Physics basis for the first ITER tungsten divertor

•Reviews the fundamental physics aspects of the first ITER W divertor and defines the required operational lifetime within the Staged Approach.•Uses the ITER divertor SOLPS simulation database to establish the target peak heat flux and neutral pressure burning plasma operating domain.•Assesses consequences of narrow SOL heat flux channels, fluid drifts, component shaping and 3D magnetic fields for ELM control.•Uses W recrystallization to define an operational budget and shows that heat fluxes ∼50% higher than previously assumed may be acceptable.•Shows that Ne and N should be equally good as seed impurities and suggests that very strong ELM mitigation will be required at high performance.•Provides a list of key outstanding R&D areas to consolidate the divertor physics basis in the period up to ITER operation.