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.