A cryotheraphy system comprising: a cryoprobe (100; 100') comprising a shaft having a closed distal end (104; 508; 604) at a probe tip (508; 604), an inlet capillary (136; 528) disposed within the shaft for providing a supply flow path for liquid nitrogen, an outlet capillary (132; 532) disposed within the shaft for providing a return flow path for the liquid nitrogen and an insulation jacket (524; 608); and a cryosystem comprising: an inlet configured to receive liquid nitrogen from a liquid nitrogen source (632, 684, 684'); and a heat exchanger and pressure controller (624, 680) configured to supply the liquid nitrogen to the cryoprobe (100; 100') through a supply line (620, 678) and to stabilize pressures within the probe (100; 100'); the cryosystem further comprising: a flow controller (158; 612, 674); and a computer processor configured to avoid vapour lock associated with freezing of the cryoprobe (100; 100'), by comprising a computer readable program including instructions configured to generate nitrogen under physical conditions near a critical point of a liquid-vapor system for the nitrogen, wherein the critical point defines a point in a phase diagram of the liquid-vapour system where molar volumes are substantially equivalent for liquid and gas, wherein the physical conditions comprise a pressure of about 3.39 MPa (33.5 atm); wherein the flow controller (158; 612, 674) is configured to be controlled by the computer processor and configured to regulate return flow of the liquid nitrogen from the cryoprobe (100; 100') to avoid vapor lock and to control flow impedance in the return flow path (132, 616). a cryoprobe (100; 100') comprising a shaft having a closed distal end (104; 508; 604) at a probe tip (508; 604), an inlet capillary (136; 528) disposed within the shaft for providing a supply flow path for liquid nitrogen, an outlet capillary (132; 532) disposed within the shaft for providing a return flow path for the liquid nitrogen and an insulation jacket (524; 608); and a cryosystem comprising: an inlet configured to receive liquid nitrogen from a liquid nitrogen source (632, 684, 684'); and a heat exchanger and pressure controller (624, 680) configured to supply the liquid nitrogen to the cryoprobe (100; 100') through a supply line (620, 678) and to stabilize pressures within the probe (100; 100'); an inlet configured to receive liquid nitrogen from a liquid nitrogen source (632, 684, 684'); and a heat exchanger and pressure controller (624, 680) configured to supply the liquid nitrogen to the cryoprobe (100; 100') through a supply line (620, 678) and to stabilize pressures within the probe (100; 100'); the cryosystem further comprising: a flow controller (158; 612, 674); and a computer processor configured to avoid vapour lock associated with freezing of the cryoprobe (100; 100'), by comprising a computer readable program including instructions configured to generate nitrogen under physical conditions near a critical point of a liquid-vapor system for the nitrogen, wherein the critical point defines a point in a phase diagram of the liquid-vapour system where molar volumes are substantially equivalent for liquid and gas, wherein the physical conditions comprise a pressure of about 3.39 MPa (33.5 atm); wherein the flow controller (158; 612, 674) is configured to be controlled by the computer processor and configured to regulate return flow of the liquid nitrogen from the cryoprobe (100; 100') to avoid vapor lock and to control flow impedance in the return flow path (132, 616). a flow controller (158; 612, 674); and a computer processor configured to avoid vapour lock associated with freezing of the cryoprobe (100; 100'), by comprising a computer readable program including instructions configured to generate nitrogen under physical conditions near a critical point of a liquid-vapor system for the nitrogen, wherein the critical point defines a point in a phase diagram of the liquid-vapour system where molar volumes are substantially equivalent for liquid and gas, wherein the physical conditions comprise a pressure of about 3.39 MPa (33.5 atm); wherein the flow controller (158; 612, 674) is configured to be controlled by the computer processor and configured to regulate return flow of the liquid nitrogen from the cryoprobe (100; 100') to avoid vapor lock and to control flow impedance in the return flow path (132, 616).