Volume Production Of Negative Ions Research Articles

A 1.5D fluid—Monte Carlo model of a hydrogen helicon plasma

Helicon plasma sources operating with hydrogen or deuterium might be attractive for fusion applications due to their higher power efficiency compared to inductive radiofrequency plasma sources. In recent years, the resonant antenna ion device (RAID) has been investigating the physics of helicon plasmas and the possibility of employing them to produce negative ions for heating neutral beam injectors (HNBs). Herein, we present a fluid Monte Carlo (MC) model that describes plasma species transport in a typical helicon hydrogen plasma discharge. This work is motivated by an interest in better understanding the basic physics of helicon plasma devices operating in hydrogen and, in particular, the volume production of negative ions. This model is based on the synergy between two separate self-consistent approaches: a plasma fluid model that calculates ion transport and an MC model that determines the neutral and rovibrational density profiles of . By introducing the electron density and the temperature profiles measured by Langmuir probes as model constraints, the densities of ion species (, , , ) are computed in a 1.5D (dimensional) geometry. The estimate of the negative ion density profile represents a useful benchmark that is comparable with dedicated diagnostics, such as cavity ring-down spectroscopy and Langmuir probe laser photodetachment. Neutral gas particles (atoms and molecules) are calculated assuming a fixed plasma background. This gas–plasma decoupling is necessary due to the different timescales of plasma (microseconds) and gas kinetics (milliseconds).

Influence of surface materials on the volume production of negative ions in a radio-frequency driven hydrogen plasma

Negative atomic hydrogen ion (H−) densities were measured in a pulsed low-pressure E-mode inductively-coupled radio-frequency (rf) driven plasma in hydrogen by means of laser photodetachment and a Langmuir probe. This investigation focuses on the influence of different metallic surface materials on the volume production of H− ions. The H− density was measured above a thin disc of either tungsten, stainless steel, copper, aluminium, or molybdenum placed onto the lower grounded electrode of the plasma device as a function of gas pressure and applied rf power. For copper, aluminium, and molybdenum the H− density was found to be quite insensitive to pressure and rf power, with values ranging between 3.6 × 1014 to 5.8 × 1014 m−3. For stainless steel and tungsten, the H− dependency was found to be complex, apart from the case of a similar linear increase from 2.9 × 1014 to 1.1 × 1015 m−3 with rf power at a pressure of 25 Pa. Two-photon absorption laser induced fluorescence was used to measure the atomic hydrogen densities and phase resolved optical emission spectroscopy was used to investigate whether the plasma dynamics were surface dependent. An explanation for the observed differences between the two sets of investigated materials is given in terms of surface reaction mechanisms for the creation of vibrationally excited hydrogen molecules.

Particle-in-cell with Monte Carlo collision modeling of the electron and negative hydrogen ion transport across a localized transverse magnetic field

The control of the electron temperature and charged particle transport in negative hydrogen ion sources has a crucial role for the performance of the system. It is usually achieved by the use of a magnetic filter—localized transverse magnetic field, which reduces the electron temperature and enhances the negative ion yield. There are several works in literature on modeling of the magnetic filter effects based on fluid and kinetic modeling, which, however, suggest rather different mechanisms responsible for the electron cooling and particle transport through the filter. Here a kinetic modeling of the problem based on the particle-in-cell with Monte Carlo collisions method is presented. The charged particle transport across a magnetic filter is studied in hydrogen plasmas with and without including volume production of negative ions, in a one-dimensional Cartesian geometry. The simulation shows a classical (collisional) electron diffusion across the magnetic filter with reduction in the electron temperature but no selective effect in electron energy is observed (Coulomb collisions are not considered). When a bias voltage is applied, the plasma is split into an upstream electropositive and a downstream electronegative regions. Different configurations with respect to bias voltage and magnetic field strength are examined and discussed. Although the bias voltage allows negative ion extraction, the results show that volume production of negative ions in the downstream region is not really enhanced by the magnetic filter.

Numerical simulation on volume production of negative ions in hydrogen plasmas (I) The mechanism of H- production.

In order to study the mechanism of H- production caused by hydrogen discharges and the dependence of H- production on plasma parameters, a comprehensive model is presented. The model considers important processes for the production of both H- and positive ions. A set of particle balance equations in a steady state hydrogen plasma is solved numerically. The calculated results have confirmed that H- ions are generated by dissociative attachment to highly vibrationally excited hydrogen molecules (vibrational level v ″= 5-10) and these molecules are produced by electrons with energies in excess of 30 eV.

Extraction of volume produced negative ions.

The volume production of negative ions are discussed with particular emphasis on the extraction of negative ions from a plasma. The dominant negative ion production mechanism recently discovered is the dissociative electron attachment to highly vibrationally excited molecules. The negative ion production and extraction experiments of the multicusp, the reflex-type and the sheet plasma sources are introduced. It seems that the investigation of the electrostatic field near the plasma-sheath boundary in a magnetic field is necessary to extract the negative ions effectively as well as suppressing electrons.