  SPIDER is the prototype ion source of MITICA, the full-size neutral beam
heating system conceived for the ITER tokamak. It includes eight drivers to
heat and sustain the inductively coupled plasma (ICP). Owing to their near
cylindrical symmetry, the coupling between the radio-frequency (RF) active
currents and the source plasma is studied using a 2D electromagnetic approach
with simplified expressions for the plasma electrical conductivity taken from
the literature. The power absorbed by the plasma and the effect of the induced
plasma currents in lowering the inductance of the driver are based on data from
the dedicated S16 experimental campaign (y.~2020) of SPIDER: plasma electron
densities on the order of $10^{18}$ m$^{-3}$, electron temperatures $\sim 10$
eV; neutral gas pressure $\sim 0.3$ Pa and up to $50$ kW of net power per
driver. It is found that the plasma conductivity cannot be explained by the
friction forces associated to local collisional processes alone. The inclusion
of an effective collisionality associated to non-local processes seems also
insufficient to explain the experimental information. Only when the electrical
conductivity is reduced where the RF magnetic field is more intense, can the
heating power and driver inductance be acceptably reproduced. We present the
first 2D electromagnetic ICP calculations in SPIDER for two types of plasma,
without and with the addition of a static magnetic field. The power transfer
efficiency to the plasma of the first drivers of SPIDER, in view of these
models, is around 50%
