Subharmonic excitation of the eigenmodes of charged particles in a Penning trap G. Tommaseo, P. Paasche, C. Angelescu and G. Werth Eur. Phys. J. D, 28 1 (2004) 39-48 Published online: 02 December 2003 DOI: 10.1140/epjd/e2003-00296-0