Galante, Bruno
(2023)
Characterization studies of carbon nanotubes as cold electron field emitters for electron cooling applications in the Extra Low ENergy Antiproton
(ELENA) ring at CERN.
Doctor of Philosophy thesis, University of Liverpool.
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Abstract
Electron emission allows the extraction of electrons from a material, commonly a metal or a semiconductor. There are three main processes for achieving electron emission: thermionic emission, photo-emission, and field emission. Electron emission is of high importance in electron cooling. Electron cooling guarantees beam quality in low-energy antimatter facilities. In the Extra Low ENergy Antiproton ring (ELENA), the electron cooler reduces the emittance growth of the antiproton beam so that a focused and bright beam can be delivered to the experiments at the unprecedentedly low energy of 100 keV. Cold emission can be beneficial in the electron cooler of ELENA for avoiding the use of a thermionic cathode. The thermionic cathode imposes constraints on the beam temperature, the fragility of the gun, and the gun design. A cold cathode may solve all these problems at once. In this thesis work, cold emission is achieved via field emission from carbon nanotubes (CNTs). For the electron cooler of ELENA, the lifetime and the current stability of the electron source play a primary role. Furthermore, it is important to pulse the cathode to promptly enable/disable the cooling process and to change the electron beam energy according to the cooling stage and the ELENA beam cycle. This work proves that a CNT array can stably emit for hundreds of hours and a record emission time of 1500 hours has been proved. The operation of a CNT cathode in current switching mode has also been tested. Rise and fall times below 1 μs have been proved with unaltered current emission stability during operation. Several constraints can arise when using CNTs as electron sources. Among these, the role of an extracting grid needs accurate evaluation. In this Ph.D. work simulations of the beam passage through micro-metric grids were performed. These simulations were aimed at studying the effect of the grid on the beam current, trajectory, and transverse energy. It was found that a grid with a squared pattern, 15 μm hole size and 3 μm bar width, does not significantly alter the beam trajectory and the beam transverse energy. The transparency of such a grid was about 68%. Finally, guidelines for mitigating possible issues related to the design of a CNT-based cold cathode electron gun were addressed. A gun layout has been proposed after the optimization of all components through electromagnetic simulations. The findings of the research carried out within the frame of this Ph.D. project are applicable to a wide range of CNT applications: from electron guns for accelerator science to X-ray tubes, CNT-based televisions, and any case where electron extraction is required. The findings from studies into extracting grids and gun design have an even wider reach and benefit any application where particle extraction and transport are involved.
| Item Type: | Thesis (Doctor of Philosophy) |
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| Divisions: | Faculty of Science and Engineering > School of Physical Sciences |
| Depositing User: | Symplectic Admin |
| Date Deposited: | 31 May 2023 08:21 |
| Last Modified: | 31 May 2023 08:22 |
| DOI: | 10.17638/03168931 |
| Supervisors: |
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| URI: | https://livrepository.liverpool.ac.uk/id/eprint/3168931 |
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