UCLA

Throughout this work, the collective electromagnetic effects that mediate the transfer of energyfrom an energetic, dense plasma species to a relatively tenuous, magnetized plasma species are studied. These are observed to play an important role in a wide variety of space and astrophysical environments such as supernova remnants, coronal mass ejections, planetary bow-shocks, and man made ionospheric explosions. Laboratory experiments can create scaled versions of these systems using smaller denser plasmas characterized by similar dimensionless parameters. These can complement in-situ measurements and validate theoretical and computational models. One of the greatest advantages of laboratory experiments lies in the direct control of parameters and in the repeatability, which allows for many-point measurements of the interaction over successive data runs.

Experiments performed at UCLA combined a high-energy laser and the Large Plasma Device(LADP) to investigate collisionless coupling between an exploding laser-produced plasma (LPP) and a magnetized helium plasma. Alaser induced fluorescence (LIF) diagnostic has been developed and optimized using collisional-radiative modeling to investigate the spatially and temporally evolving ion velocity distribution function of the LPP as it interacts with the magnetized plasma.

LIF measurements provide new insight into the two primary drivers that transfer energy: themagnetic structure feature which moves ions down magnetic field gradients and the Larmor feature which induces an E�B drift in the ambient plasma. Two experiments were conducted to investigate the different coupling regimes.

The first experiment observes the coupling when the expansion of the LPP is sub-Alfv�nic(M? = ?/?? < 1). The LIF diagnostic maps the deceleration of LPP ions in the region where large magnetic gradients are observed. Three dimensional particle-in-cell (PIC) simulations reproduce the measured quantities well and offer new insight into the electric fields responsible for coupling. Measurements in combination with PIC simulations show that energy is transferred from the energetic species to the magnetized species consistent with the magnetic structure term. Directly measuring particle distribution functions with LIF significantly improves the initialization of the simulations.

The second experiment investigates coupling when the expansion is super-Alfvenic (M? = ?/?? > 1).We observe the formation and propagation of an additional magnetic structure, or "blob", in the ambient plasma that separates a relatively large distance (∼ 0.4??) from the bulk diamagnetic cavity (∼ ??). This blob is observed to coincide with the focusing of the LPP ions into a jet-like structure which results from magnetic pressure gradients that act perpendicularly to both the magnetic field and bulk LPP direction of motion. Magnetized ambient ions are observed to accelerate along a trajectory consistent with Larmor coupling in the regions where LPP ions are observed to stream across magnetic field lines. The formation of the blob is consistent with an electron population confined between the Larmor electric fields created by the jet-like ion flow and the charge separation electric fields created from the accelerated helium ions.