An RF Coil System for the Measurement of Plasma Electrical Conductivity

摘要

An improved conductivity probe system has been designed and constructed for the measurement of plasma electrical conductivity of re-entry vehicles and for the use in conductivity measurements of simulated re-entry plasmas in the laboratory. The system involves the use of a marginal oscillator to measure the change of coil Q and inductance. Since the marginal oscillator is always on resonance and is quite sensitive to slight changes in Q, this approach is far superior to the impedance bridge method for measuring the change of coil impedance. The conductivity coil system was calibrated over a wide dynamic range of about 8 orders of magnitude of conductivity using silicon slices for calibration over the intermediate portion of the range. The operating range of the system was shown to cover the transition region where the skin depth is comparable to the soil diameter. The conductivity coil system also permits the measurement of frequency shift which itself can be calibrated in terms of the conductivity of the plasma assuming the reactive current in the plasma is small. An automatic gain control was added to the system in order to compensate for the slow changes of operating point due to temperature or aging. (Author)