Pre-breakdown phase of coplanar dielectric barrier discharge in helium Tomáš Morávek, Jan Čech, Zdeněk Navrátil and Jozef Ráhel' Eur. Phys. J. Appl. Phys., 75 2 (2016) 24706 Published online: 08 August 2016 DOI: 10.1051/epjap/2016150538