  AC losses in a high temperature superconducting (HTS) insert coil for a 25-T
cryogen-free superconducting magnet are numerically calculated during its
energization, assuming slab approximation. The HTS insert coil consists of 68
single pancakes wound with coated conductors and generating a central magnetic
field of 11.5 T, in addition to a contribution of 14.0 T from a set of low
temperature superconducting (LTS) outsert coils. Both the HTS and LTS coils are
cooled using cryocoolers, and energized simultaneously up to 25.5 T with a
constant ramp rate for 60 min. The influences of the magnitudes and
orientations of the locally applied magnetic fields, magnetic interactions
between turns, and the transport currents flowing in the windings are taken
into account in the AC loss calculations. The locally applied fields are
separated into axial and radial components, and the individual contributions of
these field components to the AC losses are summed simply to obtain the total
losses. The contribution of the axial field component to the total AC loss is
large at the early stages of the energization, whereas the total losses
monotonically increase with time after the contribution of the radial field
component becomes sufficiently large.
