A high-pressure, flowing liquid dielectric pulse-forming line

Liquid dielectrics as switching media are of special interest because of their higher breakdown field strength than that of compressed gases, especially in the case of ultrafast nanosecond charging, better thermal management, and removal of eroded electrode material while circulating through the switch. The pulse-forming line (PFL) presented here is designed to operate together with our own Marx generators. It is comprised of a peaking switch with, compared with the previous design, an increased high-pressure operability up to 4000 kPa and an additional annular crowbar electrode. The geometry of the peaking switch electrodes was optimized with computational fluid dynamics simulations, in order to ensure a laminar liquid flow through the switch with flow rates up to 55 mL/s. Galden-type HT270 perfluoropolyether fluid was chosen as the liquid dielectric. The breakdown field strength showed a significant pressure dependency, which is due to the high gas solubility of Galden. At a pressure of 3950 kPa and a gap distance of 0.25 mm, a maximum breakdown field strength of 14.3 MV/cm has been reported, an increase of 360% compared with the breakdown field strength of 3.8 MV/cm at atmospheric pressure. The minimum peaking switch rise time was 220 ps; the corresponding peak voltage rise time amounted to 1.2×10¹⁵ V/s. With the addition of the crowbar electrode, an ultrashort monopolar output pulse with a characteristic pulsewidth of 210 ps was achieved. First, the results of a repetitive operation of the PFL driven by an inductive Marx generator will be discussed.