An operating regime combining high bootstrap current fraction, reversed magnetic shear, and internal transport barriers to achieve steady-state, high-performance fusion plasmas.
The Advanced Tokamak (AT) concept refers to a class of operating scenarios that go beyond the conventional pulsed, inductively driven tokamak to achieve steady-state or near-steady-state operation with improved confinement and stability. The three defining pillars are a high fraction of self-generated bootstrap current, a tailored current profile featuring reversed or weak magnetic shear, and the presence of internal transport barriers (ITBs) that elevate core performance.1
In a conventional tokamak, the plasma current is driven by a central solenoid acting as a transformer—an inherently pulsed mechanism. The Advanced Tokamak replaces most of this inductive drive with the bootstrap current, a neoclassical self-generated current arising from pressure gradients in the trapped-particle population. At bootstrap fractions above 70–80%, the remaining current can be sustained by auxiliary non-inductive systems such as neutral beam current drive (NBCD), electron cyclotron current drive (ECCD), or lower hybrid current drive (LHCD), enabling true steady-state operation.2
The reversed-shear q-profile is central to the AT concept. By arranging for the safety factor q to have an off-axis minimum, the magnetic geometry suppresses turbulent transport (enabling ITBs) and aligns the bootstrap current density with the desired equilibrium current profile, creating a self-consistent steady state. This alignment is not automatic and requires careful control of the pressure profile shape and the locations of auxiliary current drive.3
Advanced Tokamak scenarios have been demonstrated experimentally on DIII-D, JET, JT-60U, and other devices, achieving normalized beta values βN above 3, confinement enhancement factors H98 above 1.5, and bootstrap fractions exceeding 50%. The DIII-D program in particular has sustained high-performance AT discharges for multiple current-relaxation times using real-time profile control.4
For ITER, the AT scenario (known as the “steady-state scenario”) targets Q ≥ 5 at reduced plasma current with high bootstrap fraction, complementing the baseline inductive scenario at Q = 10. Beyond ITER, compact fusion pilot plant designs increasingly rely on AT physics to minimize device size and recirculating power, making the Advanced Tokamak not merely an academic exercise but a practical pathway to economically viable fusion energy.