first wall is the innermost material surface of a 📝fusion machine — the wall that directly faces the plasma and bears its radiation and particle flux.
The first wall divides labor with the 📝divertor: the divertor takes the concentrated exhaust that magnetic geometry steers onto its targets, while the first wall covers the vastly larger remaining area and absorbs what arrives everywhere else — photon radiation from the hot core, neutral atoms born from charge exchange, and the stray edge 📝plasma of startup, ramp-down, and off-normal events. Its per-square-meter load is far gentler than the divertor's, but its area is enormous and its duty is continuous.
In a power plant the first wall is the front face of the blanket, so its materials must satisfy competing masters at once: withstand surface heat and particle erosion, survive the neutron flux passing through to the blanket behind it, hold onto as little 📝tritium as possible, and contaminate the plasma only weakly when atoms are inevitably knocked loose. The tension in that list is visible in ITER's 2023 decision to abandon beryllium armor for its first wall panels in favor of tungsten, trading beryllium's plasma-friendliness away to escape its dust, toxicity, and melting risks.