Plasma based helical undulator for controlled emission of circularly and   elliptically polarised betatron radiationJ. Vieira, J. Martins, U. Sinha
  We explore a plasma based analogue of a helical undulator capable of providing circularly and elliptically polarised betatron radiation. We focus on ionisation injection configurations and in the conditions where the laser pulse driver can force collective betatron oscillations over the whole trapped electron bunch. With an analytical model and by employing three dimensional simulations and radiation calculations, we find that circularly or elliptically polarised laser drivers can force helical betatron oscillations, which produce circularly/elliptically polarised betatron x-rays. We assess the level of polarisation numerically and analytically, and find that the number of circularly polarised photons can be controlled by tuning the laser pulse driver polarisation. We show the production of betatron radiation that is circularly polarised up to < 40% close to regions of maximum photon flux. The total flux of circularly polarised betatron radiation drops for elliptically polarised drivers, and is negligible when using linear polarisation. Our results can be tested today in current experimental facilities. 
J. Vieira, J. Martins, U. Sinha
physics.plasm-ph physics.acc-ph