Speaker: David Kelliher
Abstract: A key requirement of future neutron spallation and muon sources is flexibility of operation to best serve multiple target stations. Beam stacking allows a rapid cycling, high intensity machine to operate at lower repetition rates but with higher peak output. The effect of phase displacement and scattering on the stacked beam is described, revisiting theory first developed during the MURA years. We go on to show how beam stacking can be achieved both in the ISIS2 vFFA and its prototype ring. We also consider the choice of RF cavity technology.
2 replies on “Beam Stacking in a vertical FFA”
David, I love your slides. 1 question and 1 comment
Q: why do you employ different harmonic numbers for stacking and for acceleration. There may be a beam dynamic advantage, but you have the cost and complication of two RF systems.
Comment: Your slide #11: be aware that the thin strands of beam can be susceptible to microwave instability (of course, depending on above/below transition and whether dominant impedance is capacitive, inductive or resistive). The beam strands have much smaller delta-p/p, and Landau damping may be lost. First experimental evidence: Daniel Boussard. First explanation: Ingo Hoffman.
Thanks Shane. We chose harmonic 2 for acceleration to ensure that the RF frequency is within the operating range of the cavity. Higher harmonics are excluded by the injected bunch duration. The maximum voltage of the RF cavity for acceleration, about 6kV, is insufficient to capture more than (at a push) 2 stacked beams. Hence, we have opted for a separate fixed frequency cavity which allows higher peak voltage. Harmonic 1 makes sense as it maximises the bucket area for a given voltage and allows reasonable beam-free time for the extraction kicker.
On your comment – this is an interesting observation. We expect the microwave instability to be mitigated by the fact that the we operate below transition (the momentum compaction factor is zero in a vFFA) and because, in the case of the prototype ring, our injected beam current is low. This will be studied in more detail for the ISIS2 case.