Eddy-currrents and the Force Analysis for the Thermal Shields of the MICE Spectrometer Solenoids

MICE Note-420 Eddy-currents and the Force Analysis for the Thermal Shields of the MICE Spectrometer Solenoids H. Pan 1 , S. O. Prestemon 1 , S. Virostek 1 , R. Preece 2 and M. A. Green 1 Abstract—The MICE spectrometer solenoids contain five superconducting solenoids with different structural parameters. Two coils are each separately powered. The other three coils are connected in series. The five coils are wound on a single 6061- aluminum mandrel. If one of the coils quenches, the quench will propagate by quench-back and cause other coils quench sequentially. The decaying field will induce a significant eddy current and Lorentz force on the surrounding shields. Shield eddy current heating is non-issue for the spectrometer solenoids, but the Lorentz force becomes an important aspect for designing because the five coils are not symmetric. In this paper, the detailed eddy current simulations were performed. The net lateral forces on each part of shields and the stress distribution were analyzed. Based on the Finite-element analysis, adding cuts on shields is an effective way to avoid the collapse of the shield due to changing current path direction. The shield cuts layout is presented in detail. the radiation heat from the room temperature. The main structure of shields consist of two 9.5 mm thick neck copper shields and a 6.35 mm thick main aluminum shields including main cylinder, endplates and inner bore. Based on the previous test results, the main shields are expected to use 1100 aluminum to enhance the thermal uniformity [5]. Index Terms—Eddy current, force, quench, superconducting magnets, shields, transient stress. I. I NTRODUCTION T HE objective off the Muon Ionization Cooling Experiment (MICE) is to demonstrate the designing and engineering on a section of cooling channel capable of giving the desired performance for a Neutrino Factory [1]. Two spectrometer solenoids in the MICE cooling channel will provide a uniform magnetic field for five plane scintillating fiber trackers to analyze the changing of the muon beam emittance in the cooling channel [2]. Each of the spectrometer magnet consists of five coils wound on a 2543-mm long 6061-Al mandrel as represented in Fig.1 (not drawn to scale). The center and end coils are the tracker section of the spectrometer solenoids, providing a uniform magnetic field (0.3 percent) over a length of 1000 mm and in a diameter of 300 mm; The two match coils are to match the muon beam from the Absorber Focus Coil module (AFC) into the tracker section [3,4]. All the coils are bath-cooled in liquid helium and use cryocoolers to re-condense helium vapor. In order to reduce the heat load to the cold mass, a set of shields cooled by the first-stages of thee coolers were adopted to intercept most of Manuscript received October 9, 2012. This work was supported in part by the U.S. Department of Energy under contract DE-AC02-05CH11231. H. Pan, S. O. Prestemon, S. Virostek and M. A. Green are with Lawrence Berkeley National Laboratory, Berkeley, CA, 94720 USA (phone: 510-612- 7137; e-mail: hengpan@lbl.gov). R. Preece is with Rutherford Appleton Laboratory, Didcot, OX11 0QX UK. (e-mail: Roy.Preece@stfc.ac.uk). Fig. 1. The cold mass and shields cross section of the spectrometer solenoids Because the cold mass is asymmetric, the interaction of its local magnetic field with the eddy current induced on the conductive parts of the cryostat during a quench could introduce significant lateral forces on the mandrel and the shields. Eddy currents don’t affect the mandrel mechanical stability due to a high electrical resistivity of 6061-Al. The big concern is with the shields where their low electrical resistivity causes a large-forces during a quench. One must reduce the eddy current impact by redesigning the shields. II. MICE S PECTROMETER S OLENOIDS P ARAMETERS The Spectrometer solenoid consists of five coils wound on the slotted aluminum 6061 bobbin, and covered by Al bandage wound on top of each coil to avoid separation between coil and mandrel. Table I represents the parameters of coils for this study. TABLE I D ESIGN P ARAMETERS FOR C OILS Parameter No. turns/layer No. layers Inner radius (mm) Outer radius (mm) Axial coil build (mm) Initial current (A) M1 M2 E1 C E2