Structure of Spontaneous Ion Acceleration Regions in Expanding Plasmas

In expanding, low pressure, high-density plasmas, spontaneously forming ion beams reach speeds in excess of 10 km/s and are confined to a narrow region in the center of the expansion region. The ion beam formation process is associated with the excitation of ion acoustic-like instabilities, high-energy electrons, and large ion temperature anisotropy. Here we present high spatial resolution measurements, both perpendicular and parallel to the system axis, of the perpendicular ion velocity distribution function in an expanding argon helicon plasma. Given the typical parallel ion temperatures of approximately 0.4 eV and perpendicular ion temperatures of more than 1 eV, the thermal anisotropy of the ion velocity distribution exceeds unity. The flow field of the ions is consistent with a detached ion beam, i.e., the ion beam does not appear to be tied to the expanding magnetic field.