Speaker
Dr
Kenichiro Terasaka
(Kyushu University)
Description
A neutral depletion structure with strong inhomogeneity in the radial direction has been observed in an electron cyclotron resonance (ECR) plasma. We have measured the velocity distribution function of neutrals with a high resolution laser-induced fluorescence (LIF) system and examined the relationship between asymmetry of distribution function and flow induced by inhomogeneity. It is worth pointing out that skewness is a geometrical form factor to characterize asymmetric profile of distribution and that the relationship between the skewness and corresponding physical process has not been clarified yet. It has been revealed that the third order moment of distribution function, that is, skewness, is proportional to the inhomogeneity-induced flow, and a simple relation between the skewness and the normalized flow velocity has been obtained and confirmed in the experiment.
The experiment was performed in the HYPER-I device at National Institute for Fusion Science, Japan, and the high density ECR plasmas were produced with a 2.45 GHz microwave. In order to measure the velocity distribution function, a high resolution LIF method was utilized. The laser beam was modulated by an electro-optical modulator and a polarization beam splitter at 100 kHz, and the LIF signal was lock-in detected to improve the signal-to-noise ratio. The saturated-absorption-spectroscopy (SAS) system was installed in the LIF system, and the Lamb dip was obtained on the SAS spectrum, the position of which was used as the frequency standard and also used as the velocity origin in the laboratory frame.
We have precisely measured the velocity distribution function in an inhomogeneous neutral depletion structure. The experimental results indicate that the geometrical form factor of distribution function (skewness) is related to the flow generated by inhomogeneity. It has been found that the asymmetry of distribution function is generated in the direction parallel to the density gradient. Our experiment has clarified the physical significance of the third order moment of distribution function and related the geometrical form factor to the dynamical property (flow) in an inhomogeneous system for the first time. It is considered that the skewness is the fundamental quantity that describes inhomogeneity-induced flow.
Primary author
Dr
Kenichiro Terasaka
(Kyushu University)
Co-authors
Prof.
Atsushi Okamoto
(Nagoya University)
Prof.
Masayoshi Y. Tanaka
(Kyushu University)
Prof.
Mitsutoshi Aramaki
(Nihon University)
Prof.
Shinji Yoshimura
(National Institute for Fusion Science)
