We consider the electron-positron plasma generation processes in the magnetospheres of magnetars - neutron stars with strong surface magnetic fields, B = 10^(14) - 10^(15) G. We show that the photon splitting in a magnetic field, which is effective at large field strengths, does not lead to the suppression of plasma multiplication, but manifests itself in a high polarization of gamma-ray photons. A high magnetic field strength does not give rise to the second generation of particles produced by synchrotron photons. However, the density of the first-generation particles produced by curvature photons in the magnetospheres of magnetars can exceed the density of the same particles in the magnetospheres of ordinary radio pulsars. The plasma generation inefficiency can be attributed only to slow magnetar rotation, which causes the energy range of the produced particles to narrow. We have found a boundary in the P - Pdot diagram that defines the plasma generation threshold in a magnetar magnetosphere.
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