Effect of the vacuum vessel on magnetic measurements in TCABR tokamak

Abstract

Magnetic diagnostics in fusion experiments can be strongly affected by eddy currents induced in the vacuum vessel and in-vessel conducting components. In the present work the effect of the eddy currents on magnetic measurements in the TCABR tokamak with nearly circular plasma shape and rectangular cross section of the vacuum vessel is discussed. A method to find the magnetic field generated by eddy currents, in reply to the magnetic field of plasma currents, is based on expansion of vacuum field in series of toroidal harmonics. The conclusion is that, if the sensors are placed near the vessel, the effect of the vessel is strong and it decreases substantially in the case of sensors located near the plasma boundary at the circular surface.

Eddy currents in the vacuum vessel of the ETE spherical tokamak

“…The foregoing discussion demonstrates the importance of the inclusion of eddy currents effects in the study of transient phenomena in tokamak discharges (plasma startup, passive plates stabilization, magnetic diagnostics, halo currents), although these effects may be negligible in the steady state operation of large machines with hot plasmas. Obviously, the vessel eddy current effects are more important in the case of low aspect ratio tokamaks with a tight fitting vacuum vessel such as NSTX [10,12], GLOBUS-M [11,16] and ETE than in tokamaks with a large vessel volume [17]. The current distributions calculated in this paper will be used in future work both to model the eddy current effects in simulations of the plasma discharge in the early phase and to eliminate errorfields in the magnetic reconstruction of the plasma equilibrium in ETE.…”