  Stacks of intrinsic Josephson junctions (IJJs) in high-temperature
superconductors enable the fabrication of compact sources of coherent terahertz
radiation. Here we demonstrate that multiple stacks patterned on the same
Bi2Sr2CaCu2O8+delta crystal can - under optimized conditions - be synchronized
to emit high-power THz-radiation. For three synchronized stacks we achieved 610
microwatts of continuous-wave coherent radiation power at 0.51 THz. We suggest
that synchronization is promoted by THz-waves in the base crystal. We note
though that synchronization cannot be achieved in all samples. However, even in
these cases powers on the 100-microwatt scale can be generated.
