Reduction of electron flow current and localized anode energy deposition in transitions from coaxial feeds to a disk

Many conceptual designs for fusion energy involve combining energy from a number of feed lines to a centrally located load. One such design with 70 coaxial lines (each delivering Ia ∼ 1 MA at V ∼ 7 MV) merging into a center disk has been shown to yield large (Ie ∼ 10–40 MA) electron flow currents. The combination of large electron flow currents, abrupt transitions, and magnetic nulls (inherent in this type of current-adder geometry) cause an extreme amount of energy to be deposited into localized areas of the anode. A revised design is proposed using 10 coaxial lines delivering Ia ∼ 7 MA at V ∼ 9 MV to provide improved magnetic insulation, hence lowering the electron flow currents. Unfortunately, it is shown that reducing the electron flow currents only provides part of the solution to the problem of localized anode energy deposition. Additional modifications to the geometry of the coaxial-to-disk transitions are also necessary to obtain results where reasonable anode temperatures are maintained. Computer simulations are used to examine these designs. The 3-D electromagnetic, particle-in-cell (PIC) code QUICKSILVER developed at Sandia National Laboratories is used for the numerical simulations.