  Developing large-area inductively coupled plasma sources requires deviation
from the standard coil concepts and the development of advanced antenna
designs. First steps in this direction employ periodic array structures. A
recent example is the Inductively Coupled Array (INCA) discharge, where use is
made of the collisionless electron heating in the electric field of a periodic
array of vortex fields. Naturally, the efficiency of such discharges depends on
the how well the experimental array realizes the theoretically prescribed field
design.
  In this work two diagnostic methods are employed to measure the field
structure of the INCA discharge. Ex situ B-dot probe measurements are compared
to in situ radio-frequency modulation spectroscopy (RFMOS) and good agreement
be tween their results is observed. Both diagnostics show systematic deviations
of the experimentally generated field structure from the one employed in the
theoretical description of the INCA discharge. The subtleties in applying both
diagnostic methods together with an analysis of the possible consequences of
the non-ideal electric field configuration and ways to improve it are
discussed.
