Scattering Measurement using a Submillimeter Wave Gyrotron as a Radiation Source

I.Ogawa, T.Idehara, M.Myodo, T.Saito, T.Hori1, H.Park2 and E.Mazzucato2
Research Center for Development of Far-Infrared Region, University of Fukui
3-9-1 Bunkyo, Fukui 910-8507, Japan
Tel; +81-776-27-8662
Fax; +81-776-27-8770
e-mail; ogawa@fir.fukui-u.ac.jp
1National Institute of Information and Communications Technology,
4-2-1, Nukui-Kita, Koganei, 184-8795, Japan
2Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543
Abstract. It is important to measure density fluctuations as a port of the basic study of plasma confinement. The
scattering method with electromagnetic wave makes it possible to observe the magnitude, the frequency and the
wavenumber of the fluctuations directly with a high spatial resolution. The difficulty in obtaining effective information is
scarcity of the spatial resolution and the sensitivity of fluctuation measurements. Application of an intense, high quality,
well-collimated probe beam is effective in improving the performance of the measurement. Such probe beam is produced
by stabilization of the high frequency gyrotron output and its conversion into a Gaussian beam.
Keywords: Plasma, Scattering measurement, Gyrotron, Submillimeter wave
PACS: 07.57.Hm
INTRODUCTION
The density fluctuations are theoretically expected to enhance the energy loss of the confined plasma across the
magnetic field. It is important to measure these physical quantities as a port of the basic study of plasma
confinement. The scattering method with electromagnetic wave makes it possible to observe both the frequency and
the wavenumber of the fluctuations directly with a high spatial resolution.
If we employ a low frequency wave (millimeter wave) as a probe beam, it is difficult to focus the probe beam. A
non-collimated probe beam degrades the spatial resolution of the measurement. On the other hand, the use of a high
frequency probe beam makes the scattering angle small. Such a condition brings poor spatial and wavenumber
resolutions. There is an optimum wavelength of the probe beam in consideration of the spatial and wavenumber
resolutions. The range of the available wavelengths is in submillimeter region, which allows refractive effects to be
sufficiently small while still permitting large scattering angle.
Application of high frequency gyrotron is effective in improving the S/N ratio of the measurement because of its
capacity to deliver high powers1, 2). A final goal is to produce an optimal probe beam (high quality, intense
submillimeter wave beam) and to apply it to plasma scattering measurements with the aim of optimal performance
of the measurement.
SCATTERING MEASUREMENT IN NSTX TOKAMAK
In order to investigate ETG driven turbulence for the transport of electron energy in NSTX plasmas, high
sensitivity and spatial resolution are necessary to the measurement. We are planning to apply the submillimeter wave
gyrotron (Gyrotron FU II) as a radiation source to the scattering measurement in NSTX tokamak at Princeton
University.
The Gyrotron FU II3) is one of high frequency, medium power gyrotrons included in Gyrotron FU Series
developed in University of Fukui. In the gyrotron, as well as other gyrotrons included in the series, a narrow
resonant cavity with high Q value is installed for achieving high separation between the cavity modes. Such a
situation is important for high harmonic operation of high frequency gyrotrons. Because of this narrow cavity, our
gyrotrons could be operated in many single modes at the fundamental, the second and third harmonics of electron
cyclotron resonance. The value of high Q value confirms reductions in the starting current for operation and the
linewidth of the radiation. The lower starting current is suitable for avoiding overheating the gyrotron in case of a
long pulse operation (τ~2s).
The Gyrotrn FU II consists of an 8T superconducting magnet, water-cooled gun coils and sealed-off gyrotron
tube (Fig.1). The electromagnetic wave generated in the cavity transmits in a circular waveguide and emitted from
the vacuum window. The TE161 mode output (P=80W, f=354GHz) at the second harmonic (n=2) resonance is used
for plasma scattering measurement.
STABILIZATION OF OUTPUTS OF GYROTRON FU II
The stabilization of amplitude and frequency of gyrotron outputs is required to improve the performance of the
measurement. The fluctuations of the amplitude and the frequency are caused by the fluctuations of the cathode and
the anode voltages. We can find the operating parameters to produce the appropriate outputs with keeping the anode
voltage 0V. In such operation, gyrotron outputs will be stabilized by removing the cathode voltage fluctuations.
When the gyrotron is operated, voltage fluctuations (ΔV~40V) due to the switching noises (Fig.2) and voltage
drop during a long pulse (ΔV~300V) appear in the cathode voltage (-20kV). In order to suppress the fluctuation
level, high voltage power supply is equipped with a smoothing circuit consisting resistors, induction coils and
capacitors (Fig.3). The voltage fluctuations due to the switching noises are effectively suppressed to 0.5V by
introducing the smoothing circuit. As a result, the output power fluctuation is stabilized from 10% to MODE CONVERSION INTO GAUSSIAN-LIKE BEAM AND COUPLING WITH
CORRUGATED WAVEGUIDES
Unlike a molecular laser, the gyrotron generates spreading radiation with TEmn mode structure. It is therefore
necessary to convert the output radiation into a Gaussian beam (TEM00 mode). In this scattering measurement,
gyrotron output will be transmitted by corrugated waveguides and incidence on plasma as a probe beam. To improve
the sensitivity of the measurement, it is essentially important to realize the high coupling efficiency to the corrugated
waveguide.
The gyrotron output produced as the structure of a circular waveguide TE15 mode is converted into a linearlypolarized
beam by using a quasi-optical antenna4, 5). The far-field pattern of the linearly-polarized beam consists of
a main beam and side lobes. The main beam is similar to a Gaussian beam. The far-field pattern will appear by using
phase shift effect due to a focusing mirror (Fig.6). The radiation pattern thus obtained (Fig.7(a)) is incident on
corrugated waveguides. The radiation pattern from the end of the corrugated waveguide is shown in Fig.7(b). The
radiated power measured by using power meter is 8W. This power level is intense enough to improve the S/N ratio
of the measurement.
CONCLUSIONS
The GYROTRON FU II delivers long pulses operation (τ~2s) of suitably high power (P~80W) in submillimeter
wavelength range. The amplitude and the frequency of gyrotron outputs are stabilized by introducing the smoothing
circuit. The gyrotron output produced as the structure of TE15 mode is converted into a Gaussian-like beam and
coupled with corrugated waveguides. The amplitude and the frequency of gyrotron outputs are stable enough to
ensure the reliability of the measurement. The power coupled with corrugated waveguides.

REFERENCES
1. T.Idehara et al., Proc. of 3rd Int. Conf. on Strong Microwave in Plasmas, Moscow, Russia, 2, 634 (1997).
2. K.D.Hong et al., J. Appl. Phys., 74, 5250 (1993).
3. T.Idehara et al., Phys. Fluids, B4 (1), 267 (1992).
4. S.N.Vlasov et al., Radiofizika, 17, 115 (1974).

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