A cryogenic gas is liquefied using a refrigeration system [101] thermally coupled at an evaporator [125] to a cold end of a gas supply system [103] within a dewar [116]. The refrigerator has a minimum temperature at an evaporator [125] above the boiling point of the gas at atmospheric pressure but below the boiling point of the gas at a high pressure. Thus, the gas is compressed [128] to high pressure so it condenses when cooled by the evaporator [125]. As it expands at a flow restrictor [148], a portion evaporates and cools a fraction to the temperature of the boiling point of the gas at atmospheric pressure, producing liquefied gas. Opening a purge valve [142] sends warm gas upward through heat exchange section [146] and out through a three-way valve [138] for defrosting. To reduce clogging, the gas supply valve [138] is controlled by a gas purity sensor [158].