DetectorSolenoidIntro

Introduction

The Detector Solenoid surrounds both the Stopping Target, where muons are captured, and the tracking detector and calorimeter of the Mu2e experiment, where electrons from those muons are detected and their momenta are measured. In the region of the Stopping Target, the Detector Solenoid has a gradient field, to focus outgoing electrons toward the detector. The field is constant throughout the tracking detector and calorimeter, to allow for particle identification and the reconstruction of particle momenta with excellent resolution.

Figure 1. Overall structure of the Detector Solenoid coils and cryostat.

General Requirements

The main functions of the Detector Solenoid (DS) are to provide a graded field in the region of the stopping target and to provide a precision magnetic field in a volume large enough to house the tracker downstream of the stopping target. The inner diameter of the magnet cryostat is 1.9 m and its length is 10.9 m. The inner cryostat wall supports the stopping target, tracker, calorimeter and other equipment installed in the Detector Solenoid. This warm bore volume is under vacuum during operation. It is sealed on one side by the VSP and instrumentation feed through bulkhead, while it is open on the other side where it interfaces with the Transport Solenoid. The last section of the Transport Solenoid protrudes into the DS cryostat.

The Detector Solenoid is designed to satisfy the field and operational requirements defined in the DS requirements document[1]. The overall structure of the solenoid is shown in Figure 1. It consists of two sections: a "gradient section", which is about 4 m long, and a "spectrometer" of about 6 m. The magnetic field at the entrance of the gradient section is 2 T and it decreases linearly to 1 T at the entry to the spectrometer section, where it is then uniform over 5 m.

The Detector Solenoid does not have an iron return yoke.

Two types of conductor are required, both 20 mm in height. The "narrow" (5.25 mm in width) DS1-type conductor will be used in the DS gradient section, while the "wide" (7 mm in width) DS2-type conductor is used in the spectrometer section. The dimensions are optimized to give the required field when identical current is transported in both conductors. The conductors contain Rutherford-type NbTi cables with 12 and 8 strands, respectively. The strands have a diameter of 1.3 mm, a SC/Cu ratio of 1, and a critical current of 2750 A/mm2 (4.2 K, 5 T). As a result, the conductors have significant stability and safety margins in case of a quench.

In the baseline design, the gradient section is wound in two layers using the "narrow" DS1-type conductor (20 mm × 5.25 mm), which is necessary to obtain a field of 2 T. The field gradient is obtained by introducing several sets of spacers between coil turns. The field uniformity in the spectrometer section is achieved with a "wide" DS2-type conductor (20 mm × 7 mm), wound in a single layer coil.

It is envisaged that the DS coil will be wound in standardized modules on accurately machined collapsible mandrels. After curing, the winding mandrels are extracted and the outer aluminum support cylinders are placed over each module and the assembly epoxy bonded. The preassembled modules are then electrically connected and bolted together with spacers in a single cold mass before installation in the cryostat. The Detector Solenoid (cold mass and cryostat) weights about 42 tonnes.

Figure 2. DS coils. This figure shows the distribution of coils that form the detector solenoid. DS is composed of 11 coils. The TS coils were suppressed from the picture for clarity.

[1] Lamm, M., “Detector Solenoid Requirements Document,” (2013) Mu2e-doc-946