Probing of Toroidal Electron Plasmas Confined on Helical Magnetic Surfaces

抄録

With a skillful probing system, both large negative space potential φ_s and low electron current I_p of helical nonneutral plasmas have been measured. For the φ_s 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 I_p 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).