Open field equilibrium current and cross-field passing electrons as an initiator of a closed flux surface in EC-heated toroidal plasmas

Abstract

A model for the non-inductive initiation of a closed flux surface observed in electron cyclotron (EC) heated toroidal plasmas is presented. First, a pressure-driven equilibrium toroidal current develops under a weak external vertical field so as to counter balance the pressure-ballooning and current-hoop forces. When the self-field from the current almost cancels out the external vertical field, a forward energetic part of electrons in the velocity space begins to make cross-field passing (CFP) orbits. The CFP electrons are generated by the EC heating of bulk electrons and subsequent pitch-angle scattering, which is analyzed using the Fokker–Planck equation. They provide an additional current that closes the field lines. The model is examined for experiments in the small low aspect ratio device of LATE and in the large conventional device of JT-60U with a search for appropriate modes of EC heating. Simultaneous coincidence of the model with these two experiments is obtained in terms of microwave power and driven current. The results predict that initiation of a closed flux surface requires more and more EC power as the plasma major radius increases. In particular, careful injection of high N‖ EC waves is needed for large devices, both for initiation of a closed flux surface and for subsequent enlargement of the flux surface by the usual EC current drive onto the closed flux area.