I believe that there is a dilution refrigerator refrigerator technique using a mixture of He3 and He4 that can reach very low temperatures while dissipating a watt or two. (https://en.wikipedia.org/wiki/Dilution_refrigerator)
I believe that there is a dilution refrigerator refrigerator technique using a mixture of He3 and He4 that can reach very low temperatures while dissipating a watt or two. (https://en.wikipedia.org/wiki/Dilution_refrigerator)