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, wherein the dewar comprises
• i) a fill level that indicates full, and
• ii) a refill level;
• c) at least one cryoengine that comprises a first tank, a second tank and a tank conduit connecting the first tank to the second tank, wherein the first tank is positioned at least partially in the dewar, the first tank comprises one or more heating elements, the second tank is disposed outside the dewar and exterior surfaces of the second tank are insulated and in direct contact with a heat source, and the cryoengine is:
• i) at least partially disposed within the dewar and positioned so no more than about 40% to about 50% of the external surface area of the cryoengine that comprises the first tank, the second tank and the tank conduit connecting the first tank to the second tank is submerged within the liquid nitrogen bath in the dewar when the dewar is full, wherein when the dewar contains liquid nitrogen up to the fill level, 80% or more of the first tank of the cryoengine is submerged in the liquid nitrogen in the dewar, and when the dewar contains liquid nitrogen at the refill level, at least 5-10% the first tank of the cryoengine is submerged in the liquid nitrogen 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.