The performance of the SMC dilution refrigerator

The cooldown of the magnet from room temperature to 4 K takes about one week. The dilution refrigerator is then cooled in 8 hours to 77K and the target is loaded. The material is moved from storage socks into the cells of the target holder in a special LN2 bath to prevent the material from overheating. The target holder is pushed directly into the precooled mixing chamber of the dilution refrigerator. Purging, final cooldown and condensing the 4He-3He mixture (8700 liters STP) takes less than 8 hours.

The measured cooling performance of the refrigerator when run at optimum circulation of 3He is seen in the graph:

During DNP the temperature of the helium mixture decreases slowly from about 350 mK to 200 mK as the optimum microwave power is reduced with increasing polarization. The maximum polarizations obtained in the proton and deuteron targets are +/- 94% and +/- 46%, respectively. About 95% of the maximum polarization is typically reached within 10 hours of DNP. The target is cooled down below 100 mK by turning the microwave power off 0.5-1 hours before the field rotation. Fairly long time constants are seen during cooling to the base temperature, which is reached only after several hours of running without microwaves. This suggests that the microwaves provide heat somewhere in considerable amount and that the thermal contact is sufficiently poor to the mixing chamber to prevent a rapid removal of the heat surplus. However, practically no loss of polarization takes place during the field rotation.

peter@neuro.hut.fi

3-APR-95