Probing of Toroidal Electron Plasmas Confined on Helical Magnetic Surfaces

概要

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With a skillful probing system, both large negative space potential φs and low electron current Ip of helical nonneutral plasmas have been measured. For the φs measurement, a differential circuit with a high-voltage probe is employed. In order to prevent the signal from delaying due to the large output impedance of the high-voltage probe, an operational amplifier is used to reduce the output impedance to ideally zero. For the Ip measurement, an instrumentation amplifier is employed to precisely cancel the common-mode noise. In addition, to alleviate the thermal noise due to the large feedback resistance in the circuit, a trick of T-network resistance is utilized instead of the single large resistance. Furthermore, to cancel the significant ground loop current, the reference potential of the circuit is grounded across a small resistance (100Ω).
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