The physics basis of the ITER DRGA

A Diagnostic Residual Gas Analyzer (DRGA [1]) on ITER will sample the divertor pumping duct and thus, indirectly, the sub-divertor. Based on recent tokamak data, it is anticipated that the thus-measured, sub-divertor neutral gas composition will be representative of the corresponding ion species composition in the main plasma, over a few-seconds of plasma-wall equilibration times [2, 1]. This is a significant aspect of the DRGA physics basis. Toward its validation for ITER, SolPS-ITER simulations are performed in which the core boundary condition of ³He/(³He + D + T) concentration is abruptly changed, with the goal to deduce a measurable change in the divertor pumping gap on a timescale consistent with DRGA detection capability. The ³He concentration is a critical to an ICRH scenario that is efficient in heating T ions in the core plasma [3]. A summary of other key aspects of the physics basis behind the design of this ITER diagnostic system will also be included..