Beam Dynamics Studies of the EMMA Linear Non-Scaling FFAG

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Abstract

The development of charged particle accelerators is today reaching far beyond the realm of fundamental particle physics research. Many non-trivial social and political problems may find part of their solution lies in accelerator physics. For example, with fossil fuels becoming ever more controversial and expensive to obtain, the use of Accelerator Driven Sub-critical Reactors (ADSR) powered by rapid cycling, high current proton accelerators and thorium fuel could become part of the energy solution. Through the simplicity of the Bragg peak, cancer therapy could be enhanced through the use of high repetition rate, variable energy proton accelerators small enough to use in treatment centres. The growing problem of long lived nuclear waste storage could become a moot point through the use of high current, high power proton accelerators coupled with neutron spallation.These rapidly growing areas of study are fuelled by the development of the Fixed-Field Alternating-Gradient (FFAG) accelerator, and more recently the non-scaling FFAG. The FFAG has the ability to accelerate high current, low quality bunches of particles in very short time scales due to the fixed-field nature of its magnets. This rapid acceleration can be of the order 500 nanoseconds to 1 microsecond meaning a fast cycling rate of the machine is possible. This allows the realistic development of the ADSR, proton therapy machine and even the muon accelerator. The Electron Model with Many Applications (EMMA) accelerator is the world's first linear non-scaling FFAG and is an electron proof-of-principle accelerator based at Daresbury Laboratory, UK. EMMA can accelerate over its energy range of 10 - 20 MeV in approximately 5 - 10 machine revolutions (~275 - 500 nanoseconds) using fixed-frequency novel acceleration techniques. The accelerator contains fixed-field, constant gradient quadrupole magnets which provide all the bending and focussing to the particles. Due to the linear non-scaling nature of EMMA, many transverse integer tune values are crossed which typically cause resonant effects resulting in bunch degradation and loss. It was proposed and demonstrated that rapid crossing (in 5 - 10 turns) of integer tune values in EMMA did not result in transverse amplitude growth and particle loss. If the wider societal goals of the non-scaling FFAG are to be realised, protons and other heavy ions must be accelerated. Current technological limitations dictate that longer acceleration times of the order 1000's of turns would be necessary in proton machines of similar design to EMMA. Hence slower integer tune crossing was studied using acceleration in a synchrotron bucket in EMMA. It was found experimentally that below the nominal EMMA operating acceleration rate of 2.0 MV per turn, instabilities begin to manifest. This was indicated in the growth of closed orbit distortion (COD) and through simulation it was found that betatron amplitude growth coupled with COD resulted in eventual loss of particles to the physical aperture when crossing integer tunes. Through simulation, the amplitude growth of particles crossing integer tunes in the EMMA non-scaling FFAG was found to agree with a theory of resonance crossing proposed by R. Baartman. This theory shows that amplitude growth is proportional to $1/\sqrt(Q')$ where $Q'$ is the tune crossing rate of the particles. This means that the slower the acceleration, the slower an integer tune is crossed and hence more amplitude is gained. It was also shown that strength of the magnetic errors driving the resonant conditions was proportional to the amplitude growth.

Keyword(s)

Accelerators; Beam Dynamics; Daresbury Laboratory; EMMA; FFAG; Particle Accelerators; Resonances

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