Doctoral Degrees (Electrical and Electronic Engineering)

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Browsing Doctoral Degrees (Electrical and Electronic Engineering) by browse.metadata.advisor "Conradie, J. L."

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    A digital low-level radio frequency control system for the particle accelerators at iThemba LABS
    (Stellenbosch : Stellenbosch University, 2018-03) Duckitt, William; Niesler, T. R.; Conradie, J. L.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: This dissertation presents the design, implementation and evaluation of a digital low-level radio frequency (RF) control system for the cyclotron particle accelerators at iThemba LABS. This system replaces a 30-year old analog control system. The new system makes extensive use of state-of-the-art field programmable gate arrays (FPGA), high-speed digital to analog converters (DAC) and high-speed analog to digital converters (ADC). The presented system incorporates a modified direct digital synthesis (DDS) technique to directly convert the digital RF signals to analog RF and local-oscillator (LO) signals with 16-bit amplitude accuracy, programmable in steps of 1 µHz and 0.0001°. Down-conversion of the RF pick-up signals to an optimal intermediate frequency (IF) of 1 MHz and sampling of the IF channels by 16-bit, single samplelatency 10 MHz ADCs were implemented to allow digital high-speed low-latency in-phase/quadrature (I/Q) demodulation of the IF channels within the FPGA. This in turn allows efficient real-time digital closed-loop control of the amplitude and phase of the RF drive-signal to be achieved. The systems have been successfully integrated at iThemba LABS into the K=8 and K=10 injector cyclotrons (SPC1, and SPC2), the K=200 separated sector cyclotron (SSC), the SSC flat-topping system, the pulse-selector system and the AX, J, and K-line RF bunchers. The system has also been successfully integrated into the K=132 separated sector cyclotron at the Helmholtz-Zentrum Berlin (HZB). The described system achieves the target peak-peak amplitude and phase stabilities of 0.01% and 0.01° respectively and operates over the wide frequency range of 2-100 MHz. The system has led to a substantial improvement in the beam quality of the SSC at iThemba LABS. Prior to the integration of the presented system, it was difficult to achieve extraction current losses lower than 700-800 nA on the SPM1 and SPM2 magnetic extraction elements of the SSC for high-intensity 66 MeV proton beams with 220 µA on target. Once the presented system had been incorporated, the lowest losses achieved on both extraction elements were 13.1 nA for SPM1 and 18 nA for SPM2 with 227 µA on target. The reduction in losses by more than 90% results in less activation of the extraction components. This significantly reduces the radiation dose that personnel who have to service these components would be exposed to during servicing and emergency repair. The new system has demonstrated greatly improved performance in terms of amplitude and phase stability as well as efficiency. This places the RF control systems at iThemba LABS at the forefront of the state-of-the-art. It also extends the lifetime of this facility, and paves the way for future experimentation into the production of isotopes at higher current intensities, as well as the continued delivery of high precision particle beams for physics research and medical therapy.