Direct measurement of the ionization source rate and closure of the particle balance in a helicon plasma using laser induced fluorescence

A detailed study of the flows of ion and neutral argon populations in a helicon plasma was carried out. Understanding the principle ion sinks and sources of neutral fueling is essential to understanding the ability of helicon wave heating to create high density plasmas. This heating mechanism and the neutral fueling required to sustain it and perhaps manipulate the axial density profile in long cylindrical plasmas are an active research topic for advanced plasma wakefield accelerator concepts. Using laser induced fluorescence (LIF) of ion and neutral argon species, an ion flux of up to $2.5×1021$ m⁻² s⁻¹ was measured leaving the core of a helicon plasma with a peak electron density of $4.0×1018$ m⁻³. Taking the divergence of the axial ion flux profile yields a minimum ionization rate estimate of $2×1021$ m⁻³ s⁻¹, and including the radial divergence increases the estimated ionization rate to approximately 10²² m⁻³ s⁻¹. Neutral flow velocities measured using LIF reveal a circulatory fueling and loss mechanism with distinct zones where radial or axial sources and sinks dominate.