Self-field generation in solenoidal focusing of light ion beams

In the transport and focusing of light ion beams with a plasma filled solenoidal magnetic lens it is important to study the effect of the beams´ self-fields resulting from inadequate current neutralization. Assuming complete charge neutralization, if the characteristic beam time τ satisfies 2π/Ωe τ < 2π/Ωi, where Ωe (Ωi) is the electron (plasma ion) cyclotron frequency, the plasma can be modeled as an electron magnetohydrodynamic (EMHD) fluid. In the presence of the applied lens field B0(r, z) the linearized field equations for the perturbed azimuthal magnetic field Bθ and poloidal flux ψ = rAθ are characterized by whistler wave generation when the electron collision frequency lie νe ≪ Ωe, and by resistive diffusion when lie ν e > Ωe. We present scalings for the field evolution when the collisionality is low and current neutralization proceeds via the whistler mode, and we discuss their effect on beam transport.