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Title: A pulsed RF fischarge for a vaesium gree H‾ ion source
Author: Barnes, M.
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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Negative ion sources utilised in neutral beam injection (NBI) systems for ITER, DEMO, and future commercial reactors will be required to reliably output 40A of 1MeV D- with low co-extracted electrons for operating times of up to one hour. In order to meet these demanding criteria current fusion negative ion sources make use of caesium (Cs) in order to lower the work function of surfaces within the ion source and enhance the surface production of H- and D- ions. However it has been suggested that for DEMO/commercial fusion reactors that a similar Cs injection rate to ITER is likely to lead to NBI operational problems during the reactor's lifetime. Frequent maintenance or replacement of beamline components due to Cs contamination will result in unacceptable amounts of reactor downtime. Fusion NBI ion sources of the future will need to dramatically reduce their rate of Cs consumption, or will require alternatives to Cs for H- surface production. In order to optimise the production of negative ions in the discharge, the electron temperature must be cooled in order to reduce the collisional detachment losses due collisions between H-/D- ions and electrons in the plasma bulk. Negative ion sources planned for ITER make use of magnetic filters to reduce the electron temperature close to the extraction system of the ion source and maximise the density of negative ions in the region before they are extracted into the acceleration stage of the beamline. Time modulated discharges developed for use in damage free etching of silicon devices have shown to be able to generate a high density of volume produced negative ions without the use of Cs ovens. The pulsed nature of the discharge leads to low electron temperatures during the afterglow which result in enhanced production rates of negative ions through the dissociative attachment of rovibrationally excited hydrogen molecules. In this work characterisation of a radio frequency, time modulated inductively coupled plasma source was carried out in both argon and hydrogen using a combination of Langmuir probe and retarding field energy analyser diagnostics. Time resolved Langmuir probe IV characteristics allowed insight into the evolution of Te, density, and Vp during the modulation cycle, as well as their response to changing discharge and modulation conditions such as gas pressure and modulation frequency. Similar techniques allowed the calculation of time resolved IVDFs of ions in the discharge. During the activeglow, values of Te, density, and Vp measured during the quasi-steadystate phase of an argon discharge agreed with other studies operating at similar discharge conditions, whilst the trends of plasma properties throughout the pulse agreed with calculations reported in the literature. During the afterglow the electron temperature rapidly cooled, falling to below 0.5eV in around 50µs whilst density decreased more slowly as ions and colder electrons diffused to the walls. Decay of the plasma density in the afterglow occurs more slowly, and agreed with analytical expressions for the decay of an electropositive discharge dominated by diffusion to the walls. Time resolved measurements of plasma parameters in a hydrogen discharge also demonstrated similar responses to changing discharge or pulse parameters. However the molecular nature of hydrogen lead to reduced plasma densities during the activeglow, and more pronounced peak or overshoot of Te and Vp during the first few µs of the pulse. During the afterglow Te and Vp more rapidly decayed, decreasing to below 0.5eV on the order of 10µs. Density measurements in the afterglow suggested that the decay was mostly controlled by diffusion to the walls. Boron doped diamond samples were placed around the extraction grid, measurements using Langmuir probes and RFA diagnostics in the extraction region and below the extraction grid were conducted in an attempt to observe any enhancement in the negative ion density in the region. No significant difference was found in the extraction region plasma properties between discharges with the BDD samples present and those without. This is likely due to intrinsic negative ion densities in the discharge being too small for the measurement techniques implemented to be able to resolve. As a result it is inconclusive as to the extent BDD enhances the surface or volume production of H- ions. Whilst measurements of the effects of BDD on the discharge were unsuccessful, this work was successful in developing and characterising a time modulated ICP discharge in both argon and hydrogen. Whilst many modelling and experimental studies have been carried out in the area of pulsed argon discharges, there has been little research conducted into time modulated hydrogen discharges. The work carried out in this project into the time evolution of hydrogen plasma properties and their response to an extraction bias in the pulsed environment should form a useful contribution to the field.
Supervisor: Bowden, Mark Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral