Optimization of compact stellarator configuration as fusion devices

Abstract

Optimization of the stellarator configuration requires tradeoffs among a large number of physics parameters and engineering constraints. An integrated study of compact stellarator power plants, ARIES-CS, aims at examining these tradeoffs and defining key R&D areas. Configurations with a plasma aspect ratio of A ഛ 6 and excellent quasiaxisymmetry ͑QA͒ in both two and three field period versions were developed while reducing ␣-particle losses to Ͻ10%. Stability to linear ideal MHD modes was attained, but at the expense of reduced QA ͑and increased ␣-particle losses͒ and increased complexity of the plasma shape. Recent experimental results indicate, however, that linear MHD stability limits may not be applicable to stellarators. By utilizing a highly efficient shield-only region in strategic areas, the minimum standoff was reduced by ϳ30%. This allows a comparable reduction in the machine size. The device configuration, assembly, and maintenance procedures appear to impose severe constraints: three distinct approaches were developed, each applicable to a certain blanket concept and/or stellarator configuration. Modular coils are designed to examine the geometric complexity and to understand the constraints imposed by the maximum allowable field, desirable coil-plasma separation, coil-coil spacing, and other coil parameters. A cost-optimization system code has also been developed and will be utilized to assess the tradeoff among physics and engineering constraints in a self-consistent manner in the final phase of the ARIES-CS study.

Geometrical properties of a “snowflake” divertor

“…[1]). There are various ideas regarding the means for reaching this goal [1]. We here concentrate on the approach that would use geometrical effects for a stronger flaring of the poloidal field in the divertor.…”

Recent progress on spherical torus research

“…Even the lowest aspect ratio stellarator reactor design (ARIES-CS) has an effective aspect ratio of $4.5, and the achievable plasma beta is similar to that of a tokamak, i.e., $5%. 376 Prior to the start of ST experiments, the spheromak concept was actively investigated as a possible attractive reactor concept. This is because its fusion blanket module does not link the magnetic field coils, and thus greatly simplifies reactor engineering.…”

Accommodation of prompt alpha-particle loss in a compact stellarator power plant

“…1 [1]. Initial results indicated a high fraction of prompt alphaparticle loss (>10%) due to the appreciable magnetic field ripple associated with the non-axisymmetry of the magnetic geometry.…”

“…Fig. 2 shows an example energy spectrum of the alpha loss [1], which ranges from a few hundreds keV to 3.5 MeV. The resultant footprints on the LCMS show that the bulk of these particles are lost in a region below the mid-plane on the outboard edge of the plasma cross-section.…”