Divergence in intense ion beams caused by incomplete charge neutralization

Space charge neutralization for light ion fusion (LIF) ion beam transport is usually assumed to be perfect in the "charge-neutral" region of the diode and in the gas transport cell. However, small charge clumps in the beam will not be totally charge-neutralized, and the residual net space charge may contribute to the beam microdivergence θμ. If the net potential of the clump is limited only by electron trapping, the minimum potential will be eφ ∼ 1/2 meVi² where me is the electron mass and vi is the ion velocity. For proton beams this leads to θμ ∼ (me/MP)^1/2 ∼ 23 mrad, where Mp is the proton rest mass. For non-protonic beams, different results occur. The mechanism predicts (1) no dependence of θμ on diode voltage, (2) non-protonic θμ greater than proton θμ for proton-contaminated beams, and (3) axial energy spread Δε‖/ε‖ ∼ ±2 θμ, which are all consistent with present data. Results of analytic studies and computer simulations of this mechanism are presented. Plasma shielding reduces the effects of this mechanism but collisions and magnetic fields reduce the plasma shielding effects. 2-D PIC MAGIC simulations show that this mechanism contributes to θμ both in the "charge-neutral" region and in the gas transport region. It is concluded that this mechanism is especially important in the "charge-neutral" region.