INTENSE, NEUTRALIZED-ION BEAM FOR FUSION

Neutralized-ion beams (NIBs) are a special class of charge- and current-neutralized, intense-plasma beams characterized by accelerated-ion energies, E_ion≅ 0.15-2 MeV, ion-current densities, J_ion > kA/cm², and beam-power densities, P_beam > 0.1 GW/cm². At these levels the NIB is low magnetic beta, β = 2μ₀nkT/B² <<1, and will cross a 10 T transverse-magnetic field, propagating by the ExB drift.¹ The propagation distance will exceed the ion gyro-radius, d > ρ_ion, limited by beam expansion along the B field, shorting of the E field, or a combination of the two. Both processes can also occur in a magnetized plasma (MP), hence the NIB parameters can be adjusted to compensate for these losses, allowing the beam's energy-deposition profile to be appropriately tuned for applications in: beam heating, current drive, profile shaping, fueling, stabilization enhancement, etc., in much the same way that NPBs are currently used. Present day NIB systems use: gas-puff injectors, to produce atomically pure ion-beam species; magnetically-insulated ion-acceleration gaps, to suppress electron flow, enabling ion-current densities far beyond the Child-Langmuir limit; geometric, or magnetic, beam focusing to 10's of cm radius, increasing the beam-power density by orders of magnitude over its initial value when delivered to an injection port many meters downstream; and continuous-duty, high-power modulators that sustain MV, MW scale outputs for 10⁷ shots and more. To address the foregoing applications, NIBs must evolve to provide sustained, long-life, high-average-output power, high-system efficiency, and high reliability. The feasibility of such refinements is based on demonstrated performance on previously assembled NIBs and straightforward extrapolations of existing and near-term pulse-power technologies. This paper provides an overview summary of beam-propagation physics in a MP, key technologies used to produce and accelerate the NIB, and summarizes key-performance metrics characterizing the NIB and present day neutral-particle beam systems.