K. Sugimori

K. Tsumori

O. Kaneko

S. Shinohara

National Institute for Fusion Science (NIFS-Repository)

(a) (3 mTorr, - SO V)Dependences of ne, plasma flow (x direction, which isparallel (perpendicular) to the plate surface (magneticfield» and floating potential Vr on the biased voltage Vbwere investigated. Figure 1 shows a typical example ofradial profiles of ion saturation current I is, changing Vb.For the positively (negatively) biased case, plasma densitymoved to the positive (negative) x direction. Figure 2shows radial profiles of I is, changing the distance ybetween the plate and a probe. This asymmetry becameweaker with increasing a distance (y direction) from theplate. With the increase in the magnetic field and/or thedecrease in the pressure, this tendency became stronger. Inaddition, the density distribution was inverted by changinga polarity of the magnetic field.Applying the positively (negatively) biased voltagewith the magnetic field along the positive z direction, theplasma flowed to the positive (negative) x direction. Thisflow velocity was larger with the lower pressure andhigher magnetic field and also near the plate. The effect ofthe earthed plate was found to be small.In conclusion, magnetized plasma distribution wasinvestigated by applying a biased voltage on the metalplate. The obtained results could be understood by the EX B drift based on the normalized Larmor radius andcollision time. In order to develop a large area ion source,we must consider the electric field near the boundary.6 em0.20-..'l".:'I 0.15of0.10'-"_.l!l0.05(a) (ImTorr,1em)Improvement of spatial homogeneity of the largearea plasma production is crucial, in the high powerneutral beam injection system (NBI) utilizing negativeions developed in NIFS. The inhomogeneity issupposed to be partly due to the magnetic field andelectric field induced by the plasma potential.Nevertheless, the effect of the electric field on theplasma performance has not been investigated in detail.Here, we examined how the plasma density and bulkplasma flow changed in a magnetized plasma byapplying a voltage on the metal plate in order todevelop to a future large area ion source.The experiment was performed using Ar gasplasma (45 cm in diameter and 170 cm in axial length),produced by RF using a four-tum antenna, with apressure of P = 0.3 - 3 mTorr and the axial magneticfield B = 500 G 1,2) electron density ne andtemperature Te were < 10 0 cm-3, 3 - 8 eY, respectively. Avoltage biased plate was made of stainless steel (20 cmX 20 cm, with 0.1 cm thickness) with an insulator plateon one side. Near the bias plate, semi-circular earthedmetal plate was also located to see the boundary effects.Plasma parameters were measured by the Langmuirprobes including the Mach probe for the plasma flowmeasurement. 3)§ 20. Effects of Metal Wall and Electric Fieldon Density Profile in Large Area PlasmaSourceShinohara, S., Sugimori, K. (Interdis. Grad. Sch. Eng.Sci., Kyushu Univ.)Kaneko, 0., Tsumori, K.0.25(b)0.20-..,iii 0.15!5lof0.10'-"_.l!l0.050.00-20 -15 -10 -5 0 5 10 15 20x (em)Fig. 2. Radial profiles of ion saturation current (P = 3mTorr)References1) Matsuyama, S., Shinohara, S. and Kaneko, 0.: Trans.Fusion Technol. 39 (2001) 362.2) Shinohara, S., Matsuyama, S., and Kaneko, 0.: ThinSolid Films 407 (2002) 209.3) Shinohara, S.: Rev. Sci. Instrum. 74 (2003) 2357.0.00 .o...L.r. ..........SOV5 10 15 200.00 ...................a................a..1..I...........a...L..a...............1..L.I..........1..&..&."""""...................J-20 -15 -10 -5 00.15E'1of 0.100.050.000.200.15.-.'j.;:. 0.10-!-.!::l1-40.05(b) 25 V _- __ (1 mTorr, 2.5 em)V \r _,/. /' -:-:-.-25 V '/ " ',\' , -I '\ .\.I/;; ,' / '\ , \ ' ---- -';' " i \ \ "- ----.... '),; ,'1.\ \ '-/){ .... / ":,,,- " so V -- --... ; ,: . . . . V = -50Vbx (em)Fig. 1. Radial profiles of ion saturation current(P= lmTorr)147

Effects of Metal Wall and Electric Field on Density Profile in Large Area Plasma Source

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