Physics design for a lithium vapor box divertor experiment on magnum PSI

•The lithium vapor box is an as-yet untested concept for a tokamak divertor.•An experiment on the linear plasma generator Magnum-PSI is proposed.•An 8 cm cloud of 650 °C lithium vapor should induce plasma detachment.•Detachment will be inferred by the diversion of power away from the target.•The plasma beam should not detach without hot Li vapor present.