Design Basis for the ITER Plasma Shape and Position Control Reflectometer System

Abstract

A fundamentally new approach to plasma position control is required on ITER as a consequence of the anticipated long pulse lengths. The error in conventional magnetic plasma position control systems accumulates with time (due to drifts, etc.), such that magnetic systems are not expected to provide accurate control for discharge lengths in excess of ∼1000 s.¹ ITER will have pulse lengths exceeding 1000 s, and potentially up to 10,000 s long, such that an alternative plasma position and shape control system will be essential. The concept that edge density profile measurements via reflectometry at multiple poloidal positions could provide a position control capability emerged through the ITER design process, and was briefly discussed at the time of the 1995 Varenna conference on ITER diagnostics.² In this concept a poloidally distributed set of edge gap measurements (specifically the distance from the first wall (blanket face) to the location of a density layer lying at or inside the separatrix) will be used as an estimator of the plasma boundary shape, and utilized for feedback control. In the two years since 1995 the US Home Team has worked to investigate how this concept would work in practice, as well as to provide detailed design calculations. The conclusion of these studies is that a plasma position control system for ITER based on a poloidal set of edge density profile measurements appears to be viable, with performance levels at or close to requirements. As described in the body of this paper, this conclusion is based on: (1) a proven methodology for the design of profile reflectometer systems, (2) demonstrated performance levels of current reflectometer systems, and (3) specific design and modeling for ITER. Recent descriptions of the use of reflectometer systems for density profile measurements are available,^2,3 while a detailed design methodology for optimal system performance has also been developed.²