R. Wands

Solenoid Magnet System for the Fermilab Mu2e Experiment

The Fermilab Mu2e experiment seeks to measure the rare process of direct muon to electron conversion in the field of a nucleus. Key to the design of the experiment is a system of three superconducting solenoids; a muon production solenoid (PS) which is a 1.8 m aperture axially graded solenoid with a peak field of 5 T used to focus secondary pions and muons from a production target located in the solenoid aperture; an “S shaped” transport solenoid (TS) which selects and transports the subsequent muons towards a stopping target; a detector solenoid (DS) which is an axially graded solenoid at the upstream end to focus transported muons to a stopping target, and a spectrometer solenoid at the downstream end to accurately measure the momentum of the outgoing conversion electrons. The magnetic field requirements, the significant magnetic coupling between the solenoids, the curved muon transport geometry and the large beam induced energy deposition into the superconducting coils pose significant challenges to the magnetic, mechanical, and thermal design of this system. In this paper a conceptual design for the magnetic system which meets the Mu2e experiment requirements is presented.

Challenges and Design of the Transport Solenoid for the Mu2e Experiment at Fermilab

The Fermilab Mu2e experiment seeks to measure the rare process of direct muon to electron conversion in the field of a nucleus. The magnet system for this experiment is made of three warm-bore solenoids: the Production Solenoid (PS), the Transport Solenoid (TS), and the Detector Solenoid (DS). The TS is an “S-shaped” solenoid set between the other bigger solenoids. The Transport Solenoid has a warm-bore aperture of 0.5 m and field between 2.5 and 2.0 T. The PS and DS have, respectively warm-bore aperture of 1.5 m and 1.9 m, and peak field of 4.6 T and 2 T. In order to meet the field specifications, the TS starts inside the PS and ends inside the DS. The strong coupling with the adjacent solenoids poses several challenges to the design and operation of the Transport Solenoid. The coil layout has to compensate for the fringe field of the adjacent solenoids. The quench protection system should handle all possible quench and failure scenarios in all three solenoids. The support system has to be able to withstand very different forces depending on the powering status of the adjacent solenoids. In this paper, the conceptual design of the Transport Solenoid is presented and discussed focusing on these coupling issues and the proposed solutions.

Reference Design of the Mu2e Detector Solenoid

The Mu2e experiment at Fermilab has been approved by the Department of Energy to proceed with the development of the preliminary design. Integral to the success of Mu2e is the superconducting solenoid system. One of the three major solenoids is the detector solenoid that houses the stopping target and the detectors. The goal of the detector solenoid team is to produce detailed design specifications that are sufficient for vendors to produce the final design drawings, tooling and fabrication procedures and proceed to production. In this paper we summarize the reference design of the detector solenoid.

Mu2e Transport Solenoid Prototype Design and Manufacturing

The Mu2e Transport Solenoid consists of 52 coils arranged in 27 coil modules that form the S-shaped cold mass. Each coil is wound from Al-stabilized NbTi superconductor. The coils are supported by an external structural aluminum shell machined from a forged billet. Most of the coil modules house two coils, with the axis of each coil oriented at an angle of approximately 5° with respect to each other. The coils are indirectly cooled with LHe circulating in tubes welded on the shell. In order to enhance the cooling capacity, pure aluminum sheets connect the inner bore of the coils to the cooling tubes. The coils are placed inside the shell by the means of a shrink-fit procedure. A full-size prototype, with all the features of the full assembly, was successfully manufactured in a collaboration between INFN Genova and Fermilab. In order to ensure an optimal mechanical prestress at the coil-shell interface, the coils are inserted into the shell through a shrink-fitting process. We present the details of the prototype with the design choices as validated by the structural analysis. The fabrication steps are described as well.

Design and considerations on long Nb/sub 3/Sn high field magnets for hadron colliders

Design studies for long high field Nb/sub 3/Sn superconducting magnets for hadron colliders are described, taking a 10 meter magnet with a cosine-theta type coil as an example. The problems and complications are discussed in comparison with short magnets of 1 meter length, using MIIT calculations and quench simulation. As the stored energy in the high field Nb/sub 3/Sn magnets is quite large, close attention must be paid to all design details, and especially the extraction of the stored energy. The extensive use of heaters on the coil surface is simulated to dump the energy on the coil body itself. The MIIT value of the Nb/sub 3/Sn superconducting cable should be made large for safe operation. This is done by increasing the copper ratio. According to the results of this study, an 11.5 Tesla magnet might be the limiting case for the design of a practical accelerator high field magnet.

Complete Quench Simulation of Large Solenoid Magnet

Quench simulation method for large magnet systems has been developed. Using commercially available FEM program, ANSYS, a script program with a new algorithm was introduced to perform the complete simulation of the quench process of superconducting magnets. The method was successfully tested with ATLAS central solenoid and applied for the design of the mu2eDS magnet at Fermilab. The mu2eDS magnet is found safe against any quench situations.

A Five Tesla Solenoid With Detector Integrated Dipole for the Silicon Detector at the International Linear Collider

A conceptual design study for a 5 Tesla superconducting solenoid for the Silicon Detector (SiD) of the International Linear Collider (ILC) has been undertaken. Utilizing the existing Compact Muon Spectrometer (CMS) magnet conductor as the starting point, a winding design has been proposed for the magnet. Finite element analysis shows the resulting magnetic stresses in the coil do not greatly extrapolate beyond those of CMS, and decentering forces to the muon steel are shown to be manageable. For compensation of finite crossing angles of the ILC beams, a dipole coil integrated with the solenoid is examined

Challenges in the Design of the Detector Solenoid for the Mu2e Experiment

The Mu2e experiment at Fermilab is being designed to measure the rare process of direct muon-to-electron conversion in the field of a nucleus. The experiment comprises a system of three superconducting solenoids, which focus secondary muons from the production target and transport them to the stopping target, while minimizing the associated background. The detector solenoid is the last magnet in the transport line and it consists of an axially graded-field section at the upstream end, where the stopping target is located, and a spectrometer section with uniform field at the downstream end for accurate momentum measurement of the conversion elections. The detector solenoid has a warm bore of 1.9 m and is 10.75 m long. The stored energy of the magnet is 30 MJ. The conceptual design of the magnet is presented, in particular the challenge of achieving tight magnetic field specification in a cost-effective design.

Thermal Design of the Mu2e Detector Solenoid

The reference design for a superconducting detector solenoid (DS) for the Mu2e experiment has been completed. The main functions of the DS are to provide a graded field in the region of the stopping target, which ranges from 2 to 1 T and a uniform precision magnetic field of 1 T in a volume large enough to house a tracker downstream of the stopping target. The inner diameter of the magnet cryostat is 1.9 m and the length is 10.9 m. The gradient section of the magnet is about 4 m long and the spectrometer section with a uniform magnetic field is about 6 m long. The inner cryostat wall supports the stopping target, tracker, calorimeter and other equipment installed in the DS. This warm bore volume is under vacuum during operation. It is sealed on one end by the muon beam stop, while it is open on the other end where it interfaces with the Transport Solenoid. The operating temperature of the magnetic coil is 4.7 K and is indirectly cooled with helium flowing in a thermosiphon cooling scheme. This paper describes the thermal design of the solenoid, including the design aspects of the thermosiphon for the coil cooling, forced flow cooling of the thermal shields with 2 phase LN2 (Liquid Nitrogen) and the transient studies of the cool down of the cold mass as well.