Bootstrapping a way to a more stable fusion reactor

A promising path toward practical fusion power involves enhancing tokamak efficiency by increasing bootstrap current — the pressure gradient-generated current in a plasma that reduces the need to inject power to sustain the plasma current. Recent experiments on the DIII-D tokamak show higher energy confinement from an internal transport barrier (ITB) that spontaneously forms with high bootstrap current. This is largely a result, as detailed in Physics of Plasmas, of turbulence suppression from a large outward shift of the core plasma that increases the fraction of the total current carried by bootstrap current.

The investigators studied the transport using gyrokinetic stability analysis as well as quasi-linear turbulent transport predictive modeling. The results showed that plasma discharges with high bootstrap fractions can self-organize into a state with either a strong ITB with a weak edge transport barrier (ETB), or a weak ITB with a strong ETB. The weak ITB state could be triggered to switch into a strong ITB state by its large edge localized modes (called ELMs). The strong ITB type was more persistent in comparison due to its smaller ELMs.

Both types of discharges show the same total current and plasma to magnetic pressure ratio, as well as significantly improving energy confinement that is up to 80 percent higher than predicted by the usual empirical scalings. It’s noteworthy that discharges with the strong ITB achieve the highest energy confinement. At lower bootstrap fractions the ITB collapses and the energy confinement drops.

Transport differences between the two discharges are partially controlled by the kinetic ballooning mode instability (called KBM). Self-organization occurs by interactions between the bootstrap current, magnetic shear and KBM stability.

More research is needed to extend these discharges for future fusion energy experiments, and most importantly will be exploring ways to control ELM instabilities while optimizing global stability and transport properties, and pushing to even higher bootstrap fractions in this promising tokamak regime.

Source: “Transport barriers in bootstrap-driven tokamaks,” by G. M. Staebler, A. M. Garofalo, C. Pan, J. McClenaghan, M. A. Van Zeeland, and L. L. Lao, Physics of Plasmas (2018). The article can be accessed at https://doi.org/10.1063/1.5019282 .