2fn 3/") resonances. Frequency steptunability from 38 to 889 GHz is achieved by thefundamental, second and third harmonic operations.

(APoutlPou)/(AV,lV) = (l + Po6.lPou, )(3u2 + 4) .
The experimentally obtained efficiency becomes close

to the theoretical prediction 3d + 4, when 16 is in-hcreased so highly that Pou, is high enough and the termgsm

Po1,*lPo6 can be neglected.

(c) Frequency modulation aSlGyrotron FU IV has achieved the frequency
modulation within the limit of resonance frequency

051015width of a cavity mode. The mechanism is as follows.
The energy ofbeam electrons is modulated by variationin the body potential. The body includes the cavity andis separated electrically from the beam collector by aceramic insulator. Therefore. the electron cvclotron

Magreticfield ft@

Fig. 1 All frequencies achieved up to now by Gyrotron
FU series as functions of field intensity 8o. Solidcurves show the fundamental, the second
harmonic and the third harmonic resonances.
408

Idehara T. et al., Development of Frequency Tunable Gyrotrons in Millimeter to Submillimeter Wave Range for Plasma Diagnostics

Table 1 The operation parameters of the quasi optical gyrotron

Frequency ranges

Output power

Efficiencies
Maximum field
Electron beam parameter

80-100 GHz (fundamental operations)160-200 GHz (second harmonic operations)-100 kW (fundamental operations)-50 kW (second harmonic operations)20-30 7o (fundamental operations)10-20 7o (second harmonic operations)
4.5 T
70 kV, 10 A, I msec (first stage)
80 kV, 20 A, 10 msec (final stage)

frequency is modulated by the changing of electronmass and, as the results, a frequency modulation takes

for measurement of frequency modulation. Theplace.

several MHz and 6flf are of the order of l0-5 for several
cavity modes. The output power variations measured by
Schottky diode during ten minutes are several percent.
These variations come from the variations of anode andcathode voltages of electron gun. We are planning firstto stabilize both voltages. Then, we will try feed backmeasured frequency variations 6/during 100 msec are

The output power is transmitted by circular
waveguides and emitted to a horn antenna. The
frequencies measured by a heterodyne detection system
consisting of a sweep oscillator, a frequency counter, aharmonic mixer and a modulation domain analyzer. The
detected signal (f) is mixed with a high harmonic of thecontrol of them to achieve further high quality

local oscillator and converted to the low frequency
signal (fp). The time and frequency resolutions of thedetection system are 10 msec and l0 kHz, respectively.

stabilization.

In the preliminary experiment for amplitude
stabilization by the feed-back control of the anodevoltage of electron gun, the fluctuation level of the

In the experiment, the frequency modulation
amplitude /,f versus the body potential modulation

amplitude was decreased lower than 0.1 percent.

amplitude dV6 is observed for several values of themodulation frequency J". There is an almost linear
dependence between Af and AV6 for all values offi. The
efficiency of frequency modulation AflAVb= 0.247
MHzlY. The efficiency estimated by the computer
simulation is distributed close to the experimentallyobtained values. The submillimeter wave gyrotron isused as a radiation source for plasma diagnostics andESR experiment in our laboratory. The advantages offrequency and amplitude modulations will be useful in

3. Ouasi Optical Gyrotron
A quasi optical gyrotron is being developed by thecollaboration between National Institute for Fusion
Science and Fukui University. In the gyrotron, FabryPerot resonator is installed. It enables frequency tuning

in wide ranges near 90 GHz and 180 GHz. The
operation parameters of the gyrotron are listed in Tablel. The output powers obtained by computer simulations
are around 100 kW for fundamental operations and 50
kW for second harmonic ones.
Frequency tunability of such a high power gyrotron
will be useful for application to plasma diagnostics andhigh power spectroscopies in wide fields.

these applications.

(dl Complete cw operation with highstabilities of amplitude and frequency
One of the advantages of complete cw operation isstabilization of the frequency and the amplitude of thegyrotron output. The longest period of the operation
which our gyrotron series has achieved was 15 hours.
This means 'complete cw'. We employ a currentstabilized high voltage power supply in our experiment
to ensure stable operation. A variation in the output

4. Conclusion
We have developed frequency tunable, submillime-

ter wave gyrotrons (Gyrotron FU series) with medium
power and a quasi optical millimeter wave gyrotron withhigh power for plasma diagnostics and millimeter tosubmillimeter wave spectroscopy. The former gyrotrons

frequency was measured by a time-resolved frequency
measurement system which is the same as the one usedhave achieved frequency and amplitude modulations andhigh stability of the output power.
409

Idehara T. et al., Development of Frequency Tunable Gyrotrons in Millimeter to Submillimeter Wave Range for Plasma Diagnostics

meter Waves, 19,919 (1998).

The fluctuation level of output powers is now lower
than 0.1 percent. These advantages of the gyrotron
series enable us to apply them to high power, millimeter

I4l T. Idehara, Y. Shimizu, K. Ichikawa, S. Makino, K.
Shibutani, T. Tatsukawa, I. Ogawa, Y. Okazaki andT. Okamoto, Phys. Plasmas,2,3246 (1995).
tsl Y. Shimizu, S. Makino, K. Ichikawa, T. Kanemaki,to submillimeter wave spectroscopies in wide fields. Thegyrotrons are used for measurement of LHD in National
Institute for Fusion Science in near future.

I. Ogawa, T. Tatsukawa, T. Idehara, Phys. Plasmas,

2,2rr0 (199s).
t6l T. Idehara. Y. Shimizu. S. Makino. K. Ichikawa. T.

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