A. Fujisawa

H. Iguchi

K. Ida

K. Sugimori

S. Shinohara

National Institute for Fusion Science (NIFS-Repository)

§ 20. Study on Electric Field and DensityTransition Phenomena in MagnetizedPlasmaShinohara, S., Sugimori, K.(Interdis. Grad. Sch. Eng. Sci., Kyushu Univ.)Fujisawa, A., Ida, K., Iguchi, H.In NIFS, transport barrier and its formationmechanism have been actively investigated, andadvancing these understandings is crucial in the futurenuclear fusion studies. Plasma rotation driven by so-calledE X B drift has been also studied in relation toimprovement of the magnetic confinement. Therefore,investigations of the characteristics of the electric fieldand its effect on transition phenomena are very important.Here, we studied the density transitionphenomena1,2) along with plasma rotation and densityprofile modification,I,3-5) using ten concentric circularrings as biased electrodes. Argon plasma at a pressure P of0.1 - 10 mTorr in the cylindrical chamber, 45 cm indiameter and 170 cm in axial length, was produced by aRF wave of 7 MHz using a spiral antenna. Plasmaparameters were measured by a developed 24 ch.Langmuir probe and a 3D scanning probe,6) and theplasma flow by the Mach probe (directional probe). Datawere stored with a data logger. Using this system, detailedspatio-temporal behavior was investigated. Typical plasmadensity ne and electron temperature were 4 X 109 - 4 X 1010cm-3, 3 - 6 eV, respectively.voltage, possibilities to possess one state (state II, lowerdensity) compared to the other state (state I, higherdensity) increased. These global, self-excited, densitytransitions and back ones between two states wereaccompanied by changes of the floating potential (see Fig.1) and rotation profiles and the bias current under variousparameters.Transition time was measured as to ion saturationcurrent and floating potential, which were dependent onthe spatial position. As shown in Fig. 2 (Lissajous figure),bias current changed faster than ion saturation current, andion saturation current changed faster than floatingpotential (not shown) in the bulk plasma region. On theother hand, ion saturation current near the electrodechanged very fast (comparable to bias current change of15 while floating potential near the electrode changedslowly with a time scale of several ms. Floating potentialprofile showed that near the electrode electron can betrapped easier in the state I compared to that in the state II.Hysteresis characters such as average staying timeand staying probability, changing bias voltage, were alsofound, and it suggests the fine structure from the analysisof probability distribution function of ion saturationcurrent.In conclusion, we have investigated the detailedcharacteristics of density transitions (bistable sytstem) in awide range of operational parameters. Theseunderstandings will be expected to contribute to theplasma confmement and stability control.I to state II0.90 092. 0.94 0.96til11 0.98e (.seC) 1.02807060$023020o 10-5 0-10-20 .,.M 00 00 ill U M W W wFig. 2 Lissajous figure between bias voltage and ionsaturation current (counter clockwise rotation)Fig. 1 Time evolution of radial profile of floatingpotentialChanging biased voltage, fill pressure and biasedposition, global structural changes of plasma parameterssuch as plasma density, floating potential and azimuthalplasma rotation were investigated. Applying a bias voltage,repeated transitions phenomena with abrupt reductionsand jumps of the electron density were observed (flip-floppattern in bistable system). With the increase in the bias306References1) Matsuyama, S. and Shinohara, S.: 1. Nucl. Fusion Res.SERIES 4 (2002) 528.2) Shinohara, S. and Matsuyama, S.: Phys. Plasma 9(2002) 4540.3) Shinohara, S. et al.: Trans. Fusion Technol. 39 (2001)358.4) S. Shinohara, S. et al.: Phys. Plasma 8 (2001) 1154.5) Shinohara, S.: Phys. Plasma 9 (2002) 1834.6) Shinohara, S.: Rev. Sci. Instrum. 74 (2003) 2357.

Study on Electric Field and Density Transition Phenomena in Magnetized Plasma

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