Iter Magnetics: Key Design Issues

The ITER magnetic diagnostic system provides the main electromagnetic parameters of the plasma (eg Ip, Vloop); the shape and position of the plasma boundary; key internal parameters (eg β, li); the vertical speed of the current centroid; and analysis of the amplitude and mode number of fluctuations from quasi-static to fast (100 kHz). A subset of these measurements is used for plasma current and shape control. The application of magnetics to a burning plasma experiment poses a number of new issues: (a) The high performance cost of any wasted volume or lost shaping capability means that measurement targets (Table 1) have to be very tight; (b) The requirement for long-term (> 20 year) reliability with none or little maintenance means multiple measurement backups are essential and a backup or repair strategy has to be defined and validated in the design stage; (c) The presence of a space-filling conducting blanket in front of the sensors means that eddy currents have significant effects on physics and control — relevant time scales (a few ms); (d) In addition to the standard requirements for insulator breakdown resistance and frequency response and the issues of lifetime under irradiation, sensors and their wiring have to be optimised to fit within the limited volume, to have negligible radiation-induced errors and to dissipate the considerable nuclear heating; (e) the long- pulse targets imply that the sensors -and their wiring- have to provide high signal to noise signals and low drift integrators must be used. This paper summarises the extent to which these issues have been solved. Remaining R&D issues are highlighted.