  Accelerators magnets must have minimal magnetic field imperfections for
reducing particle-beam instabilities. In the case of coils made of
high-temperature superconducting (HTS) tapes, the field imperfections from
persistent currents need to be carefully evaluated. In this paper we study the
use of superconducting screens based on HTS tapes for reducing the magnetic
field imperfections in accelerator magnets. The screens exploit the
magnetization by persistent currents to cancel out the magnetic field error.
The screens are aligned with the main field components, such that only the
undesired field components are compensated. The screens are passive,
self-regulating, and do not require any external source of energy. Measurements
in liquid nitrogen at 77 Kelvin show for dipole-field configurations a
significant reduction of the magnetic-field error up to a factor of four. The
residual error is explained via numerical simulations, accounting for the
geometrical imperfections in the HTS screens, thus achieving satisfactory
agreement with experimental results. Simulations show that if screens are
increased in width and thickness, and operated at 4.5 Kelvin, field errors may
be eliminated almost entirely for the typical excitation cycles of accelerator
magnets.
