Betatron Radiation from an Off-axis Electron Beam in the Plasma Wakefield Accelerator

In a plasma wakefield accelerator in the nonlinear regime the accelerated bunch electrons oscillate in a pure ion column. They emit strong synchrotron, also known as betatron radiation, in the keV to MeV photon range [1]. In previous experiments with a single electron bunch the oscillating electrons where distributed symmetrically about the beam axis or the ion column axis. However, with a drive/witness bunch system, the witness bunch can be injected into the ion column with a transverse momentum component or with a radial offset. In this case the witness bunch oscillates about the beam axis resulting stronger radiation. Since the ultra-relativistic bunches do not suffer dephasing, the energy loss to radiation can be compensated for by the energy gain from the wakefield. We explore the characteristics of the witness bunch oscillations and of the betatron radiation through numerical simulations and calculations.