Zhang, YunchaoCharles, ChristineBoswell, Rod2015-12-072015-12-071070-664Xhttp://hdl.handle.net/1885/17052An ion beam generated by an annular double layer has been measured in a helicon thruster, which sustains a magnetised low-pressure (5.0 104 Torr) argon plasma at a constant radio-frequency (13.56 MHz) power of 300 W. After the ion beam exits the annular structure, it merges into a solid centrally peaked structure in the diffusion chamber. As the annular ion beam moves towards the inner region in the diffusion chamber, a reversed-cone plasma wake (with a half opening angle of about 30 ) is formed. This process is verified by measuring both the radial and axial distributions of the beam potential and beam current. The beam potential changes from a two-peak radial profile (maximum value 30 V, minimum value 22.5 V) to a flat ( 28 V) along the axial direction; similarly, the beam current changes from a two-peak to one-peak radial profile and the maximum value decreases by half. The inward cross-magnetic-field motion of the beam ions is caused by a divergent electric field in the source. Cross-field diffusion of electrons is also observed in the inner plume and is determined as being of non-ambipolar origin.5 pages© 2014 AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Physics of Plasmas and may be found at http://doi.org/10.1063/1.4885350 http://publishing.aip.org/authors/copyright-reuse http://www.sherpa.ac.uk/romeo/issn/1070-664X Author can archive publisher's version/PDF (Sherpa/Romeo as of 7/12/2015).ion beamannular double layerhelicon thrustermagnetised low-pressure (5.0 104 Torr) argon plasmaradio-frequency (13.56 MHz)300 W201410.1063/1.48853502015-12-08