Beam Stacking Experiment at a Fixed Field Alternating Gradient Accelerator

TL;DR

This work demonstrates beam stacking in a Fixed Field Alternating Gradient Accelerator (FFA) to boost peak beam power by accumulating multiple beams at extraction energy, thereby mitigating space-charge limits at injection. Using the KURNS MR, a radial-scaling FFA, the authors implement RF gymnastics controlled by an AWG to inject, stack, and recapture two beams, and assess momentum spread with Schottky spectra. The experimental results show successful stacking with only modest increases in longitudinal emittance, but a substantial reduction in the first-beam intensity, attributed to RF-knock-out, supported by 6D simulations that reproduce resonance-driven emittance growth. Mitigation strategies based on phase cancellation across RF cavities are proposed, offering a path toward higher-intensity operation in FFAs for applications such as spallation neutron sources, muon colliders, and high-power proton drivers; further experiments are planned to validate these approaches.

Abstract

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 analysed 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 knock-out is the suspected source of the beam loss.

Figures (16)

• Figure 1: Footprint of KURNS MR. Main magnets are shown as a red rectangular box. Injection and extraction orbits are shown with green and blue lines, respectively. Some equipments are also shown as indicated.
• Figure 2: Longitudinal phase space for the case of stacking at the top (a,b) and stacking at the bottom (c,d) without gap. The figures on the left (a,c) show the distribution at the point when the synchronous phase reaches zero in each case. The black contour shows the RF bucket separatrix. The figures on the right (b,d) show the distribution after the RF voltage has been gradually reduced to zero. The area between the dashed black lines on the right is equal to the bucket area in the figures on the left.
• Figure 3: Stacking at the bottom (as in Fig. \ref{['fig:sim:phasesp_c']}, Fig \ref{['fig:sim:phasesp_d']}) but in this case $A > \varepsilon_L$ and the separation between the beams is greater than $\delta E_b$.
• Figure 4: Relative momentum projection of two coasting beam distributions when the second beam (orange) is accelerated to a final energy offset $\Delta E_{sep}$ of a) -168, b) -103, c) 0 and d) 200 compared to the initial energy of the first beam (blue).
• Figure 5: Relative momentum width as a function of $\Delta E_{sep}$. The green points show the full width, including any separation between the beams. The blue and orange points show the full width of the first and second beams respectively. In cases where the first beam is split, as in Fig. \ref{['fig:sim:hist']}(b), the blue points show the sum of the two widths. The vertical lines divide the plot into the three regimes described in the text, from the left: that of no crossing, partial crossing or complete crossing of the first beam by the RF bucket.