This paper is a review on the structure of thermo field dynamics (TFD) in which the basic concepts such as the thermal doublets, the quasi‐particles and the self‐consistent renormalization are presented in detail. A strong emphasis is put on the computational scheme. A detailed structure of this scheme is illustrated by the one‐loop calculation in a non‐equilibrium time‐dependent process. A detailed account of the one‐loop calculation has never been reported anywhere. The role of the self‐consistent renormalization is explained. The equilibrium TFD is obtained as the long‐time limit of non‐equilibrium TFD.
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