Efficient teraherz radiation from electrostatic wakefields in non-uniform plasmas
2021
Abstract It is shown that electrostatic plasma wakefields can efficiently
radiate at harmonics of the plasma frequency when the plasma has a positive
density gradient along the propagation direction of a driver. The driver
propagating at a sub-luminal group velocity excites the plasma wakefield with
the same phase velocity. However, due to the positive density gradient, the
wake phase velocity steadily increases behind the driver. As soon as the phase
velocity becomes super-luminal, the electrostatic wakefield couples efficiently
to radiative electromagnetic modes. The period of time when the phase velocity
stays above the speed of light depends on the density gradient scale length.
The wake radiates at harmonics of the plasma frequency in the teraherz (THz)
band. The angle of emission depends on the gradient scale and the time passed
behind the driver. For appropriate plasma and driver parameters, the wake can
radiate away nearly all its energy, which potentially results in an efficient,
narrow band and tunable source of THz radiation.
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