Plasma expulsion may protect objects within magnetic plasma confinement devices

In the sun, gravity confines high-pressure plasma necessary for fusion. Magnetic fusion devices, on the other hand, use strong magnetic forces to confine plasma. A new particle-in-cell simulation described in Physics of Plasmas shows how magnetic forces can be used to expel plasma, not just confine it, to provide an overall better confinement strategy. The report discusses, for example, how a sufficiently strong magnetic coil embedded in a plasma allows particle recirculation in a magnetic mirror system. In an inertial electrostatic confinement (IEC) device, magnetic coils serve as electrodes. Magnetic plasma expulsion would serve to keep plasma away from cables that hold the magnetic coils in place in either system.

Currently, fusion devices use superconducting coils surrounding their chambers to confine the high-pressure plasma in a toroidal shape, limiting the plasma’s contact with the wall to specially-designed divertor regions. The high temperature of the plasma and its sensitivity to field disturbances demand significant control of what interacts with or enters the plasma, whether that be walls or injected charged particles. The use of expulsion in selected regions, with sufficient amplitude, selectively redirects magnetic field lines to inhibit plasma loss at material surfaces of objects inserted into the plasma.

The possibility of inserting objects into a plasma is explored with the Warp code, a particle-in-cell simulation. Warp simulates plasma systems by calculating a plasma’s electromagnetic field and each particle’s motion. These simulations model a region near a pair of tubes that pass through plasma, demonstrating conditions that could allow placing mechanical supports inside the plasma without significantly affecting its properties.

Reducing plasma loss to fusion device components could also enable inserting instruments to control plasma properties, such as electrodes for injecting charged particles. “Injecting charged particles of one sign of charge or the other may serve for controlling the neutrality at various locations within the plasma,” said Carlos Ordonez, co-author of the work.

Source: “Magnetic plasma expulsion,” by R. E. Phillips and C. A. Ordonez, Physics of Plasmas (2018). The article can be accessed at https://doi.org/10.1063/1.5006887 .