K. Kitachi

K. Toi

S. "Ohdachi

T." Oike

National Institute for Fusion Science (NIFS-Repository)

§9. Modification of the Edge Plasma with Po-larized Bias Electrode Ohdachi, S., Toi, K., Kitachi, K., Oike, T. (Dept. Energy Engineering, Nagoya Univ.) The importance of the edge plasma is now rec-ognized generally. The central plasma condition such as impurities and confinement can be governed by edge conditions. For example, it is well known that the improvement of the confinement of the H-mode is achieved in the very edge region and that the performance depends on the neutral density of the edge plasma. In this fiscal year, we try to control edge plasma with polarized bias electrode. The schen1atic drawing of the bias electrode system is shown in Fig.l. A Linear motion feed-through sends a probe head deeply into the plasma from a top port of JIPP-TIIU. The probe head is made of carbon-carbon composite with 20% boron. The advantage of this material is robustness for the heat load and that it can act as getter; pumping both oxygen and hydrogen. So when head is located at in the plasma; e.g., r/a=0.85, it does not make the plasma performance worse significantly. The head and the holding rod are insulated from vacuum ves-sel. With the use of the power supply system, we can apply positive/negative voltage with respect to the vacuum vessel. Since the currents from theca-pacitor banks ( -500V) are controlled by power tran-sistor array in the power supply system, we can ap-ply arbitrary wave form of voltage to the electrode. We can check the effect of the biased electrode sys-tem efficiently. After the several trials, we have not controlled the edge plasma effectively yet. On positive bias-ing case, the edge density increases slightly and the intensity of the Ho:/ Do: light changes a bit. How-ever the main plasma does not seem to be affected. As for negative biasing case, because of the volt-age drop due to the internal resistance of the power supply, applied voltage is only several tens of volt-age. We observe very litter change. In contrast to the effect for the main plasma, the bias experiment affects the edge fluctuation drastically. Time evolu-tion of the plasma parameters in the positive bias-ing experiment is shown in Fig.2. The head of the biased electrode lies at z=20 em where several cen-timeters inner from the LCFS. The fluctuation of the SOL plasma is monitored by 5-pin Langmuir probe 176 located at the next section of the JIPP-TIIU. Mea-sured fluctuation exhibits a clear reduction when bi-asing voltage are applied though the shear of the ra-dial electric shear does not established at that area. The fluctuation level of the density and the elec-tron temperature changes from 21% to 13o/o, 28o/o to 13%, respectively. The suppression effect is more efficient in the NBI-heated plasma. We continue to study fluctuation induced transports using 5-pin Langmuir probe array and the magnetic fluctuation using magnetic pick-up coils set in the electrode it-self. A electrode inserted from outward port will be also tried to see the difference. Waveform can be cantrall 0-~00V with power transistor ~ Fig.l :Schematic drawing of trw biassed electrode. 200 Time { ms] 250 300 Fig.2:Time evolution <?f the plasma current, Ip, line averaged electron density ne, the bias voltage ·u, the bias current I, the edge density nf:, edge floating potential Vf and edge electron temperature Te monitored by triple Langmuir probe. 

Modification of the Edge Plasma with Polarized Bias Electrode

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