  Hybrid magnets are currently under consideration as an economically viable
option towards 20 T dipole magnets for next generation of particle
accelerators. In these magnets, High Temperature Superconducting (HTS)
materials are used in the high field part of the coil with so-called insert
coils, and Low Temperature Superconductors (LTS) like Nb3Sn and Nb-Ti
superconductors are used in the lower field region with so-called outsert
coils. The attractiveness of the hybrid option lays on the fact that, on the
one hand, the 20 T field level is beyond the Nb3Sn practical limits of 15-16 T
for accelerator magnets and can be achieved only via HTS materials; on the
other hand, the high cost of HTS superconductors compared to LTS
superconductors makes it advantageous exploring a hybrid approach, where the
HTS portion of the coil is minimized. We present in this paper an overview of
different design options aimed at generating 20 T field in a 50 mm clear
aperture. The coil layouts investigated include the Cos-theta design (CT), with
its variations to reduce the conductor peak stress, namely the Canted Cos-theta
design (CCT) and the Stress Management Cos-theta design (SMCT), and, in
addition, the Block-type design (BL) including a form of stress management and
the Common-Coil design (CC). Results from a magnetic and mechanical analysis
are discussed, with particular focus on the comparison between the different
options regarding quantity of superconducting material, field quality,
conductor peak stress, and quench protection.
