Development and study of insulation systems for Nb$_3$Sn accelerator magnets

Fermilab is involved in the development of high field superconducting accelerator magnets for different applications based onNb$_3$Sn superconductor. Accelerator magnets have to withstand thousands of thermal and excitation cycles, significant mechanical stresses, and high voltages developed during quench while providing reliable operation for extended period of time. Insulation system is one of the most important elements of magnet design, which determines (and in some cases even limits) the electrical, mechanical, and thermal performance. Different elements of the magnet insulation system such as cable insulation, interlayer insulation, quench heater insulation, and ground insulation have been developed and studied. In this paper we will describe the insulation system used in the Fermilab shell-type dipole models followed by the high voltage test results.