Dr Max Topp-Mugglestone

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 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.

Abstract:

Electron-beam current in superconducting radiofrequency energy recovery linear accelerators (SCRF ERLs) is limited by beam break-up (BBU) instabilities disrupting beam transport and energy recuperation. The instabilities originate from the accumulation of high order modes (HOMs) in the cavity and positive feedback between the beam and HOMs which is especially evident in multi-pass ERLs. Overcoming the beam current limitations due to BBU instabilities is one of the challenges in accelerator science. A dual-axis asymmetric SCRF ERL has been proposed as a possible way to drive a high average current electron beam for next-generation light sources and beam cooling in electron-ion colliders (EICs). In this work, detailed studies on HOMs in the asymmetric dual axis cavity were carried out. The asymmetric field distribution of HOMs (i.e., confinement to one or another axis) has been demonstrated and their excitation from different axes has been studied. The results confirm the numerical predictions and theory developed to describe the asymmetric dual axis cavity.