Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Efficient narrowband teraherz radiation from electrostatic wakefields in
  non-uniform plasmas   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.

20
The categories of this paper are: acc-phys
31
The categories of this paper are: ao-sci
22
The categories of this paper are: chao-dyn
7
The categories of this paper are: hep-th
6
The categories of this paper are: astro-ph
8
The categories of this paper are: hep-ex
17
The categories of this paper are: q-fin
28
The categories of this paper are: alg-geom
13
The categories of this paper are: quant-ph
21
The categories of this paper are: adap-org
5
The categories of this paper are: gr-qc
14
The categories of this paper are: math-ph
10
The categories of this paper are: q-bio
36
The categories of this paper are: cmp-lg
30
The categories of this paper are: q-alg
27
The categories of this paper are: comp-gas