Group" CHS

K. "Ohkuni

K. Toi

National Institute for Fusion Science (NIFS-Repository)

§26. Characteristics of Edge Fluctuation Measured by Langmuir Probe Array on CHS Ohkuni, K. (Dep. Energy Eng. Science, Nagoya Univ.) Toi, K. and CHS Group In the edge plasma region, particle and energy fluxes are induced by fluctuations of plasma parameters. In the several tokamaks, the total particle flux due ~o electrostatic fluctua-tions is measured in the edge plasma region, by using Lang-muir probes. I) In order to study edge plasma fluctuations and transport induced by them in a helical device, a Langmuir probe array is installed on CHS.2) The Langmuir probe array consists of 16 electrodes, and divided into 4 sets which are arranged radially with about 6mm separation (Fig. 1). Each set which consists of 4 elec-trodes works as a standard triple probe, where floating poten-tials at two positions separated by 4.5 mm in the poloidal direction are simultaneously measured. Fluctuations of poloidal electric field are estimated by these two signals of floating potential. Edge fluctuations were measured in NBI heated plasmas, where the magnetic axis position Ru - 92 cm, the toroidal magnetic field Bt - 1.4 T, line-averaged density ne - 1.5 X 1019 m-3, and the central electron temperature Te- 0.5 keV. In the plasmas, edge plasma parameters were measured with the Langmuir probe array. The electron temperature and density near the edge are varied from 10 to 25 e V and from 0.1 to 0.4 X 1019 m-3, respectively. The particle flux in the radial direction induced by elec-trostatic fluctuations is expressed by [' = <EpoJ ne>lBt' where E~ and ne are fluctuations of poloidal electric field and elec-tron density, respectively. Figure 2 shows radial profiles of ne' Epol and ['. The direction of particle flux around r/<a> -0.94 is inward, and in the other regions, it is outward. In Fig. 2, the total flux on r/<a> - 0.92 and 0.98 are about 4.5 and 2.5 X 1019 m-2s-J, respectively. The frequency dependence of the flux at these positions is shown in Fig. 3. In the inner position, components of higher frequency (50 < f < 80 kHz) contribute to the total flux. The contribution of lower fre-quency components (f < 20 kHz) is dominant in the outer position. Detailed studies of these edge fluctuations are now being performed. Plasma radial direction Fig. 1. Schematic drawing of the Langmuir probe array "#77574 ~#77575 ~ 6 t.----.---:-.----.-~I---:-.I-··-.·ti ·-.··~··--·:···I·-··.--·I-··:-··I···:i '-' 4 ........ --.. --.... ----.-.... --.. T ... -.. -... :A:.-•... ~ .. Is:: 0) 2 t=....L..J.-J-L...L....I.---L-..L...L-L...J~...L....I.---l-..L...L-1.....J....:I ~ :~~ t.-···.·--:--·I····.····I---:-··I-··-.·it··.····I;···:·~I; ···.···1···:-··1··-·.1 0400 -.-.-.-.-•. -~--•....... -.•..... -.. -....... . I Co ~ 200 E....L.-l....J-L.J......J..---l-..L...L-1.....J-L...L....I.---l-I--I.......L-L...::I ~: ~ E.:-:-n'e-.-,n'=n: _In. -!---. "1:-.--- 1---:-1--.1 ~ ~ F::::::::~;:;-:t:;::::::~-:~-j c: -2 0.9 0.95 1 r/<3.> Fig. 2. Radial profiles of fluctuations of electron density and poloidal electric field, and particle flux induced by electrostatic fluctuations i~f~~:~~:9~ :- 1 -. ---. -.- -:.- -.- .. ···--·t-·····---·-·--······ ;"rn 0 .m .m .. _: _............... • ;8 -1 . . "0 3 r/<a>-O.98 z: ~~::::::::L::::::::::::::::j 1 I I Fig. 3. Frequency dependence of the particle flux at r/<a> -0.92 and 0.98 Reference 1) Ch. P. Ritz, et al. ,Phys. Rev. Lett. 62, 1844 (1989) 2) K. Ohkuni, et al. ,Rev. Sci. Instrum. 70,419 (1999) 249 

Characteristics of Edge Fluctuation Measured by Langmuir Probe Array on CHS

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