Characteristics of the Plasma Impedance Probe with Applied DC Bias

Summary form only given. The impedance of a small spherical probe immersed in a uniform plasma is measured by recording the reflection coefficient of an applied signal using a network analyzer. This impedance has a resonance at the plasma frequency where the imaginary part goes to zero, a feature that has made this measurement a good way of determining electron density. When the sphere is biased negatively with respect to the plasma potential a second resonance occurs at omega<omega_p due to the depleted electron density in the sheath around the probe. A greatly increased power deposition occurs at this lower resonance, whose frequency can be controlled by changing the sheath width using a DC bias. As the bias is increased the value of this frequency becomes smaller until the resonance disappears completely at V_probe=V_plasma. As the bias is further increased past the plasma potential an electron sheath forms with its own resonance which is at a lower frequency than the resonance associated with the ion sheath. The impedance of the electron sheath can be approximated using the Llewellyn-Peterson formulas for a space charge limited diode. As with the ion sheath resonance, the largest energy deposition occurs at the lower of the two frequencies.