A New Probefor Measuring small Electric Fields In Plasmas

A dipole double probe has been developed for measuring small electric fields in laboratory plasmas. The probe measures dc to ac electric fields (0 ≤ ⨍ ≤ 20 MHz) with high sensitivity (E
_min
≈ 10 μV/cm) and responds to both space-charge (E
_sc
= - ∇ φ
_p
) and inductive (E; = - ∂ A/ ∂t) electric fields. The novel feature of the probe consists of a voltage-to-frequency converter composed of a varactor diode and microwave resonator near the dipole terminals. The signal conversion circumvents two problems associated with the transmission line connected to the dipole: first, errors due to induced voltages along the line are eliminated and, second, the capacitive loading of the dipole by the line is avoided. These improvements result in high sensitivity and fast time response. Various examples of useful applications for the new probe are presented, such as measurements of (i) de ambipolar fields, (ii) ac space charge fields of ion acoustic waves, (iii) ac inductive fields of whistler waves, and (iv) inductive and space-charge electric fields in pulsed currents systems in magnetized plasmas.