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High Current Proton Fixed-Field Alternating-Gradient Accelerator Designs
[Thesis]. Manchester, UK: The University of Manchester; 2012.
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To make energy production sustainable and reduce carbon dioxide emissions it is necessary to stop using fossil fuels as our primary energy source. The Accelerator Driven Subcritical Reactor (ADSR) could provide safe nuclear power. It uses thorium as fuel, which is more abundant than uranium, and produces less long lived waste. An ADSR uses neutron spallation, caused by a high power proton beam impacting a metal target, to drive and control the reaction.The beam needs to have an energy of around 1 GeV and a current of 10 mA with a very high reliability, the combination of which is beyond the capabilities of existing particle accelerators. Cyclotrons and synchrotrons both have trouble producing such a beam, while a suitable linac would be several hundred metres long, and expensive. A more compact accelerator design would allow multiple accelerators to be combined to improve reliability.This thesis examines the use of a Fixed-Field Alternating-Gradient (FFAG) accelerator as the proton driver. FFAGs are compact, and can simultaneously achieve higher energies than a cyclotron at higher repetition rates than a synchrotron. However, it is still a challenge to reach the high currents required. A 35 to 400 MeV non-scaling FFAG was designed to demonstrate issues encountered at high currents.Two methods were investigated in order to increase the number of particle bunches that could be simultaneously accelerated. One uses multiple solutions to the harmonic conditions for acceleration, and the second injects bunches after the acceleration has started. Neither was found to give significant practical improvement in current.Space charge is a destructive force at high currents. Software was developed to simulate the effect of space charge in an FFAG using several models. Space charge tune shifts were measured for a range of energies and currents, and peak currents of above 1 A were found to be unstable. In order to provide 10 mA of average current, acceleration would need to occur in around 100 turns, which will require a very rapid RF sweep.