A. Komori

Group CHS

L. Giannone

T. Morisaki

National Institute for Fusion Science (NIFS-Repository)

§28. Langmuir Probe Measurement in CHS Komori, A., Morisaki, T., CHS Group Giannone, L. (IPP, Garching) Edge plasmas are considered to be important in determining the transport of energy and particles in toroidally confmed plasmas. Especially, there has been an increasing interest in the measurement of fluctuations in the edge region because of their overall relationship to confmement. Recent studies show that interactions and couplings between spontaneously excited fluctuations can lead to turbulence-induced particle and energy fluxes that dominate the transport in the edge of tokamaks and other toroidal devices [1, 2]. Thus, in CHS, it is of crucial importance to understand and to control the turbulence-induced transport as well as the edge plasma parameters. A Langmuir probe is adaptable to measurements in edge plasmas in which the density is below 1013 cm-3• In plasmas in which the Langmuir probe can survive, this diagnostic remains the easiest and most accurate way to make local measurements. It can obtain electron density, electron temperature, floating potential, and ion saturation current and floating potential fluctuations, .and hence, the turbulence-induced particle flux can be calculated using these data. The preliminary measurements were performed on ECRH plasmas. The Langmuir probe was introduced into the plasma in 0.5 to 1 em steps while monitoring the electron temperature and the ion saturation current. The fmal position was estimated to be 1 em outside the last closed flux surface. At this position, a temperature of about 45 e V and an ion saturation current of about 20 rnA were measured with both a single and double Langmuir probe. This implies a density of 4X 1012 cm-3 for the probe tip of A=21t r.,L=4.4 mm2 where rP=0.35 mm and L=2mm, as shown in Figs. 1 and 2. Here, the ~ axis is the same as the probe axis. The origin of the ~ axis is located on the last closed flux surface in the vacuum and the ~ direction is pointed to the wall. On the basis of this preliminary measurement, we are planning to use a floating graphite head probe with at least 5 carbon fiber tips and with 2 mm tip separation. Simultaneous measurement of electron temperature, and ion saturation current and floating potential fluctuations enables calculation of the turbulence-induced particle flux. 0.5 0.4 ,.-... '? s 0.3 (.) N 0 ........ '-" 0.2 " Q 0.1 0.0 0 2 3 ~(em) Fig. 1. Electron density profile measured with a Langmuir probe. 60 • • 40 > ~ ~" 20 0 ~--~--~--~----~--~--~ 0 2 3 ~(em) Fig. 2. Electron temperature profile measured with a Langmuir probe. References 1)Wootton, A. J., et al., Phys. Fluids B 2., 2879 (1990). 2)Tsui, H. Y. W., et al., Phys. Rev. Lett. 70, 2565 (1993). 197 

Langmuir Probe Measurement in CHS

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