23P045-P
Density Fluctuation Excitation Experiments using Supersonic Gas Puffing
超音速ガスパフによる密度揺動励起実験Akiyoshi Murakami1,2), Junichi Miyazawa3), Tsuyoshi Akiyama3), Koji Yasui3), Takanori Murase3),
Tomohiro Morisaki3), Ryuichi Sakamoto3), Hiroshi Yamada3) and the LHD Experiment Group村上昭義1,2)，宮澤順一3)，秋山毅志3)，安井孝治3)，村瀬尊則3)，森崎友宏3)，坂本隆一3)，
山田弘司3)，LHD実験グループ
1)

The Graduate University for Advanced Studies
Research Fellow of the Japan Society for the Promotion of Science3)
National institute for fusion science322-6, Oroshi-cho, Toki-shi, Gifu 509-5292, Japan1)
総合研究大学院大学，2)日本学術振興会特別研究員，3)核融合科学研究所 〒509-5292 土岐市下石町322-6
2)Density fluctuation excitation experiments using supersonic gas puffing (SSGP) has been carried out in the Large
Helical Devise (LHD). High-frequency injection (500 Hz) was confirmed in preliminary experiments on a test standusing the fast solenoid valve for SSGP. In the plasma experiment in LHD, the density fluctuation of 50 Hz was excitedby SSGP. Higher harmonics of up to 200 Hz was also observed simultaneously. Experimental results obtained in thetest stand and LHD, together with the optimum operation condition of SSGP for density fluctuation excitation will be
discussed in this presentation.

1. Introduction
Density and/or temperature fluctuations related
to the MHD instabilities and micro instabilities
are always observed in plasma experiments.
Especially, anomalous transport driven by micro
instabilities is thought to be playing an importantrole in determining the confinement property ofthe plasma. It is therefore desired to find amethod to control the instabilities of the order ofkHz to MHz to mitigate the anomalous transport
and improve the confinement property.
On the other hand, density modulation of theorder of 1 Hz using normal gas puffing has been
carried out to estimate the particle transportcoefficient [1]. Repetitive injection of smallhydrogen ice pellet of the order of 10 Hz has beenshown to be effective for controlling ELMs intokamaks [2]. It has been, however, difficult todrive the density fluctuation of the order of kHz.
Recently, a supersonic gas puff (SSGP) device
has been installed on the Large Helical Device(LHD) [3]. In SSGP, high-pressure gas is ejectedthrough a fast solenoid valve equipped with aLaval nozzle, which produces the convergent gasflow. SSGP is characterized by the high working
pressure of up to 8 MPa and the high time
response of up to 1 kHz. The former is effective
for achieving high fueling efficiency reaching~30 % [3], while the latter makes a possibility offast density fluctuation excitation by SSGP.
In this presentation, basic characteristics of theSSGP device (Section 2) and results of thedensity fluctuation excitation experiment carriedout in LHD (Section 3) are discussed.
2. The SSGP Device and Test Results
The SSGP device in LHD consists of threesolenoid valves equipped with different Laval nozzlesof 0.1, 0.3 and 0.6 mm throat diameter. By selecting
nozzles and backing pressure, one can adjust the flowrate of SSGP from ~1 to ~1000 Pam3/s. The SSGP
device is set on a lower port of LHD and therefore thegas flow is upward. The distance from nozzle toplasma is ~4 m.
Preliminary experiments using the solenoidvalves and Laval nozzles were carried out in a test
stand [4,5]. To visualize the gas flow, the working
gas was cooled by using a refrigerator and cluster
beams were formed. Continuous visible laser lightintersecting the gas flow at a few cm from the exit
3.5500 Hz

32.55060

7080
90100
Times (ms)Fig. 1. Temporal evolution of the scattered signal
from the hydrogen cluster beam ejected
through the fast solenoid valve.

of the Laval nozzle was applied and the laser lightscattered by the cluster beam was detected by a fastcamera. Figure 1 shows typical time evolution ofthe scattering signal from the supersonic hydrogencluster beam. In this case, the valve was operated at
500 Hz with the pulse length of 50 µs. Then, thescattered light fluctuating at 500 Hz was observed
as shown in Fig. 1.
3. Density Fluctuation Excitation
Figure 2(a) shows the typical waveform of theline density at the plasma edge region measured bya far infrared laser interferometer (FIR) [6]. In thisexperiment, the working gas was hydrogen and thebacking pressure was ~1 MPa. The magnetic fieldstrength on the magnetic axis was 1.5 T. The major
radius and the minor radius of the plasma torus
were 3.75 m and 0.6 m, respectively. Theline-of-sight of FIR No.1 channel passes throughthe plasma edge region including the so-called
ergodic layer in LHD. The pulse length and theinjection cycle of SSGP were set to 1 ms and 50 Hz,respectively. SSGP was injected during 3.5 - 4.3 s.
As seen in the figure, the density fluctuation wasexcited by SSGP. The increasing rate of the density
signal was changed at ~3.7 s. This might bereflecting the density dependence of the fuelingefficiency of SSGP [3]. Further investigation onthis is, however, remained for future studies.
Figure 2(b) shows the Fourier spectrum of theline density signal shown in Fig. 2(a). A large peakis recognized at 50 Hz, which corresponds to theinjection cycle of SSGP. Higher harmonics thatmight be related to the peak of 50 Hz are also
observed at 100 and 200 Hz (and presumably at 150Hz, although it is unclear).
To excite the density fluctuations, the Laval
nozzle and the backing pressure, i.e., the flow rateof SSGP, should be chosen carefully. Even at a highbacking pressure of 2.5 MPa, the 0.1 mm Laval
nozzle was not effective for density fluctuation
excitation since the density perturbation by eachpulse was so small that cannot be detected by FIR.
High frequency injection from 0.3 mm Laval nozzle
was effective as shown in Fig. 2. In the case of 0.6mm Laval nozzle, on the other hand, the density
perturbation was too large and the density of thetarget plasma increased as a result. The optimum
flow rate for density fluctuation excitation in LHD
was 40 – 100 Pam3/s, which was realized by the 0.3mm Laval nozzle with the backing pressure of 1 –2.5 MPa.
4. Summary
The density fluctuation has been successfully

(a)

(b)

Fig.2. (a) Typical temporal evolution of the line
density of FIR ch.1, and (b) the Fourier
spectrum of the density fluctuation. SSGP wasinjected from t = 3.5 s to 4.3 s.

excited by using SSGP in LHD, although theachieved frequency of 50 Hz is still smaller thanthat demonstrated in the test stand of 500 Hz.
Higher harmonics of up to 200 Hz wassimultaneously observed with the 50 Hz densityfluctuation. The optimum operation condition for
density fluctuation excitation was found at the flowrate of 40 – 100 Pam3/s.
Acknowledgments
The authors would like to thank all of the members ofLHD Experiment Group. This work has been financially
supported by the Ministry of Education, Sports, Culture,
Science and Technology, Grants-in-Aid for Scientific
Research (S) 20226018 and NIFS09ULFF006.
References[1] K. Tanaka et al: Fusion Sci. Tech. 58 (2010) 70.
[2] P.T. Lang et al.: Nucl. Fusion 44 (2004) 665.
[3] A. Murakami et al.: Proc. 38th European. Physics
Society Plasma conference, France, 2011, p4.096.
[4] A. Murakami et al.: Plasma Fusion Res. 5, (2010)
S1032.
[5] A. Murakami et al.: J. Plasma Fusion Res. SERIES,9 (2010) 79.
[6] T. Akiyama et al: Fusion Sci. Tech. 58 (2010) 352.