  Roebel cables made of HTS coated conductors can carry high currents with a
compact design and reduced AC losses. They are therefore good candidates for
manufacturing coils for HTS applications such as motors and generators. In this
paper we present the experimental DC and AC characterization of several coils
assembled from a 5 meter long Roebel cable built at KIT, which differ in the
number of turns and turn-to-turn spacing. Our experiments, supported by
finite-element method (FEM) calculations, show that a more tightly wound Roebel
coil, despite having a lower critical (and therefore operating) current, can
produce a higher magnetic field than a loosely wound one. For a given magnetic
field produced at the coil's center, all the coils have similar AC losses, with
the exception of the most loosely wound one, which has much higher losses due
to the relatively large current needed to produce the desired field. The
experiments presented in this paper are carried out on the geometry of pancake
coils made of Roebel cables, but they are exemplary of a more general strategy
that, coupling experiments and numerical simulations, can be used to optimize
the coil design with respect to different parameters, such as tape quantity,
size, or AC loss, the relative importance of which is dictated by the specific
application.
