  The use of an external transverse magnetic field to trigger and to control
electron self-injection in laser- and particle-beam driven wakefield
accelerators is examined analytically and through full-scale particle-in-cell
simulations. A magnetic field can relax the injection threshold and can be used
to control main output beam features such as charge, energy, and transverse
dynamics in the ion channel associated with the plasma blowout. It is shown
that this mechanism could be studied using state-of-the-art magnetic fields in
next generation plasma accelerator experiments.
