Study on the electrostatic potential in the formation of closed magnetic configuration

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In the Low Aspect-ratio Torus Experiment (LATE) device, there observed a spontaneous formation of closed configuration of magnetic field when a electron cyclotron (EC) resonance plasma is produced under a weak vertical field superimposed on a toroidal field. A heavy ion beam probe (HIBP) system has been developed to measure the electrostatic potential which may play an important role on the transition of the magnetic topology. It is observed at the first time that the electrostatic potential is negative at -20 ~ -30V on the midplane near the EC resonance layer when the closed magnetic field configuration is formed. The result is completely different from the open field configuration case where the electrostatic potential is positive in the entire region. This result suggests that the electrostatic potential changes drastically through the transition from the open field configuration to the closed one.