  We provide constraints on the nature of particle bunches that power fast
radio bursts (FRBs) in the coherent curvature radiation model. It has been
shown that current-induced perturbation to the motion of individual particles
results in a high-energy, incoherent component of emission. We consider
photo-magnetic interactions and show that the high-energy radiation can produce
pairs which screen the accelerating electric field. We find that to avoid
catastrophic cascades that quench emission, bunches capable of producing FRBs
must have a modest density $n_e \approx 10^{13-14}\, {\rm cm^{-3}}$, and likely
propagate along field lines with large curvature radii, $\rho > 10^8 \, {\rm
cm}$. This rules out rapidly rotating magnetars as FRB sources within the
coherent curvature radiation model.
