Direct Measurements of the Ionization Fraction in Krypton Helicon Plasmas*

Helicons are efficient plasma sources, capable of producing plasma densities of 10$^{19}$ m$^{-3}$ with only 1 kW of input RF power. But the ionization fraction in the core of a helicon is usually not well known, because the neutral density is typically inferred from indirect spectroscopic measurements or from edge pressure gauge measurements. We have developed a two photon absorption laser induced fluorescence (TALIF) diagnostic capable of directly measuring the local neutral density. We use TALIF in conjunction with a Langmuir probe to measure the neutral and ion density profiles as a function of driving frequency and magnetic field. We find that when the frequency of the driving wave is greater than the lower hybrid frequency ($f_{LH})$, the core ionization fraction is small (0.1 {\%}) and the plasma density low (10$^{17}$ m$^{-3})$. As the axial magnetic field is increased, or, equivalently, the driving frequency decreased, a transition is observed at RF = $f_{LH}$. The plasma density increases by a factor of 10 or more, the plasma density profile becomes strongly peaked, the neutral density profile becomes strongly hollow, and the ionization fraction in the core reaches $\sim $100{\%}. The dramatic neutral depletion in the core is thought to be due to a combination of increased ionization and neutral pumping. The role of the latter is quantified by a comparison of flowing and static discharges.