Investigation of the formation of a fully pressure‐driven tokamak*

A noninductive current drive concept, based on internal pressure‐driven currents in a low‐aspect‐ratio toroidal geometry, has been demonstrated on the Current Drive Experiment Upgrade (CDX‐U) [Forest et al., Phys. Rev. Lett. 68, 3559 (1992)] and further tested on DIII‐D [in Plasma Physics and Controlled Nuclear Fusion Research, 1986, Proceedings of the 11th International Conference, Kyoto (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159]. For both experiments, electron cyclotron power provided the necessary heating to breakdown and maintain a plasma with high‐βp and low collisionality (eβp∼1, ν*≤1). A poloidal vacuum field similar to a simple magnetic mirror is superimposed on a much stronger toroidal field to provide the initial confinement for a hot, trapped electron species. With application of electron cyclotron heating (ECH), toroidal currents spontaneously flow within the plasma and increase with applied ECH power. The direction of the generated current is independent of the toroid...