Date of Graduation


Document Type


Degree Type



Eberly College of Arts and Sciences


Physics and Astronomy

Committee Chair

Earl E Scime

Committee Co-Chair

V'yacheslav Akkerman

Committee Member

Paul A Cassak

Committee Member

Amy M Keesee

Committee Member

Weichao Tu


Presented here are direct, spatially resolved measurements of the spontaneous formation of a steady-state, ion accelerating structure and an ion hole in the plume of an expanding, low- pressure plasma. Ion velocity distribution function (IVDF) measurements, obtained with laser induced fluorescence, are taken parallel and perpendicular to the background magnetic field in an argon helicon plasma. The parallel IVDFs show an ion beam with v ≈ 8,000 m/s flowing downstream and confined to the center of the discharge. The ion beam is measurable for tens of centimeters along the expansion axis before the LIF signal fades, likely a result of metastable quenching of the beam ions. The parallel ion beam velocity slows in agreement with expectations for the measured parallel electric field. The perpendicular IVDFs show an ion population with a radially outward flow that increases with distance from the plasma axis. At the periphery of the ion beam, bipolar, magnetic field-aligned electric field structures, ion holes, form and accelerate ions across the ambient magnetic field. Multi-species plasmas provide further evidence that the ion acceleration mechanism is not a true double layer (DL). These measurements demonstrate that at least two-dimensional and perhaps fully three-dimensional models are needed to accurately describe the spontaneous acceleration of ion beams in expanding plasmas. A new paradigm for the spontaneous formation of ion accelerating structures in expanding plasmas is presented.