Confinement performance predictions for a high field axisymmetric tandem mirror

TL;DR

The paper tackles predicting confinement performance for a high-field, axisymmetric tandem-mirror end plug by introducing RealTwin, an integrated modelling framework that couples end-plug transport/heating, central-cell equilibrium, and POPCON-based 0D balance with Bayesian optimization to identify viable pilot-plant operating points. The approach reveals that conservative end-plug designs with HTS magnets and 240–400 keV negative-ion NBIs can yield central-cell densities around 1.5×10^20 m^-3 and central fusion power on the order of a few hundred megawatts (Pfus ~ 350 MW, Q ≈ 8.7), while alpha heating raises electron temperatures to ~100–120 keV and relaxes end-plug requirements. Key insights include the importance of alpha-driven electron heating, the significant role of classical radial transport and radiative losses at high beta, and the viability of end plugs fueled with Tritium to ease neutron and shielding challenges. The study also identifies critical gaps in stability modeling (MHD, trapped-particle modes) and edge-neutral physics, outlining future work to couple stability solvers and extend to kinetic treatments (e.g., 2D2V FP simulations) to validate and extend the presented results toward a fully integrated tandem-mirror pilot plant design.

Abstract

This paper presents Hammir tandem mirror confinement performance analysis based on Realta Fusion's first-of-a-kind model for axisymmetric magnetic mirror fusion performance. This model uses an integrated end plug simulation model including, heating, equilibrium, and transport combined with a new formulation of the plasma operation contours (POPCONs) technique for the tandem mirror central cell. Using this model in concert with machine learning optimization techniques, it is shown that an end plug utilizing high temperature superconducting magnets and modern neutral beams enables a classical tandem mirror pilot plant producing a fusion gain Q > 5. The approach here represents an important advance in tandem mirror design. The high fidelity end plug model enables calculations of heating and transport in the highly non-Maxwellian end plug to be made more accurately. The detailed end plug modelling performed in this work has highlighted the importance of classical radial transport and neutral beam absorption efficiency on end plug viability. The central cell POPCON technique allows consideration of a wide range of parameters in the relatively simple near-Maxwellian central cell, facilitating the selection of more optimal central cell plasmas. These advances make it possible to find more conservative classical tandem mirror fusion pilot plant operating points with lower temperatures, neutral beam energies, and end plug performance requirements than designs in the literature. Despite being more conservative, it is shown that these operating points have sufficient confinement performance to serve as the basis of a viable fusion pilot plant provided that they can be stabilized against MHD and trapped particle modes.

Figures (13)

• Figure 1: An example of an IPS driven workflow for calculating simple mirror plasma performance. After each code in the iteration is run the plasma state is updated and used to setup the next iteration.
• Figure 2: An example of WHAM plasma profiles and distribution functions calculated with CQL3D-m. On the left is a plot from CQL3D-m with logarithmic contours of the ion distribution in arbitrary units for the innermost $\sqrt{\psi_n} = 0.01$ normalized square root poloidal flux surface with the loss boundary shown in red. On the right are the electron (solid red) and ion (dashed red) density profiles as well as the ambipolar potential (blue) versus axial distance along the mirror $z$.
• Figure 3: An example of a magnetic equilibrium for a high $\beta$ plasma in WHAM calculated with Pleiades. On the left are the flux contours for the vacuum fields $\psi_{\mathrm{vac}}$ in blue and the flux contours after diamagnetic evolution $\psi_{\mathrm{new}}$ in red. On the right are the kinetic pressure profiles and magnetic pressure profiles as well as the paraxial equilibrium condition in green which in paraxial equilibrium is equal to $B_{\mathrm{vac}}^2/2\mu_0$.
• Figure 4: POPCON plots for tandem mirrors with $\ell_c = 50~\mathrm{m}$ at four different plasma radii through the mirror throat, (a) 0.1 m, (b) 0.15 m, (c) 0.2 m, and (d) 0.25 m using $n_p = 1.5\times10^{20}~\mathrm{m}^{-3}$ and $B_{0c} = 3.125~\mathrm{T}$ in all cases except for (a) where $B_{0c} = 5.0~\mathrm{T}$. Operating points are shown as a function of central cell density $n_c$ divided by end plug density $n_p$ versus central cell ion temperature $T_i$. Blue and red filled contours are of heating power to the central cell required at a given ($n_c$, $T_i$) operating point (blue regions indicate ignition). Regions which are off the scale for positive or negative and are inaccessible are denoted with red and blue hatching respectively. Black contour lines are fusion power in the central cell. Green contour lines are central cell vacuum $\beta$.
• Figure 5: Contours of $T_e$ for operating points at given $\langle n \rangle$ and $E_{\mathrm{NBI}}$ satisfying the constraint equations \ref{['eq:constraints_v2']}. This plot uses the $a_m = 0.15$ m POPCONs case found in Figure \ref{['fig:popcons_ini']}b. Black contour lines are fusion power in the central cell in MW.