Ion beam transport in a preionized plasma channel

We discuss a collisionless mechanism for transporting an ion beam in a pinched state. The beam is injected into a low density gas (∼ 10¹⁴–10¹⁶ cm⁻³) in which a partially ionized plasma channel, with a radius somewhat larger than that of the beam, has previously been formed, by a laser for example. The plasma electron density exceeds the beam ion density. PIC simulations show that die space charge of the beam induces an inward radial flow of plasma electrons to neutralize the beam charge, as well as a large axial current of plasma electrons in the same direction as die beam current, in the region in front of the beam. The resulting net current In actually exceeds die beam current Ib near the front of die beam. The resulting pinch effect is much stronger than that expected for self-pinched propagation in a higher gas density (resistive) regime where In ≪ Ib. Even if die channel density is initially underdense, the beam body will pinch, provided that the rate of plasma production from beam impact ionization of die neutral gas is sufficiently rapid. Analytic calculations support the simulation results. This scheme shows promise as a method for propagating heavy or light ion beams in a fusion target chamber.