Physics aspects of negative ion sources

Abstract

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 described.A 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.

PIC code for the plasma sheath in large caesiated RF sources for negative hydrogen ions

“…For lower electron temperatures other destruction channels take over. The source under investigation is based on the tandem concept of negative ion sources [27], in which a dense (n e ≥ 10 18 m −3 ) and hot plasma (T e ≥ 10 eV) is generated and then cooled down to electron temperatures below 2 eV by means of a magnetic filter field to minimize the ion destruction. As an additional effect of the magnetic filter, the electron density is reduced to around 10 17 m −3 .…”

Temperature dependence of the work function of caesiated materials under ion source conditions

“…For this purpose the converter is covered with the alkali metal caesium which reduces the surface work function χ and thus significantly increases the negative ion yield [1,2]. Quantified values for the reduction of χ for substrates covered with fractional Cs layers can be found in several publications for ultra-high vacuum conditions (background pressure below 10 −9 mbar) depending on the substrate material and its crystallographic orientation at the surface [3][4][5].…”

Comparison of measured and modelled negative hydrogen ion densities at the ECR-discharge HOMER

“…The destruction of H − is caused by collisions with electrons, positive ions and atomic hydrogen. A detailed overview and discussion of these processes is for example provided by [14]. In order to quantify the rate of each process at HOMER, the density and temperature of every reagent of the considered processes has to be obtained diagnostically.…”

“…without the application of the meshed grid suggest [8]. This leads to the creation of highly vibrationally excited hydrogen molecules by radiative decay processes [14]. Via diffusion, these metastable particles are transported through the grid to the downstream section and effectively form H − via dissociative attachment in the region accessible to the diagnostics.…”