A. Tsushima

K. Nagaoka

M. Tanaka

National Institute for Fusion Science (NIFS-Repository)

§8. Optimization of Directional Probes for Plasma Flow Measurements Tanaka, M., Nagaoka, K. Tsushima, A. (Yokohama National Univ.) Plasma flow measurements have been one of important issues related to electric field and particle transport in the boundary layers of fusion-oriented devices. In a complicated mag-netic field structure of LHD, plasma flow may change drastically in different places and be in-clined to magnetic field because of strong mag-netic shear and electric field. The purpose of this study is to investi-gate the feasibility of a directional probe for measuring plasma flow across a magnetic field. Specifically, the azimuthal drift veloc-ity v DRIFT is obtained from the ion saturation current ratio lsu / IsD of the two opposite direc-tions of the directional probe using the equa-tion from the free-fall theory[l] VDRIFT = K'ln (lsu) (1) Cs IsD ' where l{' is a constant and Cs = jkBTe/mi. This velocity VDRIFT is then compared with the azimuthal velocit"y Vo estimated from the radial potential distribution measured by an emissive probe, because the azimuthal ion ve-locity is caused by the radial electric fielq E with the axial magnetic field B [2] rWei ( Vo = -2- -1 + (2) Here, the diamagnetic ion drift is neglected because an ion temperature is very low and Wei = eB/mi and WExB = -E/(rB). Figure 1 shows the comparison between VDRIFT and Vo of an argon plasma produced in HYPER-I with a 2.4 GHz microwave power of 9.6 kW and a gas pressure of 2 x 10-4 Torr. Here, the solid curve represents VDRIFT as a function of the radius x (nun) from eq.(l) with the measured lau / laD and K' = 1.1. The closed circles represent Vo from eq.(2), where the ambiguity of Vo is due to that of the gradi-ent of the potential distribution at each posi-tion x. As shown in this figure, the agreement 262 between the two velocities is excellent and the plasma rotates counterclockwise on the axis because VDRIFT > 0 for x > 0 and VDRIFT < 0 for x < o. Under the above condition that the electrode radius a of the directional probe is smaller than the ion gyro radius Pi (a < Pi), the free-fall theory, that is eq.(l), can be used. However, in the fusion-oriented machine with a very strong magnetic field, there is no theory to be applied in the case of a » Pi so far. By using a helium plasma with a ~ Pi, thereby, how the ion satu-ration current depends on the angle relative to the magnetic field direction, that is ls(f)), was also investigated and it was found that the ra-tio In[ls(O+1r)/ Is(O)] varied sinusoidally in the same manner for a < Pi. The study of Is(O) in the case of a » Pi is to be studied in the future. 5xl0' r-r--.---.---,--.--.,---.--.,.--.---.--T-""--r--..--.---...---. 4xl0' ~ 1x10' E ~ 0 ... ~-1X10s > -2x10' -3x10s -4x10s 2x10" Torr. Ar 9.6kW. N1-115A -5x 1 05 '--'-~--'----'---'----L---,----'--"""",,---.J..--"""--..I..--,,---L-.o.....--I -20 0 20 40 60 80 100 120 X (mm) Fig. 1 Comparision between VDRIFT (solid curve) and Vo (closed circles) Reference 1) Hudis, M., and Lidsky, L. M., J. Appl. Phys. Lett. 41 (1970) 5011 2) Tsushima A. et aI., Phys. Rev. Let. 56 (1986) 1815 

Optimization of Directional Probes for Plasma Flow Measurements

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