Ion Mix Can Invert Centrifugal Confinement

Abstract

Centrifugal plasma traps, in which plasma is confined partly by centrifugal forces, represent a possible path to fusion energy production. In centrifugal plasma traps, electric fields naturally arise in the direction parallel to the magnetic field in order to ensure quasineutrality. These electric fields are sensitive to the particular mix of ion species present. This paper uses analytic and numerical calculations to consider how these mix effects could be used to improve device performance. Changing the composition of the plasma can mix or demix different ion species, improve confinement of certain ions, and can even expel certain species from the trap altogether. This makes it possible to generate an ``inverted centrifugal" end-plug with very promising properties.

Figures (5)

• Figure 1: Different distributions of protons and boron-11 along a field line, with field line position parameterized by centrifugal potential $\Delta \varphi_c$ (normalized by the temperature $T$). As the relative density of the protons drops, the self-consistent ambipolar fields push them further outwards (the top panel has the highest boron concentration; the bottom panel has the highest proton concentration). This example takes the ion and electron temperatures to be equal. Note that the overall normalization is arbitrary, so we could imagine the different cases taking place at fixed total ion charge or fixed total ion number. $n_{p0}$ and $n_{B0}$ are defined as the proton and boron densities at $\Delta \varphi_c = 0$.
• Figure 2: The introduction of a small fraction of protons can improve the confinement of deuterium and tritium. The example without screening has 50% deuterium and 50% tritium; the example with screening has 10% protons, 45% deuterium, and 45% tritium locally at the bottom of the well.
• Figure 3: A cartoon of a centrifugal end-plug, in which a barrier species (in this case, lithium) inverts the centrifugal potential in two end regions in order to create a barrier for a central species (in this case, hydrogen). The top panel shows the centrifugal potential. The second panel shows the total (centrifugal plus electrostatic) potential experienced by protons if only protons occupy the centrifugal wells. The third and fourth panels show the total potentials experienced by protons and lithium if the wells are occupied by lithium.
• Figure 4: Spatial density distributions truncated at different levels, with space parameterized by the local potential energy (including centrifugal and electrostatic parts).
• Figure 5: Different distributions of protons and boron-11 along a field line, much the same as is shown in Figure \ref{['fig:pB11']}. However, where the distributions in Figure \ref{['fig:pB11']} assume that the particles are Gibbs-distributed along field lines, the distributions used here instead assume truncated Maxwellian distributions, with a cutoff at $4T$.