Pre-breakdown processes in a dielectric fluid in inhomogeneous pulsed electric fields

Abstract

We consider the development of pre-breakdown cavitation nanopores appearing in the dielectric fluid under the influence of the electrostrictive stresses in the inhomogeneous pulsed electric field. It is shown that three characteristic regions can be distinguished near the needle electrode. In the first region, where the electric field gradient is greatest, the cavitation nanopores, occurring during the voltage nanosecond pulse, may grow to the size at which an electron accelerated by the field inside the pores can acquire enough energy for excitation and ionization of the liquid on the opposite pore wall, i.e., the breakdown conditions are satisfied. In the second region, the negative pressure caused by the electrostriction is large enough for the cavitation initiation (which can be registered by optical methods), but, during the voltage pulse, the pores do not reach the size at which the potential difference across their borders becomes sufficient for ionization or excitation of water molecules. And, in the third, the development of cavitation is impossible, due to an insufficient level of the negative pressure: in this area, the spontaneously occurring micropores do not grow and collapse under the influence of surface tension forces. This paper discusses the expansion dynamics of the cavitation pores and their most probable shape. I. Introduction.A mechanism for the rapid breakdown in the fluid, associated with the occurrence of cavitation ruptures under the influence of the electrostrictive forces near the needle electrode, was proposed in [1]. Later, a hydrodynamic model of compressible fluid motion under the influence of the ponderomotive electrostrictive forces in a non-uniform time dependent electric field was suggested in [2]. As shown in [2], if the voltage on the needle electrode is growing fast enough (a few nanoseconds), the negative stress is created in liquid, and it may be sufficient for the cavitation formation. A nanosecond breakdown, the beginning of which can be explained by the formation of cavitation in a stretched liquid due to electrostriction, was investigated experimentally in [3][4][5][6][7]. In [8], based on a theoretical model [2], it had been shown experimentally that the initial stage of development of a nanosecond breakdown in liquids is associated with the appearance of discontinuities in the liquid (cavitation) under the influence of electrostriction forces. The comparison of the experimentally measured area dimensions and its temporal development was found to be in a good agreement with the theoretical calculations. A theory of the cavitation initiation in inhomogeneous pulsed electric field was developed in [9], as well as the method that allows to determine the critical parameters, at which cavitation begins, on the basis of the comparison between the experiment and the simulation results within the framework of hydrodynamics of compressible fluids, was proposed.

Initial stage of cavitation in liquids and its observation by Rayleigh scattering

“…. Note that for the time of the order of tens of nanoseconds, pores can grow into much larger sizes (the rate of the expansion of nanopores is about 100-300 m/s [31,32]), but we disregard this fact for simplicity. If, in the process of growth, the size of the pores reaches the order of the laser wavelength, then the scattering ceases to be isotropic Rayleigh and becomes anisotropic Mie scattering (e.g.…”

Pre-breakdown cavitation nanopores in the dielectric fluid in the inhomogeneous, pulsed electric fields

“…It is seen that the pore in water as well as in transformer oil is stretched along the electric field, since the pressure at its poles is greater than the pressure at the equator. In the approximation of a spherical pore, from the equations (1) and (2) and after averaging over the angle of all the forces acting on the surface, we obtain the equation for the radius of the expanding pores [28]: It can be seen that the rate of expansion of the pores reaches hundreds of meters per second, which on one hand they are almost by two orders of magnitude greater than the velocity of the fluid near the electrode [19], but on the other they are much smaller than the velocity of sound and reach the size of about 10 nanometers within a few hundreds of nanoseconds. Thus, for accepted values of "external" field 0 E , the potential difference at the poles of pores reaches or exceeds the ionization potential, at 5 ≈ R and at 14 nm, for water and oil, respectively.…”

“…In the paper [28], the estimate of the rate of the nanopores expansion is made, and it was shown that three characteristic regions are arising in the vicinity of a needle electrode. In the first region, where the electric field gradient is the greatest, the occurring cavitation nanopores have enough time to grow to a size at which an electron can gain enough energy for the excitation and ionization of the liquid molecules on the pore wall.…”

Rayleigh scattering on the cavitation region emerging in liquids

“…A method to determine the critical parameters at which cavitation begins on the basis of the comparison between the experiment and the simulation results within the framework of hydrodynamics of compressible fluids was also proposed. The theory of nanopore generation and expansion in fluids under the influence of nanosecond pulsed electric fields was proposed and developed in [15,16].This paper shows that the Rayleigh scattering off nanopores, emerging from the negative pressure regions of the liquid, can be used to detect cavities earlier in their development than other optical methods.…”

“…It was shown in [15,16] that the average negative pressure acting on the surface of spherical nanopores in the vicinity of a needle-like electrode in water is:…”