Investigation of electron cut-off in a planar diode by the magnetic field of a planar inductor

Abstract

Abstract—In this paper, the electron cutoff is investigated in a flat diode  by the magnetic field of an external planar inductor , using vacuum switches (VS) with flat contacts as an example. The VS are chosen for research because of  the high demand for these devices in modern high-voltage power engineering. They outperform the high-voltage circuit breakers with gas and oil insulation in terms of mass-dimension and operational parameters. However, it is necessary to suppress dangerous X-ray radiation from high-voltage VS, which is generated as a result of bombardment of contact with the positive potential by electrons accelerated by the voltage applied to the contacts. Free electrons are formed in the VS during their operation due to field emission, when the distance between the contacts decreases and the electric field strength increases greatly. To prevent electronic bombardment, the cutoff effect (blocking the electron current to the anode) is used, and for its creation a radial magnetic field of the inductor is used in the form of a flat spiral located on the inoperative side of one of the contacts. The crossed fields, the axial electric field and the radial magnetic field with induction which are greater than the critical value, strongly bend the trajectories of the electrons and prevent them from bombarding contact with the positive potential. Because of the difficulties of experimental investigation of this problem, it was performed on the VS model. For this purpose, a physical topological model of VS was developed, based on a self-consistent solution of the fundamental equations that determine the electric and magnetic fields between contacts, taking into account the geometry of VS, the conductivity of the contact material and the secondary magnetic field generated by the eddy currents induced in the contact, as well as the frequency of the pulsed supply of the inductor. Calculations were made of electron trajectories emitted by a contact with a negative potential, and for those electrons that do not reach the opposite contact, the critical current of the inductor is determined, depending on the geometric parameters and operational mode. When calculating the absence or presence of one or more radial slits in contact under the inductor was taken into account. VS with a split contact is recommended. Edge effects were found at the edges of the contacts, where the orthogonality of the electric and magnetic fields is disturbed, and therefore it is proposed further to optimize the configuration of the contacts. It was suggested that the radial magnetic field of a flat inductor would be useful for cutting off electrons during the deionization of a vacuum arc plasma during the opening of contacts and for reducing the thermal effect of bombarding electrons on a positive contact. The developed model can also be used to analyse the conditions for magnetic initiation of a magnetron discharge between plane electrodes in gas discharge devices and sources of charged particles.

Ref. 13, fig. 7, tabl. 2.