A digital low-level radio frequency control system for the particle accelerators at iThemba LABS

Duckitt, William (2018-03)

Thesis (PhD)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Hierdie proefskrif bied die ontwerp, implementering en evaluering van 'n digitale laevlak radiofrekwensie (RF) beheerstelsel vir die siklotron tipe deeltjiesversnellers by iThemba LABS. Hierdie stelsel vervang 'n 30 jaar oue analoog beheerstelsel. Die nuwe stelsel maak intensief gebruik van die nuutste veldprogrammeerbare hekstruktuur (FPGA), hoëspoed syfer-analoogomsetters (SAO) en hoëspoed analoogsyferomsetters (ASO). Die voorgestelde stelsel bevat 'n gewysigde direkte digitale sintese (DDS) tegniek om die digitale RF seine direk na analoog RF en lokale ossillator seine om te skakel met 16-bis amplitude akkuraatheid, programmeerbaar in stappe van 1 µHz en 0.0001°. Afmenging van die RF terugvoer seine na 'n optimale tussenfrekwensie van 1 MHz en die meting van die tussenfrekwensie-kanale deur 16-bis, 10 MHz SAO's met enkelmonster-vertraging, is geïmplementeer. Hoë spoed lae vertraging in-fase/kwadratuur (I/Q) demodulasie van die tussenfrekwensie-kanale word deur die FPGA uitgevoer. Die tegnieke maak dit moontlik om effektiewe intydse digitale geslote lus beheer van die amplitude en fase van die RF-dryfsein uit te voer. Die stelsels is suksesvol by iThemba LABS geïntegreer in die K=8 en K=10 injektor siklotrone (SPC1 en SPC2), die K=200 oopsektor siklotron (OSS), die OSSplatkruin stelsel, die puls-selektor stelsel en die AX-, J- en K-lyn RF verdigters. Die stelsel is ook suksesvol geïntegreer in die K=132 oopsektor siklotron by die Helmholtz-Zentrum Berlyn (HZB). Die voorgestelde stelsel bereik die teiken piek tot piek amplitude en fase stabiliteit van onderskeidelik 0.01% en 0.01° en kan bedryf word oor 'n wye frekwensiebereik van 2 tot 100 MHz. Die implementering van die stelsels het bygedra tot 'n aansienlike verbetering in die bundel kwaliteit van die OSS by iThemba LABS. Voor die integrasie van die voorgestelde stelsel, was dit moeilik om ekstraksie stroomverliese laer as 700-800 nA op die SPM1- en SPM2-magnetiese ekstraksie-elemente van die OSS vir 'n hoëintensiteit 66 MeV protonbundel met 220 µA op die teiken te bereik. Na die integrasie van die voorgestelde stelsel was bundelverliese van so laag as 13.1 nA vir SPM1 en 18 nA vir SPM2 met 227 µA op die teiken gemeet. Die meer as 90% afname in bundelverliese impliseer minder aktivering van die ekstraksie komponente en gevolglik laer vlakke van bestraling waaraan personeel blootgestel word tydens onderhoud en noodherstelwerk daarvan. Die nuwe stelsel het reeds 'n groot verbetering in terme van amplitude en fase stabiliteit en gevolglike doeltreffendheid gedemonstreer. Dit plaas die RF beheerstelsels by iThemba LABS aan die voorpunt van die nuutse tegnologie. Met die stelsels word die leeftyd en volhoubaarheid van kwaliteit hoë intensiteit bundelvoorsiening deur die fasiliteit verleng. Dit baan ook die weg vir toekomstige navorsing in die verhoging van die maksimum bundelintensiteite vir die produksie van radioisotope asook die voortgesette lewering van bundels met hoë bundelkwaliteit vir kernfisika navorsing en mediese behandeling.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103569
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