  We report numerical calculations of a dynamic pairbreaking current density
$J_d$ and a critical superfluid velocity $v_d$ in a nonequilibrium
superconductor carrying a uniform, large-amplitude ac current density
$J(t)=J_a\sin\Omega t$ with $\Omega$ well below the gap frequency $\Omega\ll
\Delta_0/\hbar$. The dependencies $J_d(\Omega,T)$ and $v_d(\Omega,T)$ near the
critical temperature $T_c$ were calculated from either the full time-dependent
nonequilibrium equations for a dirty s-wave superconductor and the
time-dependent Ginzburg-Landau (TDGL) equations for a gapped superconductor,
taking into account the GL relaxation time of the order parameter $\tau_{GL}$
and the inelastic electron-phonon relaxation time of quasiparticles $\tau_E$.
We show that both approaches give similar frequency dependencies of
$J_d(\Omega)$ and $v_d(\Omega)$ which gradually increase from their static
pairbreaking GL values $J_c$ and $v_c$ at $\Omega\tau_E\ll 1$ to $\sqrt{2}J_c$
and $\sqrt{2}v_c$ at $\Omega\tau_E\gg 1$. Here $J_d$, $v_d$ and a dynamic
superheating field at which the Meissner state becomes unstable were calculated
in two different regimes of a fixed ac current and a fixed ac superfluid
velocity induced by the applied ac magnetic field $H=H_a\sin\Omega t$ in a thin
superconducting filament or a type-II superconductor with a large GL parameter.
We also calculated a nonlinear electromagnetic response of a nonequilibrium
superconducting state, particularly a dynamic kinetic inductance and a
dissipative quasiparticle conductivity, taking into account the oscillatory
dynamics of superconducting condensate and the kinetics of quasiparticles
driven by a strong ac current. It is shown that an ac current density produces
multiple harmonics of the electric field, the amplitudes of the higher-order
harmonics diminishing as $\tau_E$ increases.
