Spatial structure of ion beams in an expanding plasma

Abstract

We report spatially resolved perpendicular and parallel, to the magnetic field, ion velocity distribution function (IVDF) measurements in an expanding argon helicon plasma. The parallel IVDFs, obtained through laser induced fluorescence (LIF), show an ion beam with v ≈ 8000 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. Structures aligned to the expanding magnetic field appear in the DC electric field, the electron temperature, and the plasma density in the plasma plume. 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.

Pressure dependence of an ion beam accelerating structure in an expanding helicon plasma

“…As the downstream neutral pressure increases, the electric field at z ¼ 164 cm undergoes significant changes in amplitude, particularly along the edges of the plasma where energetic electrons are observed to stream out from the source into the expansion chamber. 17,25 The energetic electrons create an annulus of increased plasma density through enhanced ionization upstream and downstream of the source-expansion chamber junction. The radius of the annulus maps along the divergent magnetic field to a radius of one electron skin depth from the rf antenna in the plasma source.…”

“…In the new paradigm for ion beam formation in expanding helicon source plasmas recently proposed by Aguirre et al, 17 downstream plasma conditions play a critical role in determining the structure of the ion accelerating electric field a) Electronic mail: earl.scime@mail.wvu.edu 1070-664X/2018/25(2)/023503/9/$30.00…”

Plasma instability of magnetically enhanced vacuum arc thruster

“…[7][8][9][10][11][12] Due to the nonuniform configurations of equilibrium magnetic field, discharge area and plume, plasma rotation driven by Lorentz force commonly occurs in various electric thrusters. [13][14][15][16] A few analytical models have been developed or/and applied to describe this flowing phenomenon, [17][18][19][20][21] but little attention was given to the resulted plasma instability which, however, can effect the propulsion efficiency, precise control, durable reliability and life time significantly. 22,23 This paper considers an emerging plasma propulsion technology, namely magnetically enhanced vacuum a) Electronic mail: leichang@scu.edu.cn arc thruster (MEVAT), [24][25][26][27][28] as an example and studies the instability evolution caused by plasma rotation and axial flow in detail.…”

Ion beams in multi-species plasmas

“…8 Recent work suggests that ion beams created in expanding helicon plasmas are not formed by a true double layer because the length of the acceleration region is so much longer than the Debye length. 9,10 Ions entering a sheath must obey the Bohm criterion. 11 For single ion species plasmas, the Bohm criterion requires that ions entering a sheath have a velocity…”

“…The theories of Baalrud et al 14 do not appear to apply to the ion acceleration process in these plasmas, confirming that the ion acceleration is unlikely to result from a double layer. 10…”