23F02
Effects of Gas Pressure on Electron Temperature Gradient Modein Magnetized Plasma
磁化プラズマ中電子温度勾配モードに対するガス圧力の効果
Chanho Moon, Toshiro Kaneko and Rikizo Hatakeyama
文 贊鎬, 金子俊郎, 畠山力三
Department of Electronic Engineering, Tohoku University
6-6-05 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
東北大学 大学院工学研究科 電子工学専攻 〒980-8579 仙台市青葉区荒巻字青葉6-6-05
An electron temperature gradient (ETG) is formed by superimposing an electron cyclotron resonance(ECR) plasma, which passes through two different-shaped mesh grids, upon low-temperature thermionicelectrons. The formed ETG is found to excite a high-frequency fluctuation, and furthermore, the excitedfluctuation is affected by argon gas pressure PAr. The normalized amplitude of the fluctuation attains thelargest value at PAr = 0.1 mTorr, which is determined by the competition between the electron temperature
gradient and density gradient with different dependences on the gas pressure.

1. Introduction
In recent years, anomalous electron heattransports in magnetically confined plasmas are abig issue [1], which are difficult to be explained
clearly even though many researchers try tounderstand the mechanism of the heat transports
in fusion devices [2]. There are strong
experimental evidences that the anomalous heattransports are attributed to an electron
temperature gradient (ETG) driven instability(ETG mode). Actually, it is seen that theinstability is driven by the ETG and nonlinear
effects of the ETG mode generate the significant
electron transport. Although there are someearlier theoretical [3,4], numerical [5], andexperimental [6] studies on the ETG mode,
experimental observations of the ETG mode inlarge fusion devices are not sufficient to clearly
explain the excitation mechanism, which is
caused by the restricted experimental condition in
the magnetically confined fusion plasmas.
Therefore, a basic experiment on the ETG modeis urgently desired. Based on these backgrounds,
the aim of this work is to control the ETG byusing a novel plasma synthesis method
superimposing high and low temperatureelectrons in a basic plasma device and toexperimentally investigate the ETG mode withchanging the plasma parameters [7].
2. Experimental Apparatus and Method
The Experiments are carried out in a linearmachine (QT-Upgrade) of Tohoku University, theschematic of which is shown in Fig. 1. In order toform the ETG, we divide the machine into twosections. One is called as a source region where anelectron cyclotron resonance (ECR) plasma withhigh electron temperature is generated and the otheris called as an experimental region where thehigh-temperature electrons of the ECR plasma aresuperimposed on low-temperature thermionicelectrons generated from a tungsten (W) hot plate
set at the end of the machine. The argon (Ar) gasand a microwave (frequency 6 GHz and power P=10–100 W) are used to control the electron density
and temperature in the experimental region. Two
different-shaped stainless mesh grids are located atz  –40 cm and divide the source region from theexperimental region. The ETG is easily formed bycontrolling the applied bias voltage of the grids [7].
A Langmuir probe is used to measure radial profilesof plasma parameters at z = 0 cm in theexperimental region.
3. Experimental Results and Discussion
Figure 2(a) shows ∆Te (ETG), which is thedifference of the electron temperature between thecentral region (r = 0 cm) and peripheral region (r =2 cm), as a function of Vg2. It is found that theamplitude of the ETG gradually increases with aFig. 1. Schematic diagram of the experimental apparatus andan axial profile of the magnetic field strength.

0-40-200Vg2 (V)

21

1
0.1

0.2

0-3

2(b) 0.02
Ies / Ies ( )~ 0.5 MHz

0.02
0.01

~ 0.5 MHz

0.01



Ies / Ies ( )3PAr (mTorr)(b) 0.03

03002048| Te| (eV)2(a) 4| ne| (10 cm , )4 Te (eV, )(a)-40-20020Vg2 (V)Fig.2. (a) Electron temperature gradient ∆Te and (b)~normalized amplitudes I es / I es of high-frequency
fluctuation in the central and peripheral regions as afunction of Vg2 for Vg1 = –5 V, Vee1 = 0 V, Vee2 = 0 V,and P = 70 W.

decrease in Vg2 and saturates around Vg2 = – 20 V.
When the ETG is formed, the high-frequency
fluctuation is observed to be excited. In Fig. 2(b),dependence of the normalized amplitude I~es / I es ofthe fluctuation with frequency of ~0.5 MHz on Vg2,is presented. The normalized amplitude varies in asame manner as the ETG. Therefore, it is clear thatthe high-frequency fluctuation is ETG mode.
Figure 3 shows (a) the electron temperature
gradient ∆Te and density gradient ∆ne, and (b) the~normalized
amplitudesoftheI es / I es
high-frequency fluctuation as a function of argon
gas pressure PAr. When the pressure is increased,the ∆Te and ∆ne initially increase. This phenomenon
is caused by the increase in the density of the hightemperature electrons generated by the ECR
discharge with an increase in PAr until 0.04 mTorr.
However, for PAr > 0.04 mTorr, the ∆Te and ∆negradually decrease, which is affected by theelectron-neutral collisions, i.e., the electron mean
free path is almost the same as the plasma length.
As seen in Fig. 3(b), the normalized amplitude of
the high-frequency fluctuation becomes large until
PAr = 0.04 mTorr and gradually decreases for higherPAr similarly to the tendency of Te. However, thenormalized amplitude of the high-frequency
fluctuation becomes large from PAr = 0.05 to 0.1mTorr, where the ne suddenly decreases with anincrease in PAr. Since the appropriate ETG driving
parameters are e = dlnTe / dlnne, it is consideredthat the different change of ∆Te and ∆ne for the gaspressure causes the largest value of the normalized
amplitude of the high-frequency fluctuation around
0.1 mTorr.

000.1

0.2

PAr (mTorr)

Fig.3. Pressure dependences of (a) the Electrontemperature gradient ∆Te and density gradient ∆ne,~and (b) normalized amplitudes
I es / I es ofhigh-frequency fluctuation for Vg1 = –5 V, Vg2 = –30
V, Vee1 = 0 V, Vee2 = 0 V, and P = 70 W.

4. Conclusion
Anovelmethodsuperimposing
thehigh-temperature electrons on the low-temperatureelectrons has been developed to form and control
the ETG in a magnetized plasma. The formed ETG
is found to excite the high-frequency fluctuation,i.e., ETG mode, and the normalized amplitude ofhigh-frequency fluctuation becomes the largest
value when the Ar gas pressure is 0.1 mTorr. The
gas pressure yielding the maximum fluctuation
amplitude is determined by the competition
between the electron temperature gradient anddensity gradient with different dependences on thegas pressure.
Acknowledgments
The authors are indebted to H. Ishida for his
technical assistance. The work was supported by aGrant-in-Aid for Scientific Research from theMinistry of Education, Culture, Sports, Science andTechnology, Japan.
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