24P142-P
Study of plasma dynamics in attached and detached plasma
plasmass
接触・非接触プラズマ中の動的挙動に関する研究
Katsuya Okazaki1, Hirohiko Tanaka2, Noriyasu Ohno1 and Shin Kajita1
岡崎克哉1, 田中宏彦2，大野哲靖1，梶田 信1
1

Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
2
NIFS, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan1
名古屋大学 〒464-0075 名古屋市千種区不老町
2
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6

Considerable attention is given to the static and dynamic behaviors of detached plasma, because utilizationof the detached divertor is thought to provide a promising method for reducing the heat flux to plasma
facing components. In this study, we performed a electrostatic probe measurement when changing a stateof plasma from attached to detached by increasing a neutral gas pressure rapidly. As a result, it is clearlyfound that transition from an attached plasma to a detached plasma changes the phase relation between adensity and a potential.

1. Introduction
In ITER, to reduce large amounts of heat andparticle fluxes, detached divertor will be used. The
detached divertor utilizes a plasma detachmentphenomenon, which is performed by increasing aneutral gas pressure inside the divertor region andreducing a plasma heat flux by the interaction ofplasma and gas [1]. Therefore, plasma detachment
is one of the most important issues to lead a ITER
project to a successful conclusion.
A comprehensive investigation has been
performed for the static and dynamic behaviours ofdetached recombining plasmas in the linear divertor
plasma simulator NAGDIS-II [2, 3]. In this study,
we will investigate a dynamic behaviour of plasma
fluctuation characteristics by using a Langmuirprobe when changing the plasma from attached todetached by increasing a neutral gas pressure
rapidly.
2. Experimental Setup
Figure 1 shows a illustration of the NAGDIS-II.
This device has two 2000 L/s turbomolecular
pumps at the side of discharge region and divertor
test region. In this experiment, to achieve a sudden

Fig.1. Diagrammatic illustration of the linear plasma
divertor simulator NAGDIS-II.

change of the neutral gas pressure, we operated agate valve installed between the divertor test region
and the pump. When we close the gate valve, theneutral gas pressure rapidly increases fromapproximately 1 to 25 mTorr, and we can generate
the detached plasma. In this study, both themeasurements of the electrostatic fluctuations andneutral gas pressure were performed at a distance of
1.06 m from the anode. Gas species was He.
3. Experimental Result
Figure 2 shows the result of triple probe
measurement at the radius of 15 mm distance fromthe center of the plasma column when the neutral
gas pressure (P) was changed. From the probe
measurement, we obtained the time evolutions of
the moving average of electron temperature (Te),electron density (ne), floating potential (Vf) andspace potential (Vs). With increasing P rapidly at t =0.8 s, Te and Vs decreased and ne and Vf increased
considerably. After that, Te and Vs increased and neand Vf decreased gradually. Finally, detachedFig.2. Time evolutions of the moving average
deviations (a)P, (b)Te, ne, (c)Vf and Vs.

plasma was generated. From the comparison
between the attached and detached states, Te and ne
decreased.
To investigate a dependence of a phase relationbetween ne and Vf on the neutral gas pressure, weanalyzed the time evolution of the moving
cross-correlation coefficient, which is defined by
the following equation:~n~e (t )Vf (t + τ )C (τ ) =.
(1)~22n~e (t ) Vf (t )From the result of the cross-correlation coefficientin Fig. 3(b), the correlation between the ne and theVf can be divided into three characteristic time
domains, i.e. (i) t < 0.8 s, (ii) 0.8 s < t < 2.3 s, and(iii) t > 2.3 s. In period (i), attached plasma was(a)(b)generated. In this time, negative correlation wasobserved around τ = 0 s. In period (ii), although thephase relation between them does not change, theperiod of C(τ) fluctuation along τ becomes long.
After that in period (iii), transition of the phaserelation can be clearly observed. This result implies
that there is a threshold in between the attached anddetached states.
Figure 3(a) shows the power spectra of ne, S(f),under the attached and the detached plasma
conditions. Here, S(f) of the attached plasma wasobtained at (i). It is found that there is a strong peakaround 35 kHz. On the other hand, S(f) of thedetached plasma which was obtained at (iii) have apeak around 10 kHz. Therefore, by increasing the P,
the peak of S(f) shifted to the low-frequency range.
4. Summary
We measured time evolutions of an electron
temperature and a electron density, floating andspace potentials by using a Langmuir probe in thetransient state from the attached to the detachedstates. It was confirmed that an electron temperatureand density decreased by changing from an attachedplasma to a detached plasma. Result of a moving
cross-correlation coefficient between the electron
density and floating potential shows that there is asignificant difference between an attached plasma
and a detached plasma. In the future, clarification ofa detached plasma physics is expected byunderstanding a change of phase difference with a
PIC simulation and so on.
Acknowledgments
This research was supported by NIFS/NINS
under the project of Formation of International
Network for Scientific Collaborations. This work isalso performed with the support and under theauspices of the NIFS Collaboration Research
program (NIFS10KLPF003).
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Fig.3. (a) Power spectra of ne under the attached and thedetached plasma. (b) Time evolution of the moving
cross-correlation coefficient between the ne and the Vf.