Charge and Current Neutralization of an Ion-Beam Pulse Propagating in a Background Plasma along a Solenoidal Magnetic Field

Background plasma can be used as an effective neutralization scheme to transport and compress intense charged particle beam pulses and is used in many applications involving the transport of fast particles in plasmas, including astrophysics [1], accelerator applications [2], and inertial fusion, in particular, fast ignition [3] and heavy ion fusion [4]. The application of a solenoidal magnetic field allows additional control and focusing of the beam pulse. A strong magnetic lens with a magnetic field up to a few Tesla can effectively focus beam pulses in short distances of order a few tens of centimeters. However, the magnetic field can also affect the degree of charge and current neutralization. In this Letter, we show that even a small solenoidal magnetic field, less than 100 G, strongly changes the self-fields in the beam pulse propagating in a background plasma. Such values of magnetic field can be present over distances of a few meters from the strong solenoid, and affect the focusing of the beam pulse. Moreover, a small solenoidal magnetic field can be applied to optimize beam propagation through a background plasma over long distances. In Refs. [5], the response of a magnetized plasma to intense ion-beam injection was studied while neglecting electron inertia effects, which corresponded to magnetic fields of a few Tesla in ion ring devices. In the present Letter, we analyze the opposite limit, corresponding to small values of magnetic field. In the collisionless limit and without an applied solenoidal magnetic field, the return current is driven by an inductive electric field which is balanced by electron inertia effects [6]. Taking electron inertia effects into account allows us to determine the conditions under which the applied solenoidal magnetic field begins to affect the return current in the plasma, and to reveal the range of magnetic field values that strongly affect the self-electric and self-magnetic fields of a beam pulse propagating in a background plasma. In a previous study, we developed reduced nonlinear models that describe the stationary plasma disturbance (in the beam frame) excited by the intense ion-beam pulse [6]. The model predicts very good charge neutralization during quasi-steady-state propagation, provided the beam is nonrelativistic and the beam pulse duration is much longer than the electron plasma period, i.e., � b!pe � 2� . Here, !pe �� 4�e 2 np=m� 1=2 is the electron plasma frequency, and np is the background plasma density. A high solenoidal magnetic field inhibits radial electron transport, and the electrons move primarily along the mag