D. W. Posthuma de Boer

Beam stacking allows a fixed field alternating gradient accelerator (FFA) to increase the extracted beam current while also enabling a flexible time structure, making FFAs a promising candidate for future spallation neutron sources and other high intensity beam applications. For successful beam stacking, beam loss caused by rf knockout must be prevented. rf knockout can occur during beam stacking because of the finite dispersion function at the rf cavity location, which is unavoidable in a scaling FFA. In this work, the rf knockout resonance is characterized and through a series of experiments at the ISIS Neutron and Muon Source, we show that it is possible to suppress losses due to rf knockout.

A key challenge in particle accelerators is to achieve high peak intensity. Space charge is particularly strong at lower energy such as during injection and typically limits achievable peak intensity. The beam stacking technique can overcome this limitation by accumulating a beam at high energy where space charge is weaker. In beam stacking, a bunch of particles is injected and accelerated to high energy. This bunch continues to circulate, while a second and subsequent bunches are accelerated to merge into the first. It also allows the user cycle and acceleration cycles to be separated which is often valuable. Beam stacking is not possible in a time varying magnetic field, but a fixed field machine such as an fixed field alternating gradient accelerator (FFA) does not sweep the magnetic field. In this paper, we describe experimental demonstration of beam stacking of two beams at KURNS FFA in Kyoto University. The momentum spread and intensity of the beam was analyzed by study of the Schottky signal, demonstrating stacking with only a slight increase of momentum spread of the combined beams. The intensity of the first beam was, however, significantly reduced. rf knockout is the suspected source of the beam loss.