Self-defocusing/focusing of a relativistic laser pulse in a multiple-ionizing gas Naveen Kumar, V. K. Tripathi and B. K. Sawhney Eur. Phys. J. D, 32 1 (2005) 63-68 Published online: 14 December 2004 DOI: 10.1140/epjd/e2004-00184-1