  The design and operation of an electronic cooler based on a combination of
superconducting tunnel junctions is described. The cascade extraction of
hot-quasiparticles, which stems from the energy gaps of two different
superconductors, allows for a normal metal to be cooled down to about 100 mK
starting from a bath temperature of 0.5 K. We discuss the practical
implementation, potential performance and limitations of such a device.
