Hollow Toroidal Fusion Reactor

A Conceptual Design for Magnetic Confinement Fusion

Jon Thoms

Brookfield, WI, US

Abstract

This design employs two toroidal coils, inner and outer, in such a way that the inner coil creates a void in the magnetic field generated by the outer coil. This allows for a low field region in which the plasma can become homogenized. Any path leading out of the low field region requires the plasma to move in the direction of increasing magnetic field. The inner coil also provides the field otherwise generated by plasma current in a tokamak, so there is no need to induce a current in the plasma. Since the inner coil is submerged in the plasma, the turns of the coil are spaced out so that the field generated shields the inner coil from the plasma. Single particle simulations were conducted on this design that show particles being confined for long periods of time. Magneto hydrodynamic (MHD) simulations will be warranted if there is significant interest in this design.

Tokamak style fusion reactors have been the dominant type in plasma physics experiments. Many of the disruptions and drifts that plague this type of reactor tend to flow in the direction of decreasing magnetic field. Since the magnetic field is stronger nearer to the center of the tokamak, disruptions and drifts cause plasma to flow away from the center. A feature of the tokamak used to combat this effect is to have a current flowing within the plasma so that the magnetic field lines twist so that the drifts are averaged as sections of plasma spend some time close to the center and some time far from it. Since drifts and disruptions tend to flow in the direction of decreasing magnetic field, some confinement schemes have been suggested to keep the plasma in a low field region so that the plasma would have to move through a higher field region to escape the device. This design employs two toroidal coils (See Figure 1), inner and outer, in such a way that the inner coil creates a void in the magnetic field generated by the outer coil. This allows for a low field region in which the plasma can become homogenized. Any path leading out of the low field region requires the plasma to move in the direction of increasing magnetic field (See Figure 2). The inner coil also provides the field otherwise generated by plasma current in a tokamak, so there is no need to induce a current in the plasma. Since the inner coil is submerged in the plasma, the turns of the coil are spaced out so that the field generated shields the inner coil from the plasma.

Figure 1A Top View Internal/External Coils

Figure 1B Perspective View

Figure 2: Magnetic Field Magnitude at the Midplane

Figure 3: Magnetic Surfaces at the Midplane

A minimum requirement of the design is that individual charged particles are confined within the reactor and don’t interact with reactor components for long periods of time. Simulations were conducted by tracing the path that a charged particle would take within this field configuration (See Figure 4). The Ion shown in Figure 4 is a deuterium ion at 20keV kinetic energy. Please note the particles can get around the inner coil into the high field region as indicated by the path going off the image to the right. Notice that the path is a double line, which means that the particle returns along the same path back into the low field region. Tracing the particle over a long period of time (>2 seconds) shows that it doesn’t leave the reactor.

Figure 4: The Simulated Path of an Ion within the Reactor

Ultimately MHD simulations will be needed to validate the merit of the design. In addition to determining the quality of the confinement, the ratio of plasma pressure to magnetic pressure (ß) needs to be determined from simulation. Short of this, it’s possible that a scale model can be tested on lower temperature plasmas. A support system (or levitation system) for the inner coil will need to be developed. The supports may need to be magnetically shielded since they will have to pass through a region containing plasma. It’s possible the plasma density will be low enough on the inner edge of the inner coil (where the magnetic field is highest) for supports to be placed there without having to divert the plasma with magnetic shielding or otherwise.