Relaxed-state Modeling of RFP and Low-q Tokamak Plasma Formation with Programmable Power Supplies

The plasma current in a reversed-field pinch (RFP) is sustained by a combination of inductive drive and magnetic self-organization. The current profile exhibits rigidity due to the onset and nonlinear saturation of tearing instabilities, as the tendency for profile-peaking in the core is arrested by a dynamo-like emf, which simultaneously drives large poloidal current that sustains a reversed toroidal field. In the Madison Symmetric Torus (MST), the RFP equilibrium forms spontaneously (self-reversal) if the initial toroidal field is not too large. New programmable power supplies (PPS) are installed on MST that provide high-bandwidth arbitrary waveform control of the poloidal and toroidal magnetic field circuits. They greatly expand the possible ways to form and manipulate RFP plasmas, and also enable access to relatively-unexplored low-q and ultra-low-q tokamak plasmas. We provide an overview of the PPS and show comparisons with relaxed-state modeling that assumes a rigid current profile shape, with plasma resistance projected from empirical trends for the electron temperature (related to confinement scaling). For example, the modeling predicts the inductive volt-seconds required to reach maximum current depends on the initial toroidal field. A practical outcome is that MST might be capable of larger plasma current by simply optimizing startup using PPS. The study will include low-q and ultra-low-q tokamak plasmas, which exhibit some similarities to RFP self-organization.