Laboratory Astrophysics Experiments to Study Star Formation

Abstract

As a thesis project, I devised and implemented a scaled accretion shock experiment on the OMEGA laser (Laboratory for Laser Energetics). This effort marked the first foray into the growing field of laser-created magnetized flowing plasmas for the Center for Laser Experimental Astrophysical Research (CLEAR) here at the University of Michigan.

Accretion shocks form when streams of accreting material fall to the surface of a young, growing star along magnetic field lines and, due to their supersonic flow, create shocks. As I was concerned with what was happening immediately on the surface of the star where the shock forms, I scaled the system by launching a plasma jet (the “accreting flow”) and driving it into a solid surface (the “stellar surface”) in the presence of an imposed magnetic field parallel to the jet flow (locally analogous to the dipole field of the star).

Early work for this thesis project was dedicated to building a magnetized flowing plasma platform at CLEAR. I investigated a method for launching collimated plasma jets and studied them using Thomson scattering, a method which measures parameters such as temperature and density by scattering a probe beam off the experimental plasma. Although the data were corrupted with probe heating effects, I overcame this problem by finding the mass density of the jets and using it to determine they were isothermal rarefactions with a temperature of 6 eV.

Scaling an astrophysical phenomenon to the laboratory requires tailoring the parameters of the experiment to preserve its physics, rather than creating an experiment that merely superficially resembles it. I ensured this by distilling the driving physical processes of the astrophysical system—accretion shocks—into a list of dimensionless number constraints and mapping these into plasma parameter space.

Due to this project being the first magnetized flowing plasma effort at CLEAR, it suffered the growing pains typical of a young research program. Of my two primary diagnostics for the accretion shock experiment, visible light imaging was successful, but proton radiography, which was intended to probe magnetic field structure, failed twice for two independent reasons. The visible light data show that a shock forms and grows rapidly. However, there are no observable structural differences between the magnetized and un-magnetized shots. It may be that there were subtle structural differences that would have been evident in proton radiographs but did not appear in visible light images.

However, it may also be that the magnetic field was not strong enough to affect the structure; given the plasma and magnetic field parameters of the shot day, the experiment was analogous to a young star with a magnetic field of 325 Gauss, which is weaker than the roughly 1 kilo-Gauss fields typically observed. If this experimental effort continues after my departure, it would benefit from making use of one of the novel low-density plasma stream generation techniques being developed at CLEAR.