J. Plasma Fusion Res. SERIES, Vol.2 (1999) 303-306

Observation of Cyclotron Waves Resonantly Excited underElectron Cyclotron Resonance Heating at Magnetic BeachITO Kiyokazu, KIWAMOTO Yasuhitor, SAITO Teruo, TATEMATSU Yoshinori, ABE Hiraku,
KAJIWARA Ken, SUZUKI Akihiro, MASE Atsushi, TAMANO Teruo and YATSU Kiyoshi
Plasma Research Center, University of Tsukuba, Tsukuba 305-8577, JapanlFaculty of Integrated Human Studies, Kyoto (Jniversity, Kyoto 606-8501, Japan(Received: 8 December 1998 / Accepted: 17 June 1999)

Abstract
Frequency analyses are made for cyclotron radiations emitted from electrons under electron
cyclotron heating in a magnetic beach. In addition to a broad spectrum due to incoherent emissions a newspectrum is observed that is attributed to coherent emissions from spatially bunched electrons triggeredby velocity space modulation by ECH and enhanced by magnetic-mirror effect.

Keywords:
ECH, cyclotron radiation, collective motion, bunching, magnetic mirror

1. lntroduction

observed in a narrow frequency range near theAn electron cyclotron wave propagating toward amagnetic beach experiences two resonances, namely,
one is the wave-particle cyclotron resonance and theother is a singular increase of the parallel wave-number

fundamental resonance.

that is associated with strong reduction of the wave's
phase velocity. If the wave is monochromatic and of aconclude the paper in Sec. 5.

large amplitude, we expect non-stochastic accelerationof resonant electrons which remain in phase with the2. Experiment Setup
The experiment is carried out at the east plug/wave. Then, the spectrum of the cyclotron radiation

barrier cell of GAMMA10 as illustrated schematically in
Fig. l. We take the machine axis as z-axis running fromthe east to the west with z = 0 at the mid-plane of thecentral cell. At the east plug/barrier cell, a high power

The organization of the paper is as follows. The
experimental setup is given in Sec. 2, and the results aregiven in Sec. 3. After the discussion in Sec. 4, we

emitted by the electrons may bear characteristic featuresin addition to the normal spread associated with thevelocity diitribution.
We report about an experimental trial to observe
such emissions at the fundamental cyclotron resonanceand examine the observed radiation. The experiment iscarried out at one mirror cell of a tandem mirror whereelectrons are resonantly heated by an electron cyclotron
wave (ECH). Associated with the ECH, a several dB ofenhancement in the radiation level is observed over awide frequency range around the fundamentalresonance. In addition, a much stronger radiation ismicrowave, generated at 28GHz up to l50kW by agyrotron, is launched from the high field side to theresonance layer of 1T in shape of a beam of an x-mode
cyclotron wave. The electrons are heated to reachcollisionless regime (several hundred eV, = l0r7m-3)
where the mean-free-pass is about lOi times larger thanthe machine length.
The observation system is illustrated in Fig. 2. The

Corresponding author's e-mail: kiyokazu@ prc.tsukuba.acip
303@1999 by The Japan Society of Plasma
Science and Nuclear Fusion Research

ItoK. et al., Observation of Cyclotron Waves Resonantly Excited under Electron Cyclotron Resonance Heating at Magnetic Beach

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Fig. 1 A schematic view of the plug/barrier cell of theGAMMAl0tandem mirror.

Fig. 3 Time sequence of the frequency analysis (a)Frequency as a function of time. (b) The receivedpower as a function of time.

a notch filter with a 30dB rejection band from 28.0 to28.lGHz equipped for the protection of the system.fromgyrotron output power. The signals are converted tobelow 1.8GHz through a heterodyne circuit that consistsof a gunn diode local oscillator (27-29.5GH2) and abalanced mixer, and finally frequency-analyzed on aReceiverantennaheterodyne-type spectrum analyzer.

3. Experimental Results
Fig.3 illustrates the experimental scheme of thespectrum analysis. The plasma is produced by adischarge from 50 to 230ms, and the ECH is applied
from 130 to 180ms with a gyrotron power of l40kW.
The frequency of the analyzer window 5MHz width isvaried from 27.8 to 29.6GHz starting at 135ms andcompleting at 155ms as indicated in Fig. 3(a). Thepower level of the tuned microwave signal is plotted inFig. 3(b). It is experimentally confirmed that the signal
level of each frequency component in the swept domain
does not appreciably change during the time of thefrequency scanning. Therefore Fig. 3(b) shows thefrequency spectrum averaged over the time interval.

Fig. 2 A schematic diagram of the observation system. lt

consists of a movable receiver antenna, a notch

filter, a balanced mixer, a gunn diode local
oscillator and a heterodyne type spectrum
analyzer.

Associated with the application of ECH, anmicrowave emissions are collected on a receiver antenna

enhancement of the signal level is observed by several

that is radially movable at z = -9,77m. The received
microwave is propagated through an over-size circular
waveguide, and converted to TEl0 mode to be guided
through a rectangular waveguide system which includes

dB above the level detected before ECH. This
component is observed over a wide frequency range
around the fundamental resonance (26-30GHz). A
further enhancement of more than 30 dB is observed at304

Ito K. et al., Observation of Cyclotron Waves Resonantly Excited under Electron

Cyclotron Resonance Heating at Magnetic Beach

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27.5 28.0 28.5 29.0 29.5
FrequencY[GHz]Fig. 5 The characteristics parameters of the narrowcomponent vary with the ECH power. (a) The
peak power and the integrated power. {b} The 10
dB width and the Peak frequencY.

Fig. 4 Frequency spectra for different powers of ECH' (a)2OkW, (b) 60kW, {c) 100kW and (d) 140kW. Thesolid {dashed} line represents the spectrum withECH (without ECH).

results are shown in Fig. 4, in which we plot thefrequency spectrum of the received signal detected atincreasing ECH power (20, 60, 100, l40kw). In eachplate the dotted line represents the spectrum withoutl42ms (corresponding to 28.4GHz). The 1OdB
frequency width of this component is quite narrow(0.4GHz). The spike at l35ms (27.8GH2) is due to the
instrumental output associated with the initiation of theECH in the presence of the plasma. As mentioned above
the spike at 28GHz may be discarded from thediscussion. It is observed that the power level of thisfrequency sweep and at l37ms (28.0GH2) corresponds
to the heating wave. The narrow component of thecomponent increases and its width decreases withincreasing power of ECH. It is noticed that the spectral
tail in the high frequency side shrinks as the heating

emission is close to but still well separated from theheating wave frequency. Time evolutions of the twocomponents are different. The amplitude of the narrowcomponent decreases down to the level of the broad
component as soon as the ECH power is turned off. The
broad component remains observed after the ECH

power is increased. These characteristics in thefrequency spectrum are summarized in Fig. 5. Fig. 5(a)shows the peak level (closed circles) of this component
and the frequency-integrated power (open squares) as

functions of the ECH power. The integration is madefrom 28.1 to 29.5GH2. It is observed that the both

power is turned off. The total power of the broad
component is evaluated by correcting for the frequency-power levels increase almost linearly with the heatingpower. So far we have not found any threshold value inheating power. Fig. 5(b) plots the 10dB width of thiscomponent (open squares) and the frequency of the peak
power (closed circles) as functions of the ECH power.
The l0dB width is about 0.4GHz and decreases with theincreased ECH power. The peak frequency remains

response of the whole system of detection and byintegrating over the space with an approximation of
isotropic emission. The result is nearly equal to thepower of the cyclotron radiation summed overindividual electrons in the plug region. Therefore thiscomponent is attributed to thermal motion of heatedelectrons including the bulk component and hot tails inthe velocity distribution. The latter is the dominantsource of radiation for hundreds of ms after the turn-off

around 28.4GHz showing a slight decrease with theheating power.

of ECH. The narrow component shows features different
from the broad component. To study the mechanism of
the generation of this component, we examine changes

4. Discussion
It is experimentally confirmed that the frequency
spectrum does not change appreciably for differentof this component by varying the ECH power. Some305

Ito K' et al" observation of Cyclotron waves Resonantly Excited underlocations and orientations of the receiver antenna.
However, it is beyond the capability of the present
experiment to exclude a possibility that the narrowcomponent is observed as a result of a selective
transparency of the plasma against a wider frequency
spectrum of emissions originally generated deep insideElectron cyclotron Resonance Heating at Magnetic Beachmagnetic field, they are axially decelerated by the mirroreffect. This effect contributes to localize the axial
position where the azimuthal bunching appears. The
coherence of cyclotron emissions under the bunching
increases and the contributions from many electrons add

up to higher level at the frequency that is also

the plasma. To examine whether the narrow spectrum

determined by the local magnetic field of the bunching.
The shrinking tail at the high frequency side of thespectrum as observed in Fig. 5(b) can be understood interms of the stronger mirror effect against electrons

represents the spectrum of the actual emission, we study
the path and the degree of absorption of each frequency
component reaching the receiver antenna by using a ray_tracing code [1]. The ray-tracing calculation is based ongeometric optics. Following assumptions are made forwhich have acquired larger transverse energy at a higher

ECH power resulting in axial stagnation at a lower

the present calculation. The electron velocity

magnetic field.

distribution function is a bi-Maxwellian. The density

exponentially decreases as square of the radial

5. Conclusions

coordinate and also with increasing external magnetic

Under the magnetic beach heating of collisionlessfield. These simplifying assumptions are not very
contradictory to the observed results of GAMMAI0
plasma. As a result, we have confirmed that eachelectrons with a monochromatic cyclotron wave, wehave observed a radiation spectrum that is composed offrequency element of cyclotron waves in a wide range(27-3OGHz) reaches the receiver antenna without severe

attributed to incoherent cyclotron emissions of heatedelectrons. The other is limited in a narrow range in theupper side of the heating frequency but much stronger

two components. One is a broad component that isdamping as long as it is generated in the plasma.
Therefore, From the comparison between thethan the other component. The generation mechanism ofobservation and the ray-tracing, we conclude that thethis component is examined experimentally andtheoretically. The most plausible mechanism is thespatial bunching ofthe electrons under the mirror effectafter phase modulations by the heating wave. phasemodulation of gyrating particles has been discussed instrong radiations are generated only within the narrowfrequency range. Because incoherent emission should
generate a wider frequency spectrum, we expect that thenarrow component is attributed to collective motion ofelectrons. Among several candidates wave-wave
processes is not a dominant process because thefrequency separation from the heating wave does notdepend sensitively on rhe heating power and otherplasma parameters. We also exclude the possibility of
induced radiation from electrons during the accelerationby the heating wave because this component is largely
separated from the heating wave. These considerations
are consistent with our experimental confirmation that

relation to cyclotron echo [2]. It also plays an importantrole in a intermediate process of cyclotron heating [3,4].

Acknowledgments
The authors are grateful to the members of theGAMMAl0 group for collaboration and discussion in
the course of this work. This work is supported by aGrant-in-Aid for Scientific Research from the Ministryof Education, Science, Sports, and Culture of Japan.

any characteristic peaking structures are not observed in
the second harmonic range (around 56GHz). A plausible

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