  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.
