Direct measurements of the ionization profile in krypton helicon plasmas

Abstract

Single exposure three-dimensional imaging of dusty plasma clusters Rev. Sci. Instrum. 84, 023501 (2013) Hydrogen transport diagnostics by atomic and molecular emission line profiles simultaneously measured for large helical device Phys. Plasmas 20, 012514 (2013) Time-and-space resolved comparison of plasma expansion velocities in high-power diodes with velvet cathodes J. Appl. Phys. 113, 043307 (2013) Development of a diffuse air-argon plasma source using a dielectric-barrier discharge at atmospheric pressure Appl. Phys. Lett. 102, 033503 (2013) Nonmonotonic radial distribution of excited atoms in a positive column of pulsed direct currect discharges in helium Appl. Phys. Lett. 102, 034104 (2013) Additional information on Phys. Plasmas Helicons are efficient plasma sources, capable of producing plasma densities of 10 19 m À3 with only 100 s W of input rf power. There are often steep density gradients in both the neutral density and plasma density, resulting in a fully ionized core a few cm wide surrounded by a weakly ionized plasma. The ionization profile 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 neutral density profile. We use TALIF in conjunction with a Langmuir probe to measure the ionization fraction profile as a function of driving frequency, magnetic field, and input power. It is found that when the frequency of the driving wave is greater than a critical frequency, f c % 3 f lh , where f lh is the lower hybrid frequency at the antenna, the ionization fraction is small (0.1%) and the plasma density low (10 17 m À3 ). As the axial magnetic field is increased, or, equivalently, the driving frequency decreased, a transition is observed. 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 approaches 100%. Neutral depletion in the core can be caused by a number of mechanisms. We find that in these experiments the depletion is due primarily to plasma pressure and neutral pumping. V C 2012 American Institute of Physics. [http://dx.

Formation of the Blue Core in Argon Helicon Plasma

“…This gives the physical appearance of a fuzzy broad core like structure even before the complete global transition. We believe that most helicon sources work in this mode of operation [10], [20], [22], [24]. If the maximum available magnetic field in this experiment was 130 mT, we would also have thought that we work in the helicon blue core mode.…”

“…The appearance of the core is typically used as an identification of the helicon mode [4], [13], [17], [20], [22]. The formation of a bright core is not restricted to argon helicon plasma, but has been observed for other working gases such as xenon [23] and krypton [24].…”

Three-Dimensional Multispecies Distribution Functions in a Plasma Boundary With an Oblique Magnetic Field

“…Absolute densities have been measured in helicon plasmas in krypton in the CHEWIE device [84]. As expected, they show a linear increase in neutral density with fill pressure, and n n ≈ 10 20 m −3 at p = 3.6 mTorr.…”

The hot hELicon eXperiment (HELIX) and the large experiment on instabilities and anisotropy (LEIA)

“…Plasma density measurements indicate that the high-density core is approximately 1 cm in diameter. Two-photon absorption measurements of the absolute neutral density in another helicon source in our laboratory demonstrated that the core region is nearly 100% ionized in krypton helicon plasmas (Magee et al 2012a(Magee et al , 2012bGalante et al 2014). For the helium plasmas used in the Aflvén wave experiments, relative LIF measurements of the metastable neutral helium density also indicate a strong depletion of neutrals in the core.…”