Hi Pam,
As you note, for the steam tower model, there is not a cavity receiver option.
For either the molten salt or direct steam power tower models, you can remove heliostats from the solar field after running the optimization wizard by editing values in the table on the Heliostat Field page. To remove heliostats from a sector of the solar field, change the value in the table for that sector to zero: Click a cell in the table and type the number zero. (When you click a cell in the table, SAM marks the sector in the diagram with blue lines.)
If you use this approach direct steam tower after running the optimization wizard, the heliostat field would no longer be optimal, and SAM would incorrectly model the receiver as a 360 degree receiver.
Best regards,
Paul.