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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.
In this work, we present a detailed assessment of fusion-born alpha-particle confinement, their wall loads and stability of Alfvén eigenmodes driven by these energetic particles in the Infinity Two Fusion Pilot Plant baseline plasma design, a four-field-period quasi-isodynamic stellarator to operate in deuterium–tritium fusion conditions. Using the Monte Carlo codes, SIMPLE, ASCOT5 and KORC-T, we study the collisionless and collisional dynamics of guiding-centre and full-orbit alpha-particles in the core plasma. We find that core energy losses to the wall are less than 4 %. Our simulations shows that peak power loads on the wall of this configuration are approximately 2.5 MW m-
$^2$ and are spatially localised, toroidally and poloidaly, in the vicinity of x-points of the magnetic island chain 
$n/m = 4/5$ outside the plasma volume. Also, an exploratory analysis using various simplified walls shows that shaping and distance of the wall from the plasma volume can help reduce peak power loads. Our stability assessment of Alfvén eigenmodes using the STELLGAP and FAR3d codes shows the absence of unstable modes driven by alpha-particles in Infinity Two due to the relatively low alpha-particle beta at the envisioned 800 MW operating scenario.
An analysis of the divertor designs for the Infinity Two fusion pilot plant (FPP) baseline plasma design is presented. The divertor uses an 
$m=5$, 
$n=4$ magnetic island chain, where m is the poloidal number and n is the toroidal number. Two divertor designs are presented. A classical divertor that is similar to the Wendelstein 7-X island divertor is analyzed using diffusive field-line following and the fluid code EMC3-Lite. For a baseline 
$800\text{ MW}$ operating point in Infinity Two, the conditions where the heat flux on the divertor plate remains in the acceptable region are analyzed. In addition a related, but different and novel large island backside divertor (LIBD) design is shown. The LIBD promises improved neutral pumping by closing the divertor through the use of baffling and with a structure inside the island, thus preventing neutralized plasma particles from reente ring the plasma.
The magnetohydrodynamic (MHD) equilibrium and stability properties of the Infinity Two fusion pilot plant baseline plasma physics design are presented. The configuration is a four-field period, aspect ratio 
$A = 10$ quasi-isodynamic stellarator optimised for excellent confinement at elevated density and high magnetic field 
$B = 9\,T$. Magnetic surfaces exist in the plasma core in vacuum and retain good equilibrium surface integrity from vacuum to an operational 
$\beta = 1.6 \,\%$, the ratio of the volume average of the plasma and magnetic pressures, corresponding to 
$800\ \textrm{MW}$ deuterium–tritium fusion operation. Neoclassical calculations show that a self-consistent bootstrap current of the order of 
${\sim} 1\ \textrm{kA}$ slightly increases the rotational transform profile by less than 0.001. The configuration has a magnetic well across its entire radius. From vacuum to the operating point, the configuration exhibits good ballooning stability characteristics, exhibits good Mercier stability across most of its minor radius and it is stable against global low-n MHD instabilities up to 
$\beta = 3.2\,\%$.
