Plasma engineering analysis of a small torsatron reactor

This study examines the plasma physics and reactor engineering feasibility of a small, medium aspect ratio, high-beta, ℓ=2, D-T torsatron power reactor, based on the magnetic configuration of the Advanced Toroidal Facility, Oak Ridge National Laboratory. Plasma analyses are performed to assess whether confinement in a small, average radius plasma is sufficient to yield an ignited or high-Q driven device. Much of the physics assessment focuses on an evaluation of the radial electric field created by the nonambipolar particle flux. Detailed transport simulations are done with both fixed and self-consistent evolution of the radial electric field. Basic reactor engineering considerations taken into account are neutron wall loading, maximum magnetic field at the helical coils, coil shield thickness, and tritium breeding blanket-shield thickness.

In stellarator/torsatron transport modeling, the transition from an intermediate collisionality regime in which confinement decreases with increasing temperature (decreasing collisionality) to the low collisionality regime in which confinement improves with lower collisionality is shown to be of particular importance. Using a model that joins these two collisionality regimes to the resonant transition regime, where the E X B and B X ∇ B drifts cancel, electron losses are found to be the dominant factor in determining the radial electric field once the low collisionality regime is entered. Since ion confinement is greatly improved once electron losses dominate, investigations are made into the impact of the magnitude of the radial electric field, the varying effects of ion cyclotron and electron cyclotron heating methods, and the influence of additional anomalous electron losses during startup.

These analyses lead to a small, steady-state torsatron reactor of 100 cm average plasma radius, aspect ratio 7, 9% beta, 5 tesla axial magnetic field, with 2.3 MW/m² wall loading. At an assumed net efficiency of 33%, ~306 MWe is produced. Plasma performance in this small reactor is found to be very dependent on the presence of a moderate-to-strong radial electric field. Nominal values of all basic reactor engineering parameters are found which satisfy feasibility constraints, with the possible exception of completely adequate shielding directly under the helical coils.