Impurity accumulation in a Nb3Sn superconducting magnet system on the high-field tokamak TRIAM-1M

The superconducting high-field tokamak TRIAM-1M with superconducting toroidal field magnets made of Nb3Sn has been operated stably for 10 years since its first operation in 1986 and recently it successfully demonstrated steady-state 2 h tokamak discharge. At the initial stage of plasma experiment in 1986, accumulation of impurities, especially CO2 and H2O, was observed in the refrigerator for the cryogenic system. This phenomenon obliged us to turn off the refrigerator and to stop the plasma experiments within a week. In order to eliminate the impurities, we installed an adsorber (molecular sieves). Thereafter we have successfully continued to operate the superconducting system for two series of 100 days each year. In this paper we describe the phenomenon of impurity accumulation in the superconducting system of the TRIAM-1M and the effect of installing an adsorber.