Ignited Spherical Tokamaks as a Reactor Development Facility*

The talk presents the concept of Ignited Spherical Tokamaks (IST), which can serve as a neutron fusion source for the Reactor Development Facility. The IST would be uniquely consistent with three objectives of magnetic fusion, i.e., (a) high power density plasma regime (5-10 MW/m$^3$), and high neutron flux (b) for designing the ``first wall'' of the reactor (up to the fluence of 15 MW year/m$^2$), and (c) for developing the tritium cycle. The lithium-based plasma facing components (LiWalls) of an IST provide the pumping boundary conditions for the plasma. When combined with central fueling of the plasma by low energy (E=45-50 keV) neutral beam injection (NBI), the LiWall environment leads to a flat plasma temperature T=E/3. This results in a super-critical ignition regime, with ion-temperature gradient turbulence eliminated, when the energy confinement is close to neo-classical, while the high current density at the separatrix robustly stabilizes the edge-localized modes. Unlike conventional approach to magnetic fusion, the super-critical ignition regime relies on core fueling by NBI and fast expulsion of the $\alpha$-particles, rather than on their heating the plasma. In this regards, the IST configuration (for the neutron source purposes) and stellarators (as power reactors), rather than tokamaks, have similarity regarding super-critical ignition regime. A separate national program ($\simeq$\$2-2.5 B for $\simeq$ 15 years) can realistically develop an Ignited Spherical Tokamak as a fusion neutron source for reactor R\&D in 3 steps (two with DD, and one with DT plasmas), i.e., \begin{enumerate} \item A spherical tokamak, targeting achievement of absorbing wall regime with neo-classical confinement in a DD plasma and $Q_{DT-equiv}=1$, \item Full scale DD-prototype of IST for demonstration of all aspects of stationary super-critical regime with $Q_{DT-equiv}\simeq50$. \item IST itself with a DT plasma and $Q_{DT}\simeq50$ for reactor technology and $\alpha$-particle power extraction studies. \end{enumerate}