Designing A Buildable Optimized Stellarator to Confine Electron-Positron Plasmas

Abstract

In this paper, the design of the the plasma equilibrium and superconducting coils for the Electrons and Positrons in an Optimized Stellarator EPOS experiment is presented. With newly developed stellarator optimization tools, including single-stage and stochastic optimization, as well as HTS strain, this work demonstrates that it is possible to achieve key metrics for the buildability and confinement properties of the device. In particular, satisfactory quality of quasisymmetry and stellarator robustness is designed, and engineering requirements are met for eight different candidates. A feasibility study is presented that optimizes multiple candidates for different plasma major radii and coil currents, as well as the best EPOS candidate to date.

Figures (18)

• Figure 1: Top view of the engineering design concept for EPOS coils, each coil is made of a double HTS winding pack (here shown as a single brown stack) and is supported in an individual stellarator frame shown here in different colors for three coils. The last closed flux surface is shown in red.
• Figure 2: Left: Injection concept for the EPOS stellarator, a single positron trajectory is plotted in red starting at a field strength of about 6 mT and then channeled by a stray field line originating from the weave-lane coil. Right: the particles arrive at the E$\times$B electrodes and experience a drift in the radial direction as well as multiple bounces (from additional bouncing plates) until it is finally injected into the 3D shaped field. The electrons are to be injected via an emitter.
• Figure 3: Left: Diagram displaying the optimization scheme for EPOS consisting of three main optimization stages: first, a stage II optimization is performed to find relatively accurate coils; followed by the stochastic single stage method focused on improving the coil shapes and optimizing the surface to balance physics and engineering constraints. Finally, stage II optimization is required to refine the HTS strain and coil concavity. Top Right: Rendering of the EPOS configuration with LCFS and coils after the single-stage step, the heatmap corresponds to the field error $\langle B\cdot n\rangle/\langle B \rangle$, note that its maximum value is purposely still relatively high as to not end up early on the design process in a sharp local minimum. Bottom Right: Boozer plot showing the quality of quasimmetry of the fixed-boundary "cold start surface" used for the EPOS optimization.
• Figure 4: Top Left: Quasisymmetric error of the EPOS configurations plotted against the major radius for both the VMEC equilibrium and the QFM-generated one. Top Right: Squared Flux vs Quasisymmetric error for all configurations. Bottom Left: Iota profiles for all the EPOS candidates and the three most important rationals bounding the rotation transform values. Bottom Right: Quasisymmetric error radial profile for the EPOS configurations
• Figure 5: Loss fraction of positrons at 5 eV versus time of the EPOS configurations with starting conditions at s=0.05 (Left), s=0.5 (Middle), and s=0.9 (Right). Here we show that in the worst case scenario with no cyclotron cooling, at 5 eV, we are able to keep almost 100 % of the particles after 2 seconds when starting the particles at the magnetic axis. This ratio reduces to about 60 % when starting at the outer boundary.