Description
Aline is a linear and axisymmetric magnetised (maximum axial field 120 mT) plasma reactor which has been upgraded with an ICP source. A variable-frequency generator allows for coupling of up to 400 W of rf power (usually 10-40 MHz) to the plasma, for pressures between 0.1 and 10 Pa. The optical emission from the helium ICP was unexpectedly found to be strongly inhomogeneous for B > 45 mT. A bright magenta, radially localised (diameter 1-5 cm), axially elongated (5-40 cm) structure is observed in the diffusion chamber inside the hollow plasma column at low power (10 to 400 W). No mention of such a luminous structure has been found in the literature.
The plasma conditions for which the structure is observed have been studied using Langmuir probes, fast imaging, and optical emission spectroscopy. The structure has been found to be particularly sensitive to the purity of the neutral gas and to the presence of an annular density profile. While the annular plasma column collapses in the 20 µs following power-off, there is a particularly long, strong intensity overshoot seen at the position of the structure in the afterglow, with a decay time after the transient maximum of order milliseconds [1].
This contribution will present a parametric study of the plasma conditions for which we observe the structure. We will also present the preliminary modelling carried out to attempt to identify the origin of the localised, inhomogeneous electron/wave heating that could give rise to this luminous plasma structure, including lower hybrid resonance, and to describe the emission and role of the He I metastable levels in the afterglow [2]. Discussion about the possible physical origin of such a structure and on non-perturbative diagnostics to characterise it is welcomed.
References
[1] A. A. Newton, and M. C. Sexton. Journal of Physics B: Atomic and Molecular Physics 1.4 (1968): 669.
[2] L. L. Alves, G. Gousset, and C. M. Ferreira. Journal of Physics D: Applied Physics 25.12 (1992): 1713.