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To improve the low frequency spectrum of magnetic field measurements offuture tokamak reactors such as ITER, several steady state magnetic sensortechnologies have been considered. For all the studied technologies it isalways advantageous to place the sensors outside the vacuum vessel and as faraway from the reactor core to minimize radiation damage and temperatureeffects, but not so far as to compromise the accuracy of the equilibriumreconstruction. We have studied to what extent increasing the distance betweenout-vessel sensors and plasma can be compensated for sensor accuracy and/ordensity before the limit imposed by the degeneracy of the problem is reached.The study is particularized for the Swiss TCV tokamak, due to the quality ofits magnetic data and its ability to operate with a wide range of plasma shapesand divertor configurations. We have scanned the plasma boundary reconstructionerror as function of out-vessel sensor density, accuracy and distance to theplasma. The study is performed for both the transient and steady state phasesof the tokamak discharge. We find that, in general, there is a broad region inthe parameter space where sensor accuracy, density and proximity to the plasmacan be traded for one another to obtain a desired level of accuracy in thereconstructed boundary, up to some limit. Extrapolation of the results to atokamak reactor suggests that a hybrid configuration with sensors inside andoutside the vacuum vessel could be used to obtain a good boundaryreconstruction during both the transient and the flat-top of the discharges, ifout-vessel magnetic sensors of sufficient density and accuracy can be placedsufficiently far outside the vessel to minimize radiation damage.

Optimisation of out-vessel magnetic diagnostics for plasma boundary reconstruction in tokamaks