High field – low energy muon ionization cooling channel

Abstract

Muon beams are generated with large transverse and longitudinal emittances. In order to achieve the low emittances required by a muon collider, within the short lifetime of the muons, ionization cooling is required. Cooling schemes have been developed to reduce the muon beam 6D emittances to ≈300 μm-rad in transverse and ≈1-1.5 mm in longitudinal dimensions. The transverse emittance has to be further reduced to ≈50-25 μm-rad with an upper limit on the longitudinal emittance of ≈76 mm in order to meet the high-energy muon collider luminosity requirements. Earlier studies of the transverse cooling of low energy muon beams in high field magnets showed a promising performance, but did not include transverse or longitudinal matching between the stages. In this study we present the first complete design of the high field-low energy ionization cooling channel with transverse and longitudinal matching. The channel design was based on strong focusing solenoids with fields of 25-30 T and low momentum muon beam starting at 135 MeV=c and gradually decreasing. The cooling channel design presented here is the first to reach ≈50 micron scale emittance beam. We present the channel's optimized design parameters including the focusing solenoid fields, absorber parameters and the transverse and longitudinal matching.

Muon Collider Forum report

“…The cooling channels described thus far cannot reach the transverse emittance of around 25 μm (normalized) required for the beams in the collider ring. A "final cooling" system first described by R. Palmer involving an alternation between uniform, high-field solenoids and acceleration was designed and simulated during the MAP program [183]. The beamline does not so much cool the beam as "exchange" emittance to reduce the transverse emittance at the cost of a longitudinal emittance increase.…”

“…The design in [183] assumed a maximum solenoid field of 30 T; a higher field would be expected to produce lower emittances. Hybrid superconducting-resistive solenoids exist with fields as high as 45 T [184], purely superconducting solenoids exist with fields as high as 32 T [185], and an R&D project toward a 40 T purely superconducting solenoid is funded [186].…”

Magnets for a Muon Collider

“…To achieve maximum cooling efficiency and minimum emittance there is hence a clear interest in steady-state solenoid fields at the upper end of the technology reach. A design study from MAP based on a 30 T final cooling solenoid demonstrated that an emittance of about 50 micron, roughly a factor of two greater than the transverse emittance goal (25 micron), can be achieved [12]. However, other analyses [13] show that fields in the range of 50 T improve the final emittance requirements and offer further gains in beam brightness.…”

Towards a Muon Collider

“…Final cooling In the final muon cooling system, solenoids with the highest practical field are needed. A design based on 30 T solenoids -a value that has already been exceeded in a user facility using high-temperature superconductor -demonstrated that an emittance about a factor two above the target can be achieved [136]; it should be noted that the study aimed at this larger target. Several options to improve the emittance will be studied.…”