Measurement of Plasma Potential, Ion Saturation Current and Mach number using Retarding Field Energy Analyzer

This article deals about the experimental measurement of plasma potential, ion saturation current and Mach number obtained with the variation of power, operating gas pressure and radial position using retarding field energy analyzer. We employed a retarding field energy analyzer by rotating with different angles such as 0° (facing toward source), 90° (facing side walls) and 180° (facing opposite the source). The coil current is varied from 0 to 15 A to produce the magnetic field which is used to confine the plasma. The flow of plasma has been characterized which was found to be subsonic. The low-temperature plasma is produced by means of a 13.56 MHz helicon plasma source at 300-1000 kW radio frequency power. The plasma is expanding from 13.8 cm diameter source into a 150 cm long diffusion chamber of 60 cm diameter.

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Effect of permanent magnets on plasma confinement and ion beam properties in a double layer helicon plasma source

Journal of Plasma Physics ◽

10.1017/s0022377819000412 ◽

2019 ◽

Vol 85 (3) ◽

Author(s):

Erik Varberg ◽

Åshild Fredriksen

Keyword(s):

Magnetic Field ◽

Permanent Magnets ◽

Ion Beam ◽

Plasma Source ◽

Diffusion Chamber ◽

Plasma Potential ◽

Field Energy ◽

Plasma Confinement ◽

Plasma Device ◽

Field Lines

The work described in this article was carried out to investigate how permanent magnets (PM) affect the plasma confinement and ion beam properties in an inductively coupled plasma which expands from a helicon source. The cylindrical plasma device Njord has a 13 cm long and 20 cm wide stainless steel port connecting the source chamber and the diffusion chamber. The source chamber has an axial magnetic field produced by two coils, with magnetic field lines expanding into the diffusion chamber. Simulations have shown that the field lines leaving the edge of the source hit the port wall, causing a loss of electrons in this section. In the experiments performed in this work, PMs were added around the port walls near the exit of a plasma source and the effect was investigated experimentally by means of a retarding field energy analyser probe. The plasma potential, ion density and ion beam parameters were estimated, and the results with and without the PMs were compared. The results showed that the plasma density in the centre can in some cases be doubled, and the density at the edges of the plasma increased significantly with PMs in place. Although the plasma potential was slightly affected, and the beam velocity dropped by ${\sim}$ 10 %, the ion beam flux increased by a factor of 1.5.

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A comparison of ion beam measurements by retarding field energy analyzer and laser induced fluorescence in helicon plasma devices

Physics of Plasmas ◽

10.1063/1.4913990 ◽

2015 ◽

Vol 22 (3) ◽

pp. 033505 ◽

Cited By ~ 13

Author(s):

N. Gulbrandsen ◽

Å. Fredriksen ◽

J. Carr ◽

E. Scime

Keyword(s):

Ion Beam ◽

Laser Induced Fluorescence ◽

Field Energy ◽

Energy Analyzer ◽

Helicon Plasma

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Axial and radial development of the hot electron distribution in a helicon plasma source, measured by a retarding ﬁeld energy analyzer (RFEA)

Plasma Sources Science and Technology ◽

10.1088/1361-6595/ac47e5 ◽

2022 ◽

Author(s):

Lisa Buschmann ◽

Ashild Fredriksen

Keyword(s):

Magnetic Field ◽

Electron Distribution ◽

Potential Drop ◽

Plasma Source ◽

High Energy ◽

Plasma Potential ◽

Field Configuration ◽

Field Energy ◽

Hot Electron ◽

High Energy Tail

Abstract The information about the electron population of a helicon source plasma that expands along a magnetic nozzle is important for understanding the plasma acceleration across the potential drop that forms in the nozzle. The electrons need an energy higher than the potential drop to escape from the source. At these energies the signal of a Langmuir probe is less accurate. An inverted RFEA measures the high-energy tail of the electrons. To reach the probe, they must have energies above the plasma potential VP, which can vary over the region of the measurement. By constructing a full distribution by applying the electron temperature Te obtained from the electron IV-curve and the VP obtained from the ion collecting RFEA or an emissive probe, a density measure of the hot electron distribution independent of VP can be obtained. The variation of the high-energy tail of the EEDF in both radial and axial directions, in the two different cases of 1) a purely expanding magnetic field nozzle, and 2) a more constricted one by applying current in a third, downstream coil was investigated. The electron densities and temperatures from the source are then compared to two analytic models of the downstream development of the electron density. The first model considers the development for a pure Boltzmann distribution while the second model takes an additional magnetic field expansion into account. A good match between the measured densities and the second model was found for both configurations. The RFEA probe also allows for directional measurement of the electron current to the probe. This property is used to compare the densities from the downstream and upstream directions, showing a much lower contribution of downstream electrons into the source for a purely expanding magnetic field in comparison to the confined magnetic field configuration.

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