Abstract
The ITER first wall is designed for start-up and ramp-down in limiter configuration. The wall panels are toroidally shaped in order to spread the incident parallel power flux q(parallel to) uniformly, assuming a single decay length lambda(q) whose value is not known from first principles.
In order to study the scaling of q(parallel to) with plasma parameters, infra-red viewing of specially-designed limiters has been used on the COMPASS tokamak in similar to 100 discharges with scans in I-p, n(e) and for all combinations of magnetic field and I-p directions. The IR measurement clearly shows that in addition to the main SOL heat flux profile with lambda(q) > 40 mm, a steep gradient (lambda(near)(q) = 4 +/- 2 mm) dominates q(parallel to) near separatrix.
This appears independently of limiter shaping, insertion with respect to neighbors and incident field-line angles. Good agreement is found between the measured lambda(near)(q) and the prediction of a heuristic drift-based model.