Performance analysis and parametric optimization study of torsatron fusion reactors

Performance and parametric optimization issues for torsatron fusion reactors are studied. Magnetic fields and forces on torsatron magnets are calculated analytically and numerically. Velocity-space loss regions are identified using numerical orbit following techniques. Pitch-angle scattering into the velocity-space loss region is estimated analytically. Energy transport calculations are performed using a new one-dimensional transport survey code based on the spectral collocation method. Parametric optimization of the reactor nuclear island is studied with a new torsatron design code that utilizes nonlinear optimization techniques. The low-to-moderate aspect ratio torsatrons studied have a velocity-space loss region for energetic particles. This loss region provides a mechanism for removing helium ash from the reactor plasma, but also reduces the plasma heating from fusion product slowing down. However, ignition still appears possible in torsatron reactors, provided the radial electric field is large enough to reduce the neoclassical stellarator transport coefficents to acceptably small values. Electric potentials of about three times the central ion temperature are required for torsatron reactors having plasma aspect ratio of five; smaller values are required for larger-aspect-ratio torsatrons. Two modes of operation are found depending on the sign of the radial electric field. The parametric optimization results indicate no strong advantage for torsatron reactors having small (≈ 5) plasma aspect ratio compared to moderate-aspect-ratio (≈ 10) reactors. The computed costs of the torsatron nuclear islands are similiar to those of tokamak reactors.