  An optical vortex is a light wave with a twisting wavefront around its
propagation axis and null intensity in the beam center. Its unique spatial
structure of field lends itself to a broad range of applications, including
optical communication, quantum information, superresolution microscopy, and
multi-dimensional manipulation of particles. However, accessible intensity of
optical vortices have been limited to material ionization threshold. This
limitation might be removed by using the plasma medium. Here we propose the
design of suitably magnetized plasmas which, functioning as a q-plate, leads to
a direct convertion from a high-intensity Gaussian beam into a twisted beam. A
circularly polarized laser beam in the plasma accumulates an
azimuthal-angle-dependent phase shift and hence forms a twisting wavefront. Our
three-dimensional particle-in-cell simulations demonstrate extremely high power
conversion efficiency. The plasma q-plate can work in a large range of
frequencies spanning from terahertz to the optical domain.
