  While free scroll rings are non-stationary objects that either grow or
contract with time, spatial confinement can have a large impact on their
evolution reaching from significant lifetime extension [J. F. Totz , H. Engel,
and O. Steinbock, New J. Phys. 17, 093043 (2015)] up to formation of stable
stationary and breathing pacemakers [A. Azhand, J. F. Totz, and H. Engel,
Europhys. Lett. 108, 10004 (2014)]. Here, we explore the parameter range in
which the interaction between an axis-symmetric scroll ring and a confining
planar no-flux boundary can be studied experimentally in transparent gel layers
supporting chemical wave propagation in the photosensitive variant of the
Belousov-Zhabotinsky medium. Based on full three-dimensional simulations of the
underlying modified complete Oregonator model for experimentally realistic
parameters, we determine the conditions for successful initiation of scroll
rings in a phase diagram spanned by the layer thickness and the applied light
intensity. Furthermore, we discuss whether the illumination-induced
excitability gradient due to Lambert-Beer's law as well as a possible
inclination of the filament plane with respect to the no-flux boundary can
destabilize the scroll ring.
