The ion circus is a miniature storage ring formed by a segmented radiofrequency mass filter bent into a circle. The primary goal of this unique device is to perform high-resolution mass separation with small transmission loss since the resolving power is increased while the orbiting ions cool in the ring. Contrary to its linear brother, this circular Paul trap is designed to cool and mass separate the ions over a much longer flight path, thus requiring lower buffer gas pressure. Ions can be accumulated in the ring and extracted either in tangential or perpendicular directions. This way, the trap also serves as a versatile beam distribution device. Design principles are presented and the prototype instrument, under test in Orsay, is described. (c) 2008 Elsevier B.V. All rights reserved.