Charge exchange in atom-surface collisions

Negative hydrogen/deuterium ions can be formed by processes occurring in the plasma volume and on surfaces facing the plasma. The principal mechanisms leading to the formation of these negative ions are dissociative electron attachment to ro-vibrationally excited hydrogen/deuterium molecules when the reaction takes place in the plasma volume, and the direct electron transfer from the low work function metal surface to the hydrogen/deuterium atoms when formation occurs on the surface. The existing theoretical models and reported experimental results on these two mechanisms are summarized. Performance of the negative hydrogen/deuterium ion sources that emerged from studies of these mechanisms is reviewed. Contemporary negative ion sources do not have negative ion production electrodes of original surface type sources but are operated with caesium with their structures nearly identical to volume production type sources. Reasons for enhanced negative ion current due to caesium addition to these sources are discussed.

Negative hydrogen ion production mechanisms

A theoretical study of the nature and the mechanism of the bonding of an alkali metal (Cs) on a transition-metal surface [W(001)] in the high-coverage limit is presented in order to understand and explain the lowering of the work function and to elucidate the role of W surface states and surface resonance states in the adsorption process. The analysis is based on all-electron local-density-functional results obtained with our self-consistent full-potential linearized augmented-plane-wave method for thin films for (1) a five-layer slab of W, (2) an unsupported Cs monolayer, and (3) Cs in a $c(2\ifmmode\times\else\texttimes\fi{}2)$ structure on both sides of the five-layer W slab for three different Cs-W separations. We find that Cs forms a polarized-metallic rather than ionic overlayer: The Cs valence electrons originating from the atomic $6s$ states are polarized toward the W surface leading to an increase of electronic charge in the Cs/W interface region and a depletion of electronic charge on the vacuum side of the overlayer. In addition, the semicore Cs $5p$ electrons are markedly counterpolarized. The net result of these multiple surface dipoles is a lowering of the work function upon cesiation from 4.77 eV (clean five-layer W slab) to 2.77, 2.55, and 2.28 eV, corresponding to heights of the Cs atoms above the W surface of 2.60, 2.75, and 2.90 \AA{}, respectively. The Cs-induced changes in the charge density are essentially localized outside the surface W atoms. The W $d$ surface states and surface resonance states which are so characteristic of the W(001) surface are found to persist on the cesiated W(001) surface. The main effect of the Cs overlayer on these states is their energetic stabilization; this effect is most pronounced for the contamination-sensitive ${\overline{\ensuremath{\Gamma}}}_{1}$ surface state just below the Fermi energy, which is lowered in energy by 1 eV due to hybridization with Cs $s$ states.

All-electron local-density theory of alkali-metal bonding on transition-metal surfaces: Cs on W(001)

The physical phenomena important in volume and surface production negative hydrogen ion sources are reviewed The phenomenon, designated volume production, was attributed to dissociative electron attachment of low energy electrons to rovibrationally excited molecules The experimental verification in 2005 of the reality of this mechanism is reported Recent results obtained by measuring the negative ion temperature in a filament discharge ion source indicate that two negative ion populations are present Understanding the physics of volume negative ion sources operating in accelerators with H− ion density in the 100 mA cm−2 range opens the prospect of creating volume caesium-free sources for fusion The evolution of physics of surface production is also describedA measurement effected near the plasma electrode (PE) using two-laser beam photodetachment showed the presence of a directed negative ion flow The ratio of the negative ion and electron currents extracted through the transverse magnetic filter field near the PE can be predicted from a collisional flow model A specific for two negative species plasmas transport mechanism in the weak transverse magnetic filter field is described

https://iopscience.iop.org/article/10.1088/0029-5515/46/6/S05/pdf

Physics aspects of negative ion sources

Several applications of PIC simulations for understanding basic physics phenomena in low-temperature plasmas are presented: capacitive radiofrequency discharges in Oxygen, dusty plasmas and negative ion sources for heating of fusion plasmas. The analysis of these systems based on their microscopic properties as accessible with PIC gives improved insight into their complex behavior. These studies are results of joint efforts over about one decade of research groups from Greifswald University, Germany; Bari University, Italy; Keio University, Japan and Innsbruck University, Austria. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

B. Rasser

Papers

Theoretical models of the negative ionization of hydrogen on clean tungsten, cesiated tungsten and cesium surfaces at low energies

H− and D− formation by scattering H+ and D+ from a cesiated W(110) surface☆