A top-loading dilution refrigerator is permanently installed in an 18/20 T superconducting magnet and located in the Millikelvin Facility. It has a base temperature of 20 mk and 400 microwatts of cooling power at 100mK. A 5 mm hole the length of the general purpose probe allows installation of optical fibers or special leads. The sample space of this system is 25 mm. I.D.