A planar Penning trapS. Stahl, F. Galve, J. Alonso, S. Djekic, W. Quint, T. Valenzuela, J. Verdú, M. Vogel and G. WerthEur. Phys. J. D, 32 1 (2005) 139-146DOI: https://doi.org/10.1140/epjd/e2004-00179-x