The structure and stability of the ionization front, which occurs as a high intensity electron beam propagates through an insulator, are considered. It is found that, due to the electric field ionization, the velocity of the front, Vf, has a nonmonotonic dependence on the beam density, nb, and in some particular beam density range Vf increases with increasing nb. Two instabilities of the ionization front associated with the electric field ionization process of the insulator are found: a long wavelength (⪞fewμ) and relatively slow (∼1013s−1) corrugation instability and a short wavelength (∼subμ) and relatively fast (∼few×1013s−1) electric field ionization instability.

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The structure of the ionization front created by an expanding high-intensity electron beam as it travels through ∼1atm gas is considered theoretically. The velocity of the front, Vf, on the order of a few 10∧9cm∕s, is determined by the electric field ionization process. When decreasing the gas density, the velocity Vf decreases relatively slowly up to some threshold, after which it starts rapidly falling. The magnitude of Vf is higher for argon than for helium. These findings are supported by experimental observations.

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The ionization front induced by a relativistic high-density electron beam in a gas was studied in a one-dimensional (1D) approximation. The expression for the beam density corresponding to a steady front propagation was found in the limit of large (∼c) and small (⪡c) front velocities. The corresponding expression for front velocity was also found and the validity of the approximations used was verified. Finally, the values of front velocity and beam density were calculated for a wide range of beam energies and gas densities. The calculated values of the front velocity are in good agreement with the experimental data [D. Batani, S. D. Baton, M. Manclossi et al., Phys. Rev. Lett. 94, 055004 (2005).].