Helicons are efficient plasma sources, capable of producing plasma densities of 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, , where is the lower hybrid frequency at the antenna, the ionization fraction is small (0.1%) and the plasma density low (). 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.
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December 2012
Research Article|
December 17 2012
Direct measurements of the ionization profile in krypton helicon plasmas
R. M. Magee;
R. M. Magee
a)
1Department of Physics,
West Virginia University
, Morgantown, West Virginia 26506, USA
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M. E. Galante;
M. E. Galante
1Department of Physics,
West Virginia University
, Morgantown, West Virginia 26506, USA
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N. Gulbrandsen;
N. Gulbrandsen
2Department of Physics and Technology, Faculty of Science,
University of Tromsø
, N-9037 Tromsø, Norway
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D. W. McCarren;
D. W. McCarren
1Department of Physics,
West Virginia University
, Morgantown, West Virginia 26506, USA
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E. E. Scime
E. E. Scime
1Department of Physics,
West Virginia University
, Morgantown, West Virginia 26506, USA
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Phys. Plasmas 19, 123506 (2012)
Article history
Received:
September 21 2012
Accepted:
November 28 2012
Citation
R. M. Magee, M. E. Galante, N. Gulbrandsen, D. W. McCarren, E. E. Scime; Direct measurements of the ionization profile in krypton helicon plasmas. Phys. Plasmas 1 December 2012; 19 (12): 123506. https://doi.org/10.1063/1.4772060
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