Calibration of a Laboratory Plasma Impedance Probe

Erik Mateo Tejero^*George Robert GatlingMatthew Crawford Paliwoda

• Naval Research Laboratory, Washington D.C., United States

This work describes the calibration of a laboratory plasma impedance probe, a diagnostic to accurately measure the plasma density using resonances in the self-impedance spectrum that are introduced when the probe is immersed in a plasma. These probes have been used on sounding rockets and satellites to measure ionospheric densities (approximately 10 4 -10 6 cm -3 ), which has kept the resonant frequencies below 15 MHz. However, this paper focuses on calibration techniques that are essential for typical laboratory plasmas with resonant frequencies above 100 MHz, which corresponds to plasma densities above 10 8 cm -3 and have been used for plasma densities from 10 5 to 10 10 cm -3 that are found in typical laboratory plasmas. Parasitic impedances in the measurement circuit move the location of the measured resonant frequency and can lead to large errors if not calibrated appropriately. The approach uses a calibration circuit included in the measurement circuitry, along with careful characterization of the RF paths after the calibration plane to account for these parasitic impedances. The calibration algorithm is derived and an example step-by-step calibration is presented. Calibration procedures are validated with test loads, numerical simulations, and theoretical models.