Design and performance of a high pressure, flowing liquid dielectric peaking switch

A repetitive, high-voltage liquid dielectric switch for high-power microwave (HPM) applications is currently being developed and investigated at the French-German Research Institute of Saint-Louis (ISL). The switch was designed to be implemented in existing coaxial pulse line structures and is being driven by compact modular Marx generators at rise times of about 7 ns and output voltages up to 400 kV. A pumping system consisting of a gear pump and a piston diaphragm pump was installed, which allowed the operation of the switch at flow rates up to 100 ml/s and pressures up to 1800 kPa. Galden, a perfluoropolyether, was chosen as the liquid dielectric. Computational fluid dynamics (CFD) simulations of liquid flow through the switch were carried out. The results led to optimized electrode geometry with an asymmetric radial inward flow between the concave copper-tungsten electrodes and an axial outward flow through a bore hole inside the earth electrode, in order to minimize the risk of a turbulent flow through the switch. The measurements of the achievable breakdown field strength showed a significant dependency on the pressure and on the type of the Galden fluid used. A Galden fluid with a high boiling temperature is to be preferred, as it has the least tendency to contain micro gas bubbles. Maximum breakdown field strength of 10.1 MV/cm at a pressure of 2050 kPa and at a gap distance of 0.28 mm is reported using Galden HT270. This represents an increase of 270% in comparison to the breakdown field strength of 3.8 MV/cm at atmospheric pressure. The typical switch rise time was 0.49 ns; the achieved voltage rise time was 5.6×10¹⁴ V/s. Initial two-pulse experiments were carried out to investigate the achievable pulse repetition rate by charging the switch directly with a high voltage capacitor charger at a charging voltage of 50 kV and a charging time of 210 μs. The measured 90% recovery time of 4.0 ms at a pressure of 320 kPa shows the poten- ial of the liquid dielectric switch to operate at frequencies above 200 Hz.