Dry dilution refrigerator with pulse tube shutoff option

For experiments in the sub-Kelvin temperature regime with high demands for an extremely low level of vibration, a pulse tube precooled dilution refrigerator (DR) is described where the (vibration afflicted) pulse tube cryocooler (PTC) can be shut off temporarily whereas the dilution unit continues to operate at a mixing chamber temperature of Tmc < 6 mK during the entire shutoff time. Shutoff periods of 7 h were achieved with the present cryostat. Besides the dilution circuit, there is a 4He cooling circuit (1 K loop) in the cryostat; it cools and liquefies the 3He flow of the DR during normal operation and during PTC shutoffs. The 4He circuit has to be operated in single shot mode during PTC shutoffs as the two stages of the PTC warm up considerably and can no longer provide adequate precooling of the backstreaming helium gas. In its vessel enough liquid helium can be collected before a shutoff to liquefy the 3He flow of the DR in a heat exchanger for over 10 h. The temperature of the vessel is between 1.0 K and 1.3 K. After 7 h, the PTC was re-started. From here, it took about 2 to 2.5 h until the second stage of the PTC was cold enough so that the 4He circuit could be changed back from single shot mode to continuous operation. During the entire time, from the shutoff of the PTC to the resuming of the continuous operation of the 1 K loop, the DR ran undisturbed close to its base temperature. In this paper the differences between a DR with 1 K loop and commercial so-called “battery type DRs” are described.