Hello Robert,
Yes, HFSS is the proper tool for electromagnetic pulse simulation. This can be done in the 'Transient' solution type.
You can go through the "HFSS Transient" chapter of the HFSS Help document to explore more about the transient solution type in HFSS.
You can access this section through online help here - Ansys Electronics 2023 R1
Regards
Anchal