Toward new frontiers : Encounter and synergy of state-of-the-art astronomical detectors and exotic quantum beams

This research aims to develop a novel RF cavity for a flat-top acceleration system, which is a key device to produce a negative muon microbeam. A negative muon microbeam will enable us to scan a sample with a high position resolution. In addition, non-destructive 3D scan could be realized if beam energy is scanned. The improvement of the brilliance of a negative muon beam is an important milestone for muon colliders as well.

As a source of a negative muon microbeam, we use negative muons produced by pion decays. These muons have large momentum dispersion, therefore it is impossible to focus them due to chromatic aberration. To reduce the momentum dispersion, first these muons are cooled via muon catalyzed fusion (μCF) and frictional cooling as proposed by the C02 group. After the cooling process, the energy of negative muons is about 10 keV with 100 eV dispersion. Though the dispersion is still large for microbeam production, the C02 group plans to use an aberration collector which consists of several lenses. On the other hand, this research assumes to use re-acceleration of cooled muons up to E = 1–10 MeV by a flat-top RF cyclotron, in order to reduce ΔE/E down to less than 10^–4 and avoid chromatic aberration.

A flat-top acceleration is a well-established technique and used in heavy ion cyclotrons (see references), but the short lifetime of muons (2.2 μs) makes it difficult to develop its RF cavities. The RF frequency of the muon acceleration system should be about 10 times higher than conventional cyclotrons, therefore we have to develop a new type of a flat-top acceleration cavity for a muon cyclotron. In this research, the flat-top RF cavity adopts not a conventional λ/4 coaxial resonator, but a λ/2 coaxial resonator, but a center-shorted λ/2 coaxial resonator. This design is based on an open patent (No. 2002-43097), but this type of cavity has not ever been developed and fabricated actually.

In this research, we consider a cyclotron equipped with a flat-top acceleration system, as a reacceleration method for producing a ultra-slow negative muon micro beam. We will develop a new flat-top resonator, which is the most important device to be developed to realize this cyclotron.

The flat top cavity developed in this research is a λ/2 resonator (right) by shorting the central region, unlike the usual λ/4 resonator (left) used in conventional cyclotrons. The design of the resonator is based on the patent of Sumitomo Heavy Industries, Ltd., Yukio Kumada (Patent Number 2002-43097), and this is the first research to actually carry out detailed design and production.

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