Observation of weakly and strongly diverging ion beams in a magnetically expanding plasma

The spatial distribution of an ion beam created in a magnetically expanding plasma using permanent magnets is experimentally investigated for 0.35 and 1 mTorr, where the magnetic-field strength is about 100 G in the plasma source and is decreasing into a few gauss in the diffusion chamber. The beam profile for 0.35 mTorr is weakly divergent. On the other hand, the strongly diverging beam is detected for 1 mTorr. The results are discussed from the viewpoint of the plasma-potential structures and imply the beam divergence caused by the radial electric fields in the ion acceleration region and the diffusion chamber.