Cryogenic system and methods

A cryogenic system for cooling an exterior surface of a cryoprobe shaft using nitrogen as a cryogen, the system comprising:

• a) a cryoprobe comprising:
• i) a cryoprobe shaft
• ii) a cryoprobe supply lumen,
• iii) a cryoprobe return lumen, and
• iv) a freeze zone in fluid communication with the cryoprobe supply lumen and the cryoprobe return lumen, the freeze zone having an interior surface in thermal communication with an external surface of the shaft;
• b) a dewar adapted to contain a liquid nitrogen bath;
• c) at least one cryoengine, the cryoengine
• i) at least partially disposed within the dewar and positioned so about 40% to about 50% of the cryoengine is submerged within the liquid nitrogen bath in the dewar, and
• ii) capable of generating sub-cooled, pressurized nitrogen at a pressure of 1000 psi or greater and a temperature of about −160° C. or colder using one or more heat sources to pressurize the nitrogen, and heat exchange between the cryoengine and liquid nitrogen bath to sub-cool heat pressurized nitrogen in the cryoengine;
• and
• d) a supply line through which sub-cooled, pressurized nitrogen is capable of flowing from the cryoengine to the freeze zone of the cryoprobe,
• wherein the system is adapted to provide continuous flow of sub-cooled, pressurized nitrogen from the cryoengine to the freeze zone of the cryoprobe at decreasing pressures starting at an initial pressure of about 1000 psi or greater, and a temperature of about −160° C. or colder and continuing until the pressure of the sub-cooled, pressurized nitrogen flowing from the cryoengine to the freeze zone drops to about 400 psi or lower, the temperature of the sub-cooled, pressurized nitrogen at the freezing zone being no warmer than about −158° C. throughout the continuous flow of sub-cooled, pressurized nitrogen from the initial pressure of about 1000 psi or greater to the final pressure of about 400 psi or lower.