to account for the observed optical emission. That is, most of
the discharge power is dissipated in a much smaller volume
when the vapor layer and plasma are formed compared to the
low-voltage ohmic conduction mode.
Above this threshold voltage, which is characteristic of a
given probe configuration as well as saline concentration, the
impedance rapidly increases, accompanied by the formation
of a vapor layer as well as plasma breakdowns that cause a
luminous discharge to be seen. At higher voltages, the voltage
and current waveforms are more complex, reflecting the
combined nonlinear nature of the plasma impedance as well
as the saline impedance. During portions of the waveforms
when both current and voltage are reasonably constant, the
impedance is largely reflective of local equilibrium conditions,
where the ohmic dissipation by the discharge is balanced by
heat conduction and other loss processes. Since the average
slope of the impedance versus voltage increases when strong
driving voltages are used, this indicates that the high impedance
of a vapor layer is not completely compensated by the high
conductivity of the gaseous ions produced by electron-impact
processes.
In multielectrode probe configurations, each electrode operates more or less independently of the others. The plasma discharges from separate electrodes do not fire on all cycles but
fire with a duty cycle that increases with voltage. To a large extent, electrode position in the probe also determines how intense
the discharges are. At low voltages, well below the breakdown
threshold, the total dissipated power is well correlated with the
power from each electrode multiplied by the number of electrodes. At higher voltages, the variability of vapor layer dynamics and intermittent firing of individual electrodes causes
the total dissipated power to fluctuate by a factor of approximately three. This behavior is illustrated in Fig. 6, which shows
the maximum and minimum power dissipated by a probe operating in isotonic saline at each voltage setting. At the highest
voltage available, some electrodes fire on nearly every cycle, but
the total power is reduced from the power dissipated at intermediate voltages.
In many surgical procedures, the electrodes are placed in
close proximity to, or physically touching, tissue or other