Electrostatic probes for measuring the boundary plasma in tokamaks are reviewed and presented. Transport properties in JFT-2, the ion temperature and the magnetic surface in JFT-2M and floating potential fluctuations during the strong additional heating in JT-60 are measured by several types of electrostatic probe the above-mentioned purposes. The Langmuir probe including the double probe is applied to measure the spatial profile of boundary plasma in JFT-2. The ion sensitive probe, the rotating cylindrical double probe, the asymmetric double probe and the differential double probe are applied to measure the ion temperature and magnetic surface in JFT-2M. The reciprocating Langmuir probe applied to JFT-2M observes the potential and density fluctuations and a new type probe is proposed for the quick diagnostic of core hot plasmas as a development of this probe. The fluctuation observed in JT-60 is identified to be the ion cyclotron instability of the hot plasma caused by the strong anisotropy of the ion distribution function.
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