The Infinity Two fusion pilot plant baseline plasma physics design

We provide an assessment of the Infinity Two fusion pilot plant (FPP) baseline plasma physics design. Infinity Two is a four-field period, aspect ratio $A = 10$ , quasi-isodynamic stellarator with improved confinement appealing to a max- $J$ approach, elevated plasma density and high magnetic fields ( $ \langle B\rangle = 9$ T). Here $J$ denotes the second adiabatic invariant. At the envisioned operating point ( $800$ MW deuterium-tritium (DT) fusion), the configuration has robust magnetic surfaces based on magnetohydrodynamic (MHD) equilibrium calculations and is stable to both local and global MHD instabilities. The configuration has excellent confinement properties with small neoclassical transport and low bootstrap current ( $|I_{bootstrap}| \sim 2$ kA). Calculations of collisional alpha-particle confinement in a DT FPP scenario show small energy losses to the first wall ( ${\lt}1.5 \,\%$ ) and stable energetic particle/Alfvén eigenmodes at high ion density. Low turbulent transport is produced using a combination of density profile control consistent with pellet fueling and reduced stiffness to turbulent transport via three-dimensional shaping. Transport simulations with the T3D-GX-SFINCS code suite with self-consistent turbulent and neoclassical transport predict that the DT fusion power $P_{{fus}}=800$ MW operating point is attainable with high fusion gain ( $Q=40$ ) at volume-averaged electron densities $n_e\approx 2 \times 10^{20}$ m $^{-3}$ , below the Sudo density limit. Additional transport calculations show that an ignited ( $Q=\infty$ ) solution is available at slightly higher density ( $2.2 \times 10^{20}$ m $^{-3}$ ) with $P_{{fus}}=1.5$ GW. The magnetic configuration is defined by a magnetic coil set with sufficient room for an island divertor, shielding and blanket solutions with tritium breeding ratios (TBR) above unity. An optimistic estimate for the gas-cooled solid breeder designed helium-cooled pebble bed is TBR $\sim 1.3$ . Infinity Two satisfies the physics requirements of a stellarator fusion pilot plant.