Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments

Abstract

Negative ions in low-pressure plasma sources are created either in the plasma volume by dissociative attachment or, at the plasma surface interaction due to surface ionization of backscattered or sputtered particles. Negative-ions formed on surfaces are accelerated towards the plasma by the sheath. They can influence the plasma kinetics through collisions with plasma species, or are self-extracted from the plasma thanks to the energy acquired in the sheath. Self-extraction of negative-ions can affect processes like sputtering, where the negative-ions formed on the cathode bombard the layer being deposited. In applications such as negative-ion sources for accelerator or fusion devices, it is taken advantage of negative-ion surface production. A low work-function material (usually caesium-covered metals) is in contact with the plasma and greatly enhances negative-ion production because of the low energy required to extract an electron from the surface. However, caesium free negative-ion sources would be greatly valuable for fusion applications because of the strong maintenance constraints induced by caesium injection.

Alternative solutions to caesium in negative-ion sources: a study of negative-ion surface production on diamond in H2/D2 plasmas

“…Surface negative ion production measurements are performed in the diffusion chamber of a plasma reactor. We only briefly describe the experiment here, as details can be found elsewhere [83,84]. The plasma is generated at 1 or 2 Pa, either by capacitive coupling from an external antenna using a 20 W 13.56 MHz generator, or at 1 Pa by an Electron Cyclotron Resonance antenna driven by a 60 W, 2.45 GHz generator.…”

“…Both modelling methods that will be presented later require knowledge of ion trajectories inside the sheaths in front of the mass spectrometer and in front of the sample as a function of ion emission angle and energy. The experimental arrangement has been designed in order to ensure planar sheaths in front of mass spectrometer and sample (the biased surface is much larger than the sample surface to prevent edge effects) [83]. In this situation, the local electric field in the sheaths can be calculated using Child Langmuir law knowing the electron density and temperature (from Langmuir probe measurements) and the surface bias.…”

“…For the sake of simplicity, trajectories in sheath 1 are not correctly represented (electric field is much lower in sheath 1). context this has been found to be an acceptable assumption for carbon materials [83,84]. The input for the SRIM computations were the positive ion distribution functions, as measured by the mass spectrometer and the surface parameters, namely the hydrogen surface coverage and the hydrogen surface binding energy.…”

Magnetized retarding field energy analyzer measuring the particle flux and ion energy distribution of both positive and negative ions

“…In addition, absolute ion flux measurements are very complicated due to a nontrivial dependence of the total system transparency for ions having different mass and energy. 20 Another tool used in negative ion beam measurements is the magnetized Faraday cup [21][22][23] using transversal magnetic field for electron suppression. This compact size device can be easily manufactured and integrated into a wide variety of experimental setups.…”

In‐plane and out‐of‐plane defects of graphite bombarded by H, D and He investigated by atomic force and Raman microscopies

“…D ion are produced in a low‐pressure plasma source and subsequently extracted and accelerated. Recently, it has been demonstrated that carbon materials are interesting candidates to enhance negative ion production in hydrogen plasmas . In a series of papers, negative ion surface production mechanisms on hydrogenated carbon materials have been investigated, pointing out how Raman spectroscopy can be a useful tool for relating surface modification and ion production.…”