Finding the optimal RF voltage ramp to capture coasting beams in high intensity rings has been the subject of ongoing study for many decades. We are motivated to revisit the topic with a view to capturing coasting, stacked beams in a future high intensity, high power FFA. However, the results have general applicability. We compare various voltage laws including linear, bi-linear and iso-adiabatic through simulation and experimentally, making use of the ISIS synchrotron. Making use of longitudinal tomography, we seek to establish the voltage program that minimises the captured beam emittance.

Beam stacking is a key advantage of Fixed Field alternating gradient Accelerators (FFAs) for high-intensity applications. During stacking, one beam is stored as a coasting beam at the extraction energy while another, incoming beam is accelerated. However, the beam loss mechanism termed RF knockout can occur during stacking and undermine gains in extracted beam current. The accelerating RF program of the incoming beam can cause cumulative displacements in the stored coasting beam and result in significant beam loss. To ensure that beam stacking is a viable technique to extract highest intensities from an FFA, methods to avoid the loss from RF knockout must be established. This study presents results from a series of experiments at the ISIS proton accelerator to characterise and, crucially, to mitigate RF knockout and ensure successful beam stacking with no loss.

The quarter wave resonator (QWR, a.k.a. λ/4 resonator) for the new ISIS MEBT is a bunching cavity that longitudinally compresses the H- beam into smaller bunches. It has two gaps with a distance of βλ/2 between mid-gaps, and works in π mode at the resonant frequency of 202.5 MHz, with a phase angle of -90 degrees, and a maximum voltage per gap (E0L) of 55 kV. The detailed RF and thermal design was developed, followed by the manufacturing of a prototype, all being presented elsewhere. Several mechanical issues were noticed with the RF finger strips and tuners during the assembly of the prototype cavity. The manual tuner (to account for the manufacturing tolerances and the vacuum load) was machined to the final dimension to achieve the desired resonant frequency, according to the Vector Network Analyser (VNA) measurements. The measured quality factor was found to be much lower than expected, which required a redesign of some of the RF seals. The cavity was powered and conditioned in a relatively short time up to a nominal power, but severe multipacting was observed, initially only at low power, but later also at medium power levels, which required a creative approach to be fixed without a major cavity redesign.

Fixed Field Alternating gradient accelerators (FFA) hold promise for pulsed high intensity applications. No such FFA has been constructed to date; therefore a prototype - the Front End Test Stand-FFA (FETS-FFA) has been pro- posed to explore the feasibility of using FFA technology for the next generation spallation neutron source, ISIS-II. A key component of this prototype is its main magnets, which must meet several critical requirements: maintaining zero chro- maticity during acceleration, offering tune point flexibility, and providing a large dynamic aperture. The selected lattice incorporates a doublet spiral magnet design for more flexi- ble operations in the tune space. The magnetic field profile is generated by distributed conductors wound over the pole face; a 3D analysis using OPERA software was conducted to evaluate the settings necessary to produce the desired field. The cell tune variation was found to be within ±0.0015 hor- izontally and ±0.002 vertically, for four different working tune points.