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NON-SCALING FFAGs: questions needing answers Andy Wolski The Cockcroft Institute, and the University of Liverpool Department of Physics. BASROC-CONFORM Project Open Day Daresbury Laboratory 11 May 2009

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What makes an FFAG different? Promises: High average current, high energy beams Relatively compact, low-cost accelerator Challenges: Difficult technical subsystems Complicated beam dynamics Non-equilibrium operation

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Injection and Extraction Fields must be turned on and off in less than one revolution period. Fields must be large enough to deflect bunches through large angles. Components must fit into a very confined space. Field quality, stability and overall reliability are all issues. Can we construct injection/extraction systems that meet the operational requirements within the space constraints?

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Magnets Lattice needs a large number of identical unit cells. Accelerator components, including the magnets, need to achieve high quality in a very compact space. Can we control the field quality in the magnets well enough to avoid detrimental nonlinear effects in the beam dynamics? Can we predict these effects with sufficient accuracy?

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RF System Revolution period varies with energy. Three options for acceleration: Asynchronous Fixed rf frequency and relative phase. Simplifies rf system, but leads to complicated longitudinal dynamics. Synchronous RF frequency and phase synchronised with time of flight. Simplifies longitudinal dynamics, but requires high bandwidth cavities, with complicated controls. Harmonic jump Fixed frequency; voltage and or phase maybe variable. Can we accelerate? What are the real advantages/disadvantages with each option?

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Diagnostics Diagnostics are essential for commissioning, tuning and operation. Basic measurements include: Bunch charge. Bunch position. Charge distribution. Transverse and longitudinal diagnostics will be needed. Issues include: Dynamic range. Accuracy. Bandwidth. Stability. What are the essential diagnostics? What functionality and performance are needed? 100 MeV proton beam extracted from KEK FFAG. Y. Mori, M. Aiba, ICFA Beam Dynamics Newsletter 43 (2007) 27-37.

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Beam Dynamics We need to: avoid particle loss; maintain stability of the trajectory; preserve beam quality. Can we model the beam dynamics with sufficient accuracy? How do we best control the beam with the complicated dynamics of a nsFFAG? Variation of betatron tunes with energy in a nsFFAG. (Resonance crossing.)

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Collective Effects Charged particles in a bunch interact with each other and with their environment in a variety of different ways: Space charge. Wake fields, and beam loading. Ion trapping. Intrabeam scattering. Incoherent and coherent synchrotron radiation. What effects will be important in a non- scaling FFAG? How do we design and operate a nsFFAG to avoid detrimental collective effects?

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Pushing the limits What are the limits on performance of a non- scaling FFAG, and how can we overcome them? Acceleration – What is the largest acceleration factor that could be achieved in a non-scaling FFAG? Beam current – Is multibunch operation feasible? Beam quality and stability – What issues are associated with resonance crossing? Reliability

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Summary Non-scaling FFAGs will present challenges for design, commissioning and operation. To maximize the potential of nsFFAGs, we will need a better understanding of: performance possibilities of technical subsystems, including injection/extraction, magnets, rf, and diagnostics; non-equilibrium, nonlinear beam dynamics; design and construction for stable, reliable operation of non-equilibrium accelerators.

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