Performance of the COMPASS polarized target dilution refrigerator

The dynamic nuclear polarization (DNP) of 6LiD requires 1 mW/g or more microwave power in the beginning of the process. With the material of 350 g, this gives more than 350 mW in total for the COMPASS polarized target. A temperature around 300 mK or below is needed for an efficient polarization. These low temperatures can only be achieved with a dilution refrigerator designed to operate with a 3He flow of 100 mmol/s. In order to keep the polarization in the frozen mode, temperatures of about 65 mK are used with typical magnetic relaxation times of more than 1400 h at 0.42 T and of more than 15 000 h at 2.5 T. Low lattice temperature is important in achieving high nuclear polarization. The base temperature is limited by the heat brought to the mixing chamber by the inlet 3He and by radiation and conduction of heat.