Parallel Ion-Beam Instabilities at LAPD and in Space

Parallel collisionless shocks, from the Earth's bow shock to supernova-remnant shocks, are formed when two ion-beam instabilities interact nonlinearly. These electromagnetic instabilities go under different names in different communities; whereas space physicists talk of the nonresonant instability (NRI) and the resonant right-hand instability (RHI), cosmic-ray astrophysicists refer to Bell's instability and the streaming instability. Although some parameters like the Alfvénic Mach number and the beam density may differ between both scenarios, the principal kinetic physics of counter-streaming ion beams and how they excite both left- and right-handed electromagnetic waves is identical.

An experiment at the Large Plasma Device (LAPD) laboratory at UCLA studies these instabilities with a kilojoule-class laser system, accelerating a beam of carbon ions in a magnetized 17-meters-long helium plasma. This easily controlled environment allows us to measure the magnetic-field profile with in-situ probes and resolve both left- and right-handed instabilities for a wide range of beam parameters. Langmuir probes detect fluctuations in the ion density. The beam composition is characterized with a time-resolved fluorescence monochromator and reaches M_A≈5.

After an overview of the parallel ion-beam instabilities that linear theory predicts, we will discuss the role of density filamentation for their saturation in the various regimes. We will show three-dimensional LAPD measurements of the RHI evolving in which filaments are beginning to form. Our conclusion will address the role of the NRI for the LAPD experiment.