J. Plasma Fusion Res. SERIES, Vol.5 (2002) 205-209

High Ouality Operation of a Submillimeter Wave Gyrotron
for Plasma Diagnostics Application
OG,\WA Isamu, IDEHARA Toshitakal, OKADA Tomokazu, MAEDA Satoshi, IWATA Yosuke,
PAVLICHENKO Rostyslavl, MITSUDO Seitaror, WAGNER Dietmar2 and THUMM Manfred3
Faculty of Engineering, Fukui University, Fukui 910-8507, Japan
I Research Center
for Development of Far-Infrared Region,Fukui University, Fukui 910-8507, Japan2
Max-Planck-Institut fiir Plasmaphysik,Boltzmannstrasse 2, D-85748 Garching, Germany'' Institut fiir Hochleistungsimpuls- und Mikrowellentechnik, Forschungsryntrum Karlsruhe,P.O. Box 3640, D-76021 Karlsruhe, Germany(Received: ll December 2OOl / Accepted: 28 August 2002)

Abstract
Thr: outputs (cw operation,,f = 301 GHz, P - 20 W) of the Gyrotron FU IV are stabilized (MlP-|Vo, A,f - 50 kHz) by decreasing the fluctuations of the potentials (Ayk - 0.6 V, A% - 0.2 V), via
introducing smoothing circuits consisting of a resistor, an induction coil and a capacitor. Furtherstabilization of output frequency (A/- I kHz) is attained by applying phase lock loop. Gyrotron FU VA
can produce high mode purity outputs. The gyrotron has a nonlinear cavity which is designed using ascattering matrix code and exquisitely fabricated.

Keywords:
submilhmeter wave gyrotron, high power source, stabilization, phase lock, high mode purity, radiation
pattern

1. Introductionof their capacity to deliver high powers [5,6]. We havealready carried out plasma scattering measurements
using a high frequency gyrotron (Gyrotron FU II) [7]. Itturns out that the stabilization of gyrotron output andhigh quality probe beam are required to improve theperformance of the measurement.
In order to apply the gyrotron to plasma scattering
measurement, we have stabilized the output (P = 20 W,f = 301 GHz) of gyrotron up to the level (APIP < | Vo,
A/ < l0 kHz) established by molecular vapor lasers.

Plasma scattering measurement is effective
technique to observe low frequency density fluctuations(f < | MHz, k - 10 cm-l) excited in plasma. The spatialand wave nunrber resolutions and the S/N ratio ofmeasurement depend on the wavelength range, the size

and the intensity of a probe beam. A well-collimated,submillimeter wave beam to offer small scattering
volume and relatively large scattering angle is suitablefor improving l.he spatial and wave number resolutions.
Up to now, molecular vapor lasers [1,2] and backwardwave oscillator:s [3,4] have been used as the principal
power sources. However, their output powers are lower

The gyrotron output can be stabilized by decreasing thefluctuations of the cathode potential and the anode
potential. Because a gyrotron is characterized by voltage

than 0.5 W and 10 mW, respectively.

controlled oscillator [8], a high frequency stability canApplication of high frequency gyrotron is effectivein improving the S/N ratio of the measurement because

be attained by introducing phase lock loop.
@2O02 by The Japan Society of Plasma

Corresponding author's e -mail : ogawa@ maxwell.apphy.fukui-u.ac.jp
205

Science and Nuclear Fusion ResearchOgawa I. et al., High Quality Operation of a Submillimeter Wave Gyrotron for Plasma Diagnostics ApplicationUnlike a molecular vapor laser, a gyrotronsensitivity, we introduce an amplifier between a phaselock module for gunn oscillator and a control electrode(Fig. 3). The phase lock signal is fed back to the bodyelectrode of the gyrotron across a load resister of I kQ.
The body electrode is connected with cavity region andits potential is close to grounding potential. Frequency

generates spreading radiation with TE., mode structure.

It is therefore necessary to convert the output radiation
into a Gaussian beam (TEM00 mode), which is suitable

for an effective transmission and a well-collimatedprobe beam. High purity mode operation is convenient

for effectively converting the output radiation into theGaussian beam (TEMes mode). We have constructed a(alnew gyrotron, Gyrotron FU VA, which has a carefully

2lo'2

designed resonant cavity developed in collaboration
with the Max-Planck Institute for Plasma Physics andthe Institute for High Power Pulse and Microwave
Technology at the Research Center (FZK) Karlsruhe.

d 0l0'-2 l0'22. High Stabilization of the Gyrotron Output
We have achieved cw operation of thesubmillimeter wave gyrotron, Gyrotron FU IV. This0too2loo41ooTime t (ms)2 101gyrotron consists of sealed-off tube and magnet system.
The superconducting magnet produces the main field in-the cavity region whose intensity can be raised up to12 T. Three subsidiary copper coils are used in theelectron gun region. Both fields can be adjusted
independently to control the formation of the electron
beam produced by the Magnetron Injection Gun (MIG).
The cw operation (TEo, mode,/= 301 GHz, P = 20 W)
is obtained under the following conditions: magnetic
field intensity in the cavity region Bs = 10.8 T, cathodepotential Vr = -16 kV, anode potential V^ = -2.9 kY.
The output power of cw operation was not stable (- 5Vo) dae to the fluctuation of the cathode potential (AVs 40 V). The evolution of operation parameters with timeis shown in Fig. l. The fluctuation of the output power
correlates with that of the cathode potential AV1. When
AV1 decreases to the minimum value, AP|P reaches its
maximum as shown in Fig. l. In order to suppress thefluctuation level of the cathode potential and the anode
potential, high voltage power supplies are equipped withsmoothing circuits consisting of a resistor, an induction
coil and a capacitor (Fig. 2). The fluctuation level was

0 100-2 l0l
0 100

2 100
Time r (ms)(b)

I 10-2d0lo"-1 10'0 100

5 10r
Time t(ms)5 l0-t0 100-510'l
U100

5 10t

Time t (ms)decreased (Ayk - 0.6 V, LVu - 0.2 V) by introducing the2 10'lsmoothing circuit. Accordingly, the fluctuations of theoutput power were decreased from 4 Vo to I Vo.

0 100

A gyrotron functions as a voltage controlled
oscillator. This enables us to attain a high stability ofoutput frequency by introducing a phase lock loop.
Compared to conventional sources (gunn oscillator,BWO), a gyrotron has small frequency modulation
sensitivity. The value ofthe gyrotron FU IV obtained bychanging the potential of body electrode is 0.016 MHzlV. In order to compensate the mismatch of modulation-2 lof0loo5lor

tlo2

Time r (ms)Fig. 1 Time evolution of fluctuations of gytotron output
power AP, cathode potential AV, and anode potential AV". (a) without a smoothing circuit (b) with asmoothing circuit.
206

Ogawa I. et al., High Quality Operation of a Submillimeter Wave Gyrotron for Plasma Diagnostics Application

stabilization is effectively improved by introducing
phase lock loop from A,f - 50 kHz to

3. High Purity Mode Operation of a

A/- I kHz (Fig.

Gyrotron
Gyrotron FU VA (Fig. 5) is constructed using ahelium-free superconducting magnet. This magnet can
produce a magnetic field up to 8 T without using liquid
helium. The tube is demountable, because we will try to
optimize all components, the cavity, the transmissionwaveguide and the output window. The window is madeby quartz plate with the thickness of 3.175 mm andrelative dielectric constant of 3.83.
In order to avoid conversion of the cavity mode tospurious modes, the cavity (Fig. 6) has an optimized
design with nonlinear up-tapers and a rounded iris at theoutput. The resonance calculation using a scatteringmatrix formalism (SM-code) was performed taking into
account the complete gyrotron geometry including thepumping sections (slots) and the window [9]. Table I
shows output mode purities and quality factors fordifferent modes.

4).

LI/40V 2kO

2ko

7H48H

Fig. 2 A smoothing circuit to suppress output voltage

fluctuations.

WindowIon purry

Collector

Turbo molecular pump

CavityS.C. rnagnet

Glrotson FU IVIvoltage

ffimContlol

AnplifierIon purp

Fig. 3 Block diagram of a phase lock system.

Anodec"rrroce

f =301j28GH2-J

unnFig. 5 A schematic drawing of Gyrotron FU VA.

SWP: 600 ms
10 kHz / div

clg -50{3l0-60o)

orio-7oI-eo3Fo-)

-100
39.96

39.98

40

40.02

40.04-1040.06

Intermediate frequency, {, [MHzlFig. 4 A logarithmic plot of frequency spectrum of intermediate freguency signal under phase-locked sta-050100150z (mm)Fig. 6 The shape of the cavity installed in Gyrotron FUVA.

bilization.
207

Ogawa I. et al., High Quality Operation of a Submillimeter Wave Gyrotron for Plasma Diagnostics Application

Table 1 Calculated resonance frequencies f,"", diffractive(a)quality factors Oo and output mode purities 4for the complete Gyrotron FU VA.
modei", (GHz)73.85

Qo251rl (7o)

TEor

TErz

02.36

1020

98.54

TEoz

34.40

2910

97.26

TEr:

63.37

3109

99.42

TEor

94.57

2603

99.02

TEo+

254.63

1 1569

98.49

TEro

344.13

2'77t2

87.65

TEoo

3'74.65

10106493.83

9'1 .99v (mm)The radiation patterns are measured by two-x (mm)dimensionally (-r-y plane) moving pyro-electric detector
array over the gyrotron window. The intensity profilesof radiation pattern for TE32-, TEs3- and TE13-mode areshown in Fig. 7. These patterns are not so affected by

the diffraction at the output aperture of waveguidebecause they are measured in the far-field region (z > zr

1.0= ka*2 - 200 mm).
The patterns for these modes agree well with theintensity profiles calculated. This demonstrates that
Gyrotron FU V can produce outputs of high purity
mode. As can be seen from Figs. 7(a) and (c), this

0.5

gyrotron does not produce a rotating mode but a nonrotating mode. Such a feature is favorable for converting
gyrotron output into a Gaussian beam, because a quasioptical antenna is available for a set of TE6, mode

x (mm)v (mm)

outputs and non-rotating TEln mode outputs.

4. Gonclusions
The output of the Gyrotron FU IV (f = 301 GHz, P- 20 W) obtained by cw operation is stabilized up toI Vo and 50 kHz by decreasing the fluctuations of thepotentials (Ayk - 0.6 V, A% - 0.2 V), via introducing asmoothing circuit consisting of a resistor, an induction
coil and a capacitor. As a result of phase lock loop, we
succeeded to reduce the frequency fluctuation within
I kHz.
To produce high purity mode outputs, a carefully
designed cavity is installed in Gyrotron FU VA. The
measurement of radiation patterns demonstrates highx (mm)purity non-rotating mode operations. The resonancecalculation for a complete gyrotron geometry using
scattering matrix formalism (SM-code) was carried out
and the results were compared with the measurement.

v (mm)

Fig. 7 Intensity profiles of radiation pattern. (a) intensity
profile for TE' mode (Bo=5.5 T, f=151 GHz, 2=730mm), (b) intensity profile for TEo. mode (Bo=7.0T,
f=194 GHz, z=350 mm), (c) intensity profile forTE'. mode (8.=5.8 T, f=163 GHz, 2=730 mm)'

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Ogawa I. et al., High Quality Operation of a Submillimeter Wave Gyrotron for Plasma Diagnostics Application

Acknowledgments
This work was partially supported by a Grant-in-[4] F. Lewen et al.,Rev. Sci.,Instrum. 69,32 (1998).
[5] T. Idehara et al., Proc. of 3rd Int. Conf. on Strong

Aid from the Ministry of Education, Science, Sports andMicrowave in Plasmas, Moscow, Russia 2, 634

Culture of Japan and the Murata Science Foundation.

(1997).

t6l K.D. Hong et al., J. AppL Phys. 74, 5250 (1993).
[7] L Ogawa et al., Fusion Eng. Des. 34-35, 455
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