Fast dielectric volume breakdown in liquid nitrogen

Abstract

Miniaturization of electrical components along with growing superconductor technology requires a better understanding of the phenomenology of breakdown in liquid nitrogen. It is known that the time delay between breakdown-onset and final impedance-limited arc current can occur within a few nanoseconds. For a temporal resolution down to several 100 ps. a discharge apparatus was built and tested that uses a cable discharge into a coaxial system with axial discharge, and a load line to simulate a matched terminating impedance. Main experiments are done in self-breakdown mode in supercooled liquid nitrogen, pulsed breakdown at high over-voltages in standard electrode geometry is investigated as well. Transmission line type current sensors and capacitive voltage dividers with fast amplifierslattenuators cover an amplitude range of 0.1 mA to 1 kA with a time resolution of 300 ps, providing complete information about discharge voltage and current. The light emission is measured with fast photomultiplier tubes (risetime 800 ps). and these optical measurements will be supplemented by high-speed photography and spectroscopic investigations on a nanosecond time scale. Preliminary results on self-breakdown in the surface flashover mode with a gap width of 2 mm and electrodes with 5 mm radius of curvature (breakdown voltage -60 kV) show a three-phase development: the current rises from an unknown level to several mA during 2 ns, stays approximately constant for 100 ns with superimposed ns-duration spikes, and shows a final exponential rise to the full impedance limited current amplitude during several nanoseconds. The detailed optical and spectroscopic diagnostics along with the high-speed electrical diagnostics will in particular address the physical mechanisms initiatinglassisting the liquid nitrogen volume breakdown, such as bubble formation during the pre-breakdown phase.A repetitive pulsed power modulator, which uses high voltage static induction thyristors as main switching devices, has been designed and constructed for the application to a discharge light source.The main components of modulator are a pulse forming network (PFN) with 20 stages, a magnetic switch with a reset circuit and a stacked semiconductor switch.The PFN consists of 100 ceramic capacitors (each capacitance is 2M)OpF. 30kV) connected in parallel. The resulted electrical length, capacitance and impedance of the PFN are 213 ns, 0.283 uF and 0.75 ohm, respectively.A current source, which provides 8 A, is used for resetting the magnetic switch. The leakage current flowing when the magnetic switch is unsaturated can be used as a pre-ionization current.The semiconductor switch is made of 3 high voltage static induction thyristors connected in series. In order to get a high di/dt, it is important to supply a high gate current with a short risetime. Therefore, an inductive storage type gate driver, which is driven by an extemal optical signal, was made to provide a very large di/dt current to the gate. The significant feature of the static induction thyristo...