A Monitor of the Focusing Strength of Plasma Lenses Using MeV   Synchrotron Radiation
  The focusing strength of plasma lenses used with high energy electron or positron beams can give rise to synchrotron radiation with critical energies in the MeV range. A method is described for measuring the characteristic energy of this radiation as a way of monitoring the strength of the focus. The principle has been implemented in a plasma lens experiment with a 28.5 GeV positron beam. 
Clive Field, Gholam Mazaheri and Johnny S.T. Ng