Beam dynamics of muon acceleration for neutrino factory

Abstract

A conceptual design of a muon acceleration scheme based on recirculating superconducting linacs is proposed. In the presented scenario, acceleration starts after ionization cooling at 245 MeV/c and proceeds to 20 GeV, where the beam is injected into a neutrino factory storage ring. The key technical issues are addressed; such as: the choice of acceleration technology (superconducting versus normal conducting) and the choice of RF frequency, and finally, implementation of the overall acceleration scheme: capture, acceleration, transport and preservation of large phase space of fast decaying species. Beam transport issues for large-momentum-spread beams are accommodated by appropriate lattice design choices. The proposed arc optics is further optimized with a sextupole correction to suppress chromatic effects contributing to emittance dilution. Muon Acceleration SchemeA neutrino factory [1] is aimed to produce narrow neutrino beams via decay of muons in long straight sections of a storage ring. As illustrated schematically in Figure 1, a proposed muon accelerator complex features a 0.2-to-2.8 GeV straight pre-accelerator linac and a 2.8-to-20 GeV four-pass recirculating linac accelerator (RLA).The pre-accelerator captures a large muon phase space coming from the cooling channel and accelerates them to relativistic energies of about 2.8 GeV. It makes the beam sufficiently relativistic and adiabatically decreases the phase-space volume, so that effective acceleration in recirculating linacs is possible. The RLA further compresses and shapes the longitudinal and transverse phase spaces, while increasing the energy. Accelerating Technology -Design ChoicesTo ensure adequate survival rates of short-lived muons, acceleration must occur at high average gradient. Initial estimate [2] shows that a "real estate average" RF gradient of 15 MV/m will allow survival of about 80% of source muons throughout the RLA. Since muons are generated as a secondary beam they occupy large phase-space volume. The accelerator must provide high average gradient, while maintaining very large transverse and longitudinal accelerator acceptances. The above requirement drives the design to low RF frequency, e.g. 200 MHz. If normal-conducting cavities at that frequency were ♣

Interim Design Report

“…The proposed "dog-bone" configuration facilitates simultaneous acceleration of both µ + and µ − species through the requirement of mirror-symmetric optics in the return "droplet" arcs. The linac consists of superconducting RF cavities and iron-shielded solenoids grouped in cryomodules [276], while the recirculating linacs RLA I and RLA II consist of superconducting RF cavities and quadrupoles. The linac is required to accelerate 0.22 GeV/c muons coming from the muon frontend to 0.9 GeV/c and, given these relatively low energies, solenoidal transverse focusing has been chosen so that the beam preserves its initial horizontal-vertical phase-space coupling.…”

“…To maximise the longitudinal acceptance, the initial position of the bunch is shifted relative to the centre of the bucket, to keep the beam boundary inside the separatrix [276], as illustrated in figure 44d. The synchrotron motion also suppresses the sag in acceleration for the bunch head and tail.…”

Muon Acceleration-RLA and FFAG

“…The large acceptance of the preaccelerator requires large aperture and tight focusing at its front-end. Given the large aperture, tight space constraints, moderate beam energies, and the necessity of strong focusing in both planes, we have chosen solenoidal focusing for the entire linac [1] …”

Recirculating Linac Acceleration—End-to-End Simulation

“…The large acceptance of the preaccelerator requires large aperture and tight focusing at its front-end. Given the large aperture, tight space constraints, moderate beam energies, and the necessity of strong focusing in both planes, we have chosen solenoidal focusing for the entire linac [1]. To achieve a manageable beam size in the linac front-end, short focusing cells are used for the first 6 cryo-modules.…”

“…The accelerator must also accommodate the phase-space volume occupied by the beam after the cooling channel. The need for large transverse and longitudinal acceptances [1] drives the design of the acceleration system to low RF frequency, e.g. 201 MHz.…”