  Micro-refrigerators that operate in the sub-kelvin regime are a key device in
quantum technology. A well-studied candidate, an electronic cooler using Normal
metal - Insulator - Superconductor (NIS) tunnel junctions offers substantial
performance and power. However, its superconducting electrodes are severely
overheated due to exponential suppression of their thermal conductance towards
low temperatures, and the cooler performs unsatisfactorily - especially in
powerful devices needed for practical applications. We employ a second NIS
cooling stage to thermalize the hot superconductor at the backside of the main
NIS cooler. Not only providing a lower bath temperature, the second stage
cooler actively evacuates quasiparticles out of the hot superconductor,
especially in the low temperature limit. The NIS cooler approaches its ideal
theoretical expectations without compromising cooling power. This cascade
design can also be employed to manage excess heat in other cryo-electronic
devices.
