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

Abstract

Both Laser-Induced Fluorescence (LIF) and Retarding Field Energy Analyzers (RFEA) have been applied to the investigation of beams formed in inductively coupled helicon plasmas. While the LIF technique provides a direct measurement of the velocity distribution in the plasma, the RFEA measures ion flux as a function of a retarding potential. In this paper, we present a method to compare the two techniques, by converting the LIF velocity distribution to an equivalent of a RFEA measurement. We applied this method to compare new LIF and RFEA measurements in two different experiments; the Hot Helicon Experiment (HELIX)-Large Experiment on Instabilities and Anisotropies (LEIA) at West Virginia University and Njord at University of Tromsø. We find good agreement between beam energies of the two methods. In agreement with earlier observations, the RFEA is found to measure ion beams with densities too low for the LIF to resolve. In addition, we present measurements of the axial development of the ion beam in both experiments. Beam densities drop exponentially with distance from the source, both in LIF and RFEA measurements. The effective quenching cross section from LIF in LEIA is found to be r b;Ã ¼ 4 Â 10 À19 m 2 , and the effective beam collisional cross sections by RFEA in Njord to be r b ¼ 1:7 Â 10 À18 m 2. V

RFEA Measurements of High-Energy Electrons in a Helicon Plasma Device with Expanding Magnetic Field

“…The vertical dotted line at z = 55 cm marks the axial position of the probes. The horizontal dashed lines at r = ±7 cm represent the observed width of the beam [29]. This type of ion detachment from magnetic field lines has earlier also been observed by Takahashi et al [18] and Cox et al [20].…”

“…The high-energy electrons seen at ±7 cm correspond to the width of the source and the width of the ion beam, while the high-energy electrons at r = 14 cm corresponds well with a magnetic field line emerging from the edge of the source [29]. We have already seen increased ion densities in this position (Figure 4A), possibly due to ionization from the high-energy electrons.…”

“…The background distribution (around V b = 40 V) has a maximum in the center (r = 0 cm) and another peak at r = 14 cm. The ion beam RFEA measurements are further discussed in Gulbrandsen et al [29].…”

“…In this case the plasma potential was around 40 V, so that the electron energies in the plasma would be about 40 eV higher than what the discriminator voltage shows. Figure 4A shows radial profiles of RFEA Ion Distribution Functions (IDF) [29] measured at z = 55 cm. A beam can be seen at around 60 V. The beam flux decreases with radius and disappear around r = 12 cm.…”

“…Figure 2A shows the grid configuration for ion measurements. The repeller, R, was biased to −80 V and the discriminator, D, was scanned from 0 to 100 V. S is the secondary electron repeller and was kept at −18 V. The collector was kept at −9 V. The ion RFEA measurements is further described in Gulbrandsen et al [29]. Figure 2B shows the grid configuration for electron measurements.…”