Efficiency of spark discharge in electrohydraulic lithotripsy.

Abstract

Electrohydraulic lithotripters and SWT devices generate shock waves by discharge of a high-voltage capacitor through submerged electrodes. As the electrodes age, the interelectrode gap widens. How this affects the efficiency of spark generation was studied using a research HM3-clone lithotripter. Widening of the interelectrode gap (∼0.3 mm with new electrodes; ∼2.5 mm after 4000 discharges) increased the lag-time to breakdown (∼0 to ∼30 μs, respectively). Increased lag-time as electrodes aged was associated with partial discharge of the capacitor (leakage promoted by ∼0.6 mS conductivity of the surrounding water), such that the average energy remaining at the capacitor at the moment of breakdown was reduced four times compared to new electrodes. However, with new electrodes almost 90% of the energy was lost in the circuitry rather than in the spark, as the resistance of the spark (R∼0.03 Ω) was much smaller than the resistance of the remainder of the circuit—including the high-voltage switch and connective wires (r∼0.23 Ω). With old electrodes, most of the energy was released in the spark, as the electric efficiency R/(R+r) approached ∼90%. The product of electric efficiency and capacitor energy agreed closely with both the acoustic energy and the volume of the primary bubble produced at spark discharge. [Work supported by NIH-DK43881.]