Discharge duration limits of contemporary tokamaks and stellarators

The realization of long-time discharges in magnetic confinement devices (MCDs), namely, in tokamaks and stellarators, is a key issue in the development of controlled fusion energy. Experiments demonstrate that the discharge duration achieved in contemporary MCDs is limited and the normal operation is terminated after 0.3 to 3600 s depending on plasma, discharge and device parameters. This paper is devoted to the analysis of physical mechanisms which may be responsible for the discharge duration limit in MCD operation. The impact of heat transfers to plasma facing components from the plasma, coolants and thermal radiation is evaluated and compared with the available experimental database. The critical temperature T-cr 2300 K of plasma facing components is considered as the key parameter that limits the discharge duration. The regimes of the first wall temperature growth governed by both the heat conductivity and heat capacity are identified experimentally and analytically. Heat removal from wetted areas by means of thermal surface radiation and linear heat transfer to the coolant is identified as the key physical mechanism that determines the boundary of time-limited discharges in MCDs. The principal role of localized wetted areas with a size of 0.2-0.6 m(2) is revealed for operation of contemporary devices. This means that in further development of fusion reactors major attention should be devoted to the organization of a more uniform and distributed heat exhaust. It is shown that the proposed semi-analytical approach explains the experimentally discovered trends in the MCD operation and may be used for the evaluation of the discharge duration limit of new facilities designed to obtain steady-state discharges.