  To improve the low frequency spectrum of magnetic field measurements of
future tokamak reactors such as ITER, several steady state magnetic sensor
technologies have been considered. For all the studied technologies it is
always advantageous to place the sensors outside the vacuum vessel and as far
away from the reactor core to minimize radiation damage and temperature
effects, but not so far as to compromise the accuracy of the equilibrium
reconstruction. We have studied to what extent increasing the distance between
out-vessel sensors and plasma can be compensated for sensor accuracy and/or
density 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 of
its magnetic data and its ability to operate with a wide range of plasma shapes
and divertor configurations. We have scanned the plasma boundary reconstruction
error as function of out-vessel sensor density, accuracy and distance to the
plasma. The study is performed for both the transient and steady state phases
of the tokamak discharge. We find that, in general, there is a broad region in
the parameter space where sensor accuracy, density and proximity to the plasma
can be traded for one another to obtain a desired level of accuracy in the
reconstructed boundary, up to some limit. Extrapolation of the results to a
tokamak reactor suggests that a hybrid configuration with sensors inside and
outside the vacuum vessel could be used to obtain a good boundary
reconstruction during both the transient and the flat-top of the discharges, if
out-vessel magnetic sensors of sufficient density and accuracy can be placed
sufficiently far outside the vessel to minimize radiation damage.
