Collisionless Larmor Coupling of a laser plasma with a magnetized ambient plasma*

We experimentally investigate the collisionless coupling leading to the cross-field acceleration and subsequent rotation of the ambient plasma during the perpendicular expansion of a laser-produced plasma in a magnetized background plasma.

In our experiment performed on the Large Plasma Device (LAPD), a super-alfvenic, laser-produced carbon plasma expands in a magnetized ambient helium plasma and forms a diamagnetic cavity. The streaming of the super-Alfvenic debris ions leads to the transverse energization of background helium ions via collisionless Larmor coupling. The magnetic fields are mapped out in 2D using a magnetic flux probe. The energized background plasma is observed and tracked using filtered self-emission of excited He+ ions at 468.6 nm, observed with filtered fast-gate imaging. Spatially and temporally resolved Doppler-shift spectroscopy shows that He+ ions are accelerated radially outward and then rotate in a motion consistent with Larmor coupling. Additionally, electric potential measurements with an emissive probe indicate that the transverse dynamics of the background ions cannot be explained by the electrostatic field.