A global model of high current negative hydrogen ion source

Abstract

Summary form only given. Dissociative electron attachment to rovibrationally excited hydrogen molecules is one of the key mechanisms of volume negative hydrogen ion formation. Usually production of high-lying vibrational states of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> molecules is attributed to collisions with energetic electrons <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> (> 20eV). At the same time these electrons are effective to destroy negative ions. Therefore, the volume sources are mostly based on space separation of vibrationally excited molecules formation and negative hydrogen ions generation regions. A new concept for negative ion production is investigated. The production of vibrationally excited molecules is accomplished in a high pressure discharge followed by the generation of negative hydrogen ions in a second chamber connected with a nozzle. This concept has an advantage over existing negative ion sources, by keeping the electron temperature low thus eliminating the need of magnetic filter. A global model of the high pressure discharge chamber is presented. The chemical composition is assumed to contain ground state hydrogen molecules and atoms, 14 vibrationally excited hydrogen molecules, three positive hydrogen ions (H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> , H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> , H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ), two negative species (H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> and electrons). The volume-averaged continuity equations with assumed space profiles are solved in conjunction with electron and neutral energy equations assuming drift diffusion approximation for particle fluxes. Compared to conventional global models ours does not assume a neutral temperature but obtains it through the energy equation. The number densities of all species, electron and neutral temperatures are obtained as a function of absorbed power and volume flow rate to the discharge chamber. This new global model was verified, validated and used in a parametric study in order to obtain optimum operational parameters of the high pressure discharge.