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arXiv · 2307.06747

Helicon waves in a converging-diverging magnetoplasma

Félicien Filleul·Antonella Caldarelli·Kazunori Takahashi·Rod Boswell·Christine Charles·John Cater·Nicholas Rattenbury
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Abstract

Waves propagating along a converging-diverging rf magnetoplasma having the characteristics of a bounded m=0 helicon mode are reported and characterised. The discharge features a 30 cm separation between the region of radiofrequency energy deposition by a single loop antenna and the region of maximum magnetic field applied by a pair of coils. With 200 W of rf input power, the resulting plasma exhibits a strong axial plasma density gradient peaking at the magnetic mirror throat where an Ar II blue-core is observed. Two dimensional B-dot probe measurements show that the rf magnetic fields are closely guided by the converging-diverging geometry. The wave is characterised as a m=0 mode satisfying the helicon dispersion relation on-axis with radial boundary conditions approximately matching the radii of the plasma column. Analysis of the wave phase velocity and wave axial damping failed to identify collisionless or collisional wave-plasma coupling mechanisms. Instead, the wave axial amplitude variations can be explained by local wave resonances and possible reflections from localised rapid changes of the refractive index. A Venturi-like effect owing to the funnel-shaped magnetoplasma and conservation of the wave energy may also explain some level of amplitude variations.

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Félicien Filleul, Antonella Caldarelli, Kazunori Takahashi, Rod Boswell, Christine Charles, John Cater, Nicholas Rattenbury. 2023-07-13. Helicon waves in a converging-diverging magnetoplasma. https://doi.org/10.1088/1361-6595%2Fad0b96

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