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Deliverables
The objective of the project was the investigation of fundamental processes of electron-vibration kinetics in Cs-H2 plasmas as a source of negative hydrogen ions (H-). It is known that the mechanism of H- ion generation in volume plasma sources is dissociative electron attachment of low energy plasma electrons to rovibrationally excited molecules. The research was developped in two different discharge conditions:
a) the low-voltage Cs-H2 discharge at a relatively high pressure (1 Torr), studied at the Ioffe Physical-Technical Institute (St. Petersburg);
b) the low-pressure (milliTorr range) hydrogen discharges seeded with cesium.
The theory of the low-voltage Cs-H2 mixture, as a new type of H- volume-plasma source, was created. It was shown that the optimum electron temperature for H- generation due to dissociative electron attachment is approximately 1eV. High level of H2 vibrational excitation, Evib = 0.5 eV/molecule, and very high H- concentration, N(H-)> 1013 cm³ could be achieved in the discharge plasma. The theoretical optimization of the discharge was fulfilled for a maximum H- yield in stationary and moving plasma. Experiments using a diode with a plane-parallel electrode geometry confirmed that a plasma having the parameters needed for H- ion generation can be achieved in the low-voltage Cs-H2 discharge. An indication of the effects associated with vibrationally excited hydrogen molecules was obtained by studying the discharge afterglow. It was shown that a long-lived plasma exists in the afterglow of the discharge. This plasma is produced due to cesium ionization by H2 molecules which were previously vibrationally excited in the discharge. Experiments have also shown that that the plasma density in the afterglow depends significantly on the electrode material. The reason for this is the influence of hydrogen atom density upon the process of vibrational relaxation.
The laser photodetachment technique has been applied for the measurement of the negative ion density in the crossed (electric-and magnetic) field ion source with cesium hollow cathode, developed at Kurchatov Institute (Moscow), as well as in the "tandem" type negative ion sources, studied at Queen's University (Belfast) and Ecole Polytechnique (Palaiseau). A new method for measuring the extracted negative ion current, based on the measure of the beam momentum (beam thrust), was applied at Kurchatov Institute. The hollow cathode physics has been separately studied at the Institute for Plasma Research (University of Stuttgart).
In the low-pressure tandem H- ion sources studied at Queen's University (Belfast) and Ecole Polytechnique (Palaiseau) measurements of plasma parameters, including H- densities, have been made in conjunction with emission spectroscopy. Surface work function and surface temperature were measured at Queen's University, while the cesium neutral pressure was measured at Ecole Polytechnique. These measurements gave similar results. These results indicate, in the opinion of the scientists from Queen's University, that in their experimental situation surface effects dominate the enhanced negative hydrogen ion production. The scientists from Ecole Polytechnique consider that negative ions formed on the wall by surface processes are likely to be significantly depleted on their way to the center of their large chamber. Having also observed the resonance radiation emitted by cesium neutral atoms, which proves that cesium is indeed present in the plasma bulk, the enhancement of the negative ion density is attributed by these scientists to volume processes occurring in the cesium seeded plasma.
Two outstanding problems remained to be solved:
a) Why does the electron density drop when cesium is added to the discharge?
b) In the experiment at Queen's University there was no pronounced dependence of the negative ion density on the distance from the wall.
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