Negative ion based neutral beam injection (N-NBI) systems which produce powerful high energy hydrogen atom beams are currently used on fusion reactor devices for plasma heating and current drive. To produce sufficient negative ions (NIs), sources with a large surface area are needed for reactor relevant injectors. Plasma source non-uniformity is a recurring issue in the present N-NBI systems, it is mainly due to the transverse magnetic field (filter field) at the front of the source which generates plasma gradients. This paper presents experimental results on a large size arc driven ion source at CEA, called Cybele, the purpose of the experiment is to compare the plasma density distribution (uniformity) between two magnetic confinements, (1) the transverse (horizontal) magnetic filter which is implemented in the present negative ion (NI) sources and (2) a new concept based on the magnetic mirror principle. Experiments show that in Cybele the standard configuration with the horizontal filter field (1) leads to a significant vertical plasma drift with a measured plasma density seven times higher at the top of the source, while the source with the vertical field and mirror-like confinement (2) is vertically uniform within ±10% and can operate at very low filling pressure (0.1 Pa).