Ion phase space vortices in 3 spatial dimensions L. K. S. Daldorff, P. Guio, S. Børve, H. L. Pécseli and J. Trulsen Europhys. Lett., 54 2 (2001) 161-167 Published online: 01 December 2003 DOI: 10.1209/epl/i2001-00290-6