Focusing of intense ion beams with a plasma-filled solenoidal magnetic lens including self-fields

We consider the potential of a solenoidal magnetic lens to transport and focus intense ion beams for the purpose of light ion driven ICF. We present results of numerical simulations of a short pulse, intense, Li⁺³ ion beam propagating through a single focusing, plasma-filled, solenoidal magnetic lens. The beam evolution is treated self-consistently by including the effects of self-electromagnetic fields which arise because the plasma electrons and energetic beam ions have very dissimilar dynamics in the presence of the strong applied magnetic lens fields. By considering the beam ions as particles and the plasma electrons as an inertialess fluid obeying a generalized Ohm´s law, we derive equations for the evolution of the azimuthal magnetic field Bθ and the poloidal flux function ψ. The motion of background plasma ions has been neglected because beam pulse lengths and transit times are much shorter than the local Alfvén transit time. The beam particle dynamics are then solved self-consistently with a 2–1/2 dimensional, axisymmetric (δ\\δ θ = 0), hybrid PIC code. For specified beam and plasma parameters, the results are expressed in terms of the power/cm² delivered to the target as a function of time. Attention is also paid to beam emittance growth. We study the cases when the plasma conductivity and density are held constant and when the conductivity and density evolve according to beam induced ionization of a gas.