including those studied here, increases with temperature. The
observed increasing impedance suggests that the solution conductivity is decreasing, which could be due to a local reduction in ionic concentration in the solution near the electrodes
due to extraction of hydrated ionic components Na (H O) and
Cl (H O) to the electrode surfaces at high voltages. The extracted ions form a double layer on the electrode. The reduced
local ion concentration near the electrode would explain the positive slope of the impedance curve at low voltage.
Table I and Fig. 5 both show clearly that a transition in the
single-electrode impedance occurs when the output voltage
exceeds about 175 V. Before the transition, relatively large
power is dissipated in the saline solution, between the active
electrode and the current return electrode, increasing linearly
with power setting. The specific power density (in watts per
cubic centimeter of saline, or watts per square centimeter of
electrode area) remains relatively low in this mode. Above
an output voltage of 175 V, luminous plasma discharges are
observed, and the total power dissipated is reduced, which is
a consequence of the increased impedance of the vapor layer
formed over the surface of the probe. Although the power
dissipated is reduced at higher voltages, the specific power
density dissipated in the thin plasma vapor layer must be higher