K. Adati

K." Toi

S. "Ohdachi

"Ohdachi,  S.

Adati,  K.

Toi,  K."

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§15. LHD Langmuir Probe System Ohdachi, S. ,Adati, K., Toi, K. Langrnuir probes have been proved to be useful tool for n1easuring edge plasn1a of Toka-nlaks. Since the edge plasma affects the per-formance of the fusion plasma for various rea-sons, it is now very irnportant to study. In L-HD, which is Heliotorn/Torsatron type device, it is not easy to access edge region frorn avail-able ports; LHD plasrna is surrounded by helical winding and insertable probe rnust go through the divertor legs (FSP in Fig.1), where probe systern can be damaged by the heat load. Fig.1 Concept of LHD probe systen1s. VVe are planning to build a Langmuir probe systern (FLP) which is fixed in the vacuun1 vessel to study the SOL region of LHD plasrna. This systern should be n1ovable if we want to avoid the darnage of the probe, since the thickness of the S 0 L plasma will be less than 10 111111 due to the the short connection length ( rv 1 m) of the LHD magnetic configuration. Magnetic field can be used for driving source there, whereas other type of drivers, e.g. n1agnetic rnotor or air cylinder, will not function in harsh circurn-stances (strong n1agnetic field rv 4 T and high ternperature frorn the radiation). A prototype of our system is shown in Fig.2. A probe head (right upper part of Fig. 2) is supported by a coil shaped frame. The j x B force from the cur-rent flows in this coil rotates the systern. If we control the current properly, we can control the position of the probe head. A prirnitive calcu-lation of the needed current when we swing the pro be by 30 degrees ( rv 10 Clll at pro be head) is 136 shown in Fig. 3. This calculation is based on the standard magnetic field in the 3 T operation. It is shown that we only need 20 A at rnaxirnurn if we want to insert/pull out the probe systern within 100 ms. This value is not difficult to be realized. Developrnents of the current controller with feed back and the test of the whole system is scheduled in 1998. Fig.2: Prototype of LHD Langmuir probe. 90 Q) e(t) --Q) 60 lo... 0) Q) ~ 30 <:D 0 20 2 .......... <t: 10 E z c 0 Q) lo... o'Q;' ::J lo... -10 ::J o-lo... (.) -20 0 -21-0 2 3 4 5 6 time [ s] Fig.3: Angle of probe and needed currents for this system. 

LHD Langmuir Probe System

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