  With increasing laser peak power, the generation and manipulation of
high-power laser pulses becomes a growing challenge for conventional
solid-state optics due to their limited damage threshold. As a result,
plasma-based optical components which can sustain extremely high fields are
attracting increasing interest. Here, we propose a type of plasma waveplate
based on magneto-optical birefringence under a transverse magnetic field, which
can work under extremely high laser power. Importantly, this waveplate can
simultaneously alter the polarization state and boost the peak laser power. It
is demonstrated numerically that an initially linearly polarized laser pulse
with 5 petawatt peak power can be converted into a circularly polarized pulse
with a peak power higher than 10 petawatts by such a waveplate with a
centimeter-scale diameter. The energy conversion efficiency of the polarization
transformation is about $98\%$. The necessary waveplate thickness is shown to
scale inversely with plasma electron density $n_e$ and the square of magnetic
field $B_0$, and it is about 1 cm for $n_e=3\times 10^{20}$ cm$^{-3}$ and
$B_0=100$ T. The proposed plasma waveplate and other plasma-based optical
components can play a critical role for the effective utilization of
multi-petawatt laser systems.
