Observation of Return Current Effects in a Passive Plasma Lens

Abstract

Observations of relativistic beam focusing by a passive plasma lens have demonstrated a reduction in focusing strength due to plasma return current. A 50 MeV beam was propagated through a 1-3 cm long plasma with density around 10 14 cm 23. Beam size was measured as a function of propagation distance. For a ratio of collisionless plasma skin depth to beam spot size k p s r 0.33, no significant reduction in focusing was observed. Reduced focusing was measured for k p s r 1.1, where a significant fraction of the inductively driven return current in the plasma flows within the beam. The observations are in good agreement with an envelope equation model and with particle-in-cell simulations.

Controlling charge and current neutralization of an ion beam pulse in a background plasma by application of a solenoidal magnetic field: Weak magnetic field limit

“…The plasma electrons can effectively neutralize the beam charge, and the background plasma can provide an ideal medium for beam transport and focusing. Neutralization of the beam charge and current by a background plasma is an important issue for many applications involving the transport of fast particles in plasmas, including astrophysics, 1-4 accelerators, 4,5 and inertial fusion, in particular, fast ignition 6 and heavy ion fusion, 7,8 magnetic fusion based on field reversed configurations fueled by energetic ion beams, 9 the physics of solar flares, 10 as well as basic plasma physics phenomena. 11 Previous studies have explored the option of ion beam pulse neutralization by passing the beam pulse through a layer of plasma or a plasma plug.…”

Beam hosing instability in overdense plasma

“…In the narrow beam limit r 0 1, μ 1 − (0.366 + ln[1/r 0 ])r 2 0 /2. The factor μ(r 0 ) describes the plasma return current; for r 0 ∼ 1, the plasma return current partially flows through the bulk of the beam neutralizing the current, reducing the focusing [19] and instability coupling. In the long-beam, early-time regime such that |ζ | k β z (i.e., |∂ zxc | k β and |∂ ζxc | 1), Eq.…”

The quantum plasma lens concept: A preliminary investigation

“…In an overdense plasma lens where n p n b , the space charge of the electron beam is fully neutralized by the plasma through the displacement of plasma electrons by the beam electrons, resulting in beam self-focusing through its own magnetic field [13][14][15][16][17]. In the underdense lens, where n p < ∼ n b , all plasma electrons are displaced by the beam electrons, and the focusing force is due to the remaining plasma ions [11,18].…”