Focusing of 28.5 GeV electron and positron beams in meter-long plasmas

Plasmas can focus both electron and positron beams. However, while the incoming Gaussian transverse profile of an electron beam is preserved upon focusing, the focusing of a positron beam by a long, high-density plasma results in a tightly focused core with the formation of a charge halo. The plasma acts on the positron beam as focusing element with transverse and longitudinal aberrations. The analysis of the focused positron beam transverse profiles is presented here.