High Resolution Ion Velocity Distribution Map Downstream of Expanding Magnetic Field*

High spatial resolution measurements, both perpendicular and parallel to the system axis, were carried out to investigate the ion velocity distribution function (IVDF) in an expanding argon helicon plasma. Several hundred 2 mm resolution measurements of the IVDF, electron temperature, and electron density were performed over 5 cm along the system axis and from -5 cm to -10 cm along the radial axis. Previous laser induced fluorescence (LIF) measurements of IVDFs revealed surprisingly broad perpendicular IVDFs (consistent with perpendicular ion temperatures in excess of 1.5 eV). With typical parallel temperatures of approximately 0.4 eV, the thermal anisotropy of the ion velocity distribution greatly exceeds unity. These new high spatial resolution measurements display significant structure at sub-ion-gyroradius scales are consistent with the previous measurements and demonstrate that two-dimensional or possibly three-dimensional models are needed to describe the evolution of the IVDF in expanding plasmas.